Self Assessment and Review of Biochemistry Rebecca James Perumcheril
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1Amino Acids and Proteins
CHAPTER OUTLINE
  1. Chemistry and Metabolism of Amino Acids
  2. Proteins
  3. Enzymes
2

Chemistry and Metabolism of Amino Acids1

 
CHEMISTRY OF AMINO ACIDS
 
General Structure of Alpha Amino Acid
zoom view
Fig. 1.1: General structure of alpha amino acid
 
Alpha Amino Acid
Amino group and carboxyl group attached to the alpha carbon atom.
Most of the amino acids are alpha amino acid.
 
Non Alpha Amino Acid
Unlike alpha amino acids either carboxyl group or Amino group is not attatched to the alpha carbon atom.
Non alpha amino acids present in tissues in free form are:
  • β Alanine
  • β Amino Isobutyrate
  • γ-Amino Butyrate
 
Imino Acid
In an imino acid amino group is not free.
The nitrogen of amino group is seen inside the Pyrrolidine ring.
Still it can form a peptide bond.
Proline is an imino acid
zoom view
Fig. 1.2: Structure of imino acid, proline
 
CLASSIFICATION OF AMINO ACIDS (VERY IMPORTANT TOPIC)
 
BASED ON VARIABLE SIDE CHAINQ
 
I. Aliphatic Amino Acid
  1. Simple Amino Acid
    • GlycineQ
    • Alanine
  2. Branched Chain Amino AcidQ
    • Leucine
    • Isoleucine
    • Valine
Mnemonic for Branched Chain Amino Acid-LIV Amino Acid4
  1. Sulphur Containing Amino Acid
    • CysteineQ
    • MethionineQ
  2. Amino Acid with Hydroxyl Group
    • SerineQ
    • ThreonineQ
  3. Amino Acid with Amide Group
    • Asparagine
    • Glutamine
  4. Acidic Amino Acid
    • Aspartic Acid (Aspartate)
    • Glutamic Acid (Glutamate)
  5. Basic Amino Acid
    • Arginine (Most Basic Amino acid)Q
    • Lysine
 
II. Aromatic Amino Acid
  • PhenylAlanineQ
  • TyrosineQ
With heterocyclic aromatic ring (Ring structure contain more than one type of atom).
  • TryptophanQ
  • Histidine (Basic Amino Acid)Q
 
III. Imino Acid
  • Proline
 
BASED ON SIDE CHAIN CHARACTERISTIC (POLARITY)Q
zoom view
Fig. 1.3: Classification of amino acid based on polarity
 
1. Polar Amino AcidsQ (Hydrophilic)
Charged
  • Acidic Amino acids-Aspartic Acid (Aspartate), Glutamic Acid (Glutamate)
  • Basic Amino Acids-Histidine, Arginine, Lysine
Uncharged
  • Aliphatic amino acid with hydroxyl group as side chain-Serine, Threonine
  • Aliphatic amino acids with amide group-Asparagine, Glutamine
  • Simple Amino acid-Glycine alone
  • Sulphur containing Amino acid- Cysteine alone
 
2. Nonpolar Amino AcidQ (Hydrophobic)
  • Simple amino acid –Alanine alone
  • Sulphur containing amino acid-Methionine alone
  • Aromatic Amino acids except Histidine (Think as it is basic amino acid it is already included among polar amino acid)
  • All branched chain amino acids-Leucine, Isoleucine and Valine
  • Imino acid-Proline
 
BASED ON METABOLIC FATEQQQ
Classification of Amino
Amino Acid
Purely Ketogenic
LeucineQ
Both Ketogenic and Glucogenic
Phenyl Alanine
Isoleucine
Tyrosine
Tryptophan
Lysine* (Predominantly Ketogenic)
Glucogenic
Any amino acid that do not belong to the above groups
 
BASED ON NUTRITIONAL REQUIREMENTQQQ
  • Essential: Those amino acids which cannot be synthesised in the bodyQ. Hence these amino acids are to be supplied in the diet.
  • Semiessential: Growing children require them in the food, but not essential in adults.
  • Nonessential: Amino acids which can be synthesised in the bodyQ, hence not required in the diet.
Essential
Semiessential
Nonessential
Methionine
Arginine
All the other amino acids
Threonine
Tryptophan
Valine
Isoleucine
Leucine
6
Phenyl Alanine
Lysine
Histidine***
 
*Frequently Asked Doubts-Practical Tips
Is histidine essential or semiessential.
Although histidine is considered essential, unlike the other essential it does not fulfill the criteria of inducing negative nitrogen balance promptly upon removal from the diet.
How to approach such question?
Example 1:
  • Which of the following amino acid is semiessential?
    1. Lysine
    2. Tyrosine
    3. Arginine
    4. Histidine
For single response type of question choose Arginine as the answer NOT Histidine.
Example: 2:
  • Which of the following amino acid is semiessential?
    1. Histidine
    2. Glycine
    3. Tyrosine
    4. Glutamate
      From these options, Histidine is the single best answer.
 
Special Groups Present in Amino Acids
Amino acid
Special group
Arginine
GuanidiniumQ
Phenyl Alanine
Benzene
Tyrosine
Phenol
Histidine
ImidazoleQ
Proline
Pyrrolidine
Methionine
Thioether Linkage
Tryptophan
Indole
Cysteine
Thioalcohol (SH)
 
Conservative (Homologous) Substitution
One aminoacid replaced by another aminoacid of similar characteristics.
Examples of homologous substitution is shown in the diagram given below.
Conservative Mutation
Hydrophilic, Acid
Asp
Glu
Hydrophilic, Basic
His
Arg
Lys
Polar, Uncharged
Ser
Thr
Gln
Asn
Hydrophobic
Ala
Phe
Leu
Ile
Val
Pro
 
Nonconservative (Nonhomologous) Substitution
One amino acid replaced by another amino acid of different characteristics.
 
ABBREVIATIONS OF AMINO ACIDS
  • Amino acids with unique first letter
Amino acid
Three letter Abbreviation
One letter Abbreviation
Cysteine
Cys
C
Histidine
His
H
Isoleucine
Ile
I
Methionine
Met
M
Serine
Ser
S
Valine
Val
V
Amino Acids which do Not have Unique First Letter
Abbreviated based on the commonly occuring amino acids.
Amino acid
Three letter Abbreviation
One letter Abbreviation
Glycine
Gly
G
Alanine
Ala
A
Leucine
Leu
L
Proline
Pro
P
Threonine
Thr
T
Abbreviated based on phonetically sounding letters
Amino acid
Three letter Abbreviation
One letter Abbreviation
Arginine
Arg
R (aRginine)
Asparagine
Asn
N (asparagiNe)
Aspartatic Acid
Asp
D (asparDic acid)
Glutamic Acid
Glu
E (glutEmic acid)
Glutamine
Gln
Q (Qtamine)
Phenyl Alanine
Phe
F (Fenyl alanine)
Tyrosine
Tyr
Y (tYrosine)
Tryptophan
Trp
W (tWiptophan)
Abbreviated based on letter close to initial letter
Amino acid
Three letter Abbreviation
One letter Abbreviation
Lysine
Lys
K (letter close to L)
7
 
Image-Based Informations-Structure of Amino Acids
Name
Symbol
Structural Formula
With Aliphatic side Chains
Glycine
Gly [G]
Alanine
Ala [A]
Valine
Val [V]
Leucine
Leu [L]
Isoleucine
Ile [I]
With side chains containing hydroxylic (OH) Groups
Serine
Ser [S]
Threonine
Thr [T]
With side chains containg sulfur atoms
Cysteine
Cys [C]
Methionine
Met [M]
With side chains containing acidic groups or their amides
Aspartic acid
Asp [D]
Asparagine
Asn [N]
Glutamic acid
Glu [E]
Glutamine
Gin [Q]
With side chain containing Basic groups
Arginine
Arg [R]
Lysine
Lys [K]
Histidine
His [H]
Containing Aromatic Rings
Histidine
His [H]
See above
Phenylalanine
Phe [F]
Tyrosine
Tyr [Y]
Tryoptophan
Trp [W]
Imino Acid
Proline
Pro [P]
8
 
DERIVED AMINO ACIDS
Classified into:
  • Derived amino acids seen in proteins
  • Derived amino acids not seen in proteins
 
Derived Amino Acid seen in ProteinQ
4-Hydroxy Proline
  • Found in Collagen
  • Vitamin C is needed for hydroxylation.
5-Hydroxy Lysine
  • Vitamin C is needed for hydroxylation.
Methyl lysine
  • Found in Myosin
Gamma carboxy glutamate
  • Found in clotting factors, like Prothrombin that bind Ca2+
  • Vitamin K is needed for Gammacarboxylation
Cystine
  • Found in proteins with disulphide bond.Q 2014 DNB
  • Two cysteine molecules join to form cystine
  • For example Insulin, Immunoglobulin
Desmosine
  • Found in ElastinQ AIIMS Nov 2014
 
Derived Amino Acid not seen in ProteinQ
Ornithine
Intermediates of Urea Cycle
Arginosuccinate
Citrulline
Homocysteine
Derived from MethionineQ
Homoserine
Product of Cysteine Biosynthesis
Glutamate-γ Semialdehyde
Serine Catabolite
 
Recent Updates
Extra Terrestrial Amino Acids
In February 2013 following explosion of approximately 20,000 metric ton meteor in the skies in chelynabinsk, west siberia extra terrestrial amino acids like alanine, aspartic acid, glutamic acid, isoleucine, leucine, phenyl alanine, serine, threonine, tyrosine, valine, n methyl glycine, β alanine were found in the remnants of meteors. These findings demonstrated potential insights to existence of extraterrestrial life.
 
PROPERTIES OF AMINO ACID I. A GENETIC CODE SPECIFIES AN AMINO ACID
More than 300 naturally occurring amino acids exist in nature out of which 20 amino acids constitute monomer units of proteins.
 
II. AMINO ACID EXIST IN THREE CHARGED STATE, POSITIVE, NEGATIVE OR NEUTRAL
Depends on the two factors:
  • Isoelectric pH of the amino acid.
  • pH of the surrounding medium.
 
Isoelectric pH of Amino Acids
  1. At pH = Isoelectric pH
  2. At pH < Isoelectric pH
  3. At pH > Isoelectric pH
  1. At pH = Isoelectric pH(pI)
    • The amino acid carry equal number of positive and negative charge, i.e. NO NET CHARGE.
    • Amino acid exist as ZWITTER ION (AMPHOLYTE)
9
Zwitter Ions or Ampholytes
Molecules which carry equal number of ionizable groups of opposite charge and therefore bear no net charge are called Zwitter ions or ampholytesQ. Zwitter is a german word which means hermaphrodite.
Properties of Amino acid at Isoelectric pH(pI)
  • No mobility in electric field.Q
  • Minimum solubility.
  • Maximum precipitabilityQ.
  • Minimum Buffering capacity.
  1. At pH less than isoelectric pH(pI)
    • Amino acid exists as protonated or positively charged.
  2. At pH greater than isoelectric pH(pI)
    • Amino acid exists as deprotonated or negatively charged.
 
III. AMINO ACIDS EXHIBIT ISOMERISM
Amino acids have asymmetric (chiral) alpha carbon atom. The mirror images produced with reference to alpha carbon atom, are called D and L forms or enantiomers.
zoom view
Fig. 1.4: L and D amino acid
  • Almost all naturally occurring Amino Acids are L-Isomers
  • Some naturally occurring Amino acids are D Amino acids.
 
Naturally Occurring D Amino Acid
  • Free D Aspartate and Free D Serine in brain tissue
  • D-Alanine and D Glutamate in cell walls of gram positive bacteria
  • Bacillus subtilis excretes D-methionine, D-tyrosine, D-leucine, and D-tryptophan to trigger biofilm disassembly
  • Vibrio cholerae incorporates D-leucine and D-methionine into the peptide component of their peptidoglycan layer.
 
Potentially Toxic L-Amino Acids
  • Certain L α Amino acids present in the in plants can adversely affect human health.
  • Present in the seeds of certain species of Lathyrus.
 
Examples of Toxic L-Amino Acids
L- Amino Acids
Clinical Implication
L-Homoarginine cleaved by Arginase to L Lysine & Urea
Causes Neurolathyrism in humans
β N Oxalyl Diamino Propionic Acid (β ODAP)
Neurotoxin
Causes Neurolathyrism in humans
β N Glutamyl Amino Propiono Nitrite (BAPN)
An Osteolathyrogen10
2,4-Diaminobutyric acid
Inhibits ornithine transcarbamylase, resulting in ammonia toxicity.
β-Methyl amino alanine
(Present in Cycad seeds)
Possible risk factor for neurodegenerative diseases, like
  • Parkinson's Disease
  • Amyotropic Lateral Sclerosis (ALS)
 
IV. AMINO ACID ABSORB UV LIGHTQ
Amino Acids which absorb 250-290 nm (Maximum at 280 nm) uv light are tryptophan, phenylalanine, tyrosine.
Maximum absorption of UV light by tryptophan.Q
 
BETA-ALANINE
Very important topic for national board pattern exams
Formed from Cytosine and Uracil.QDNB/AIPGMEE
Other sources of Beta Alanine is hydrolysis of Beta alanyl dipeptides.
Beta Alanine is seen in.Q
  • Pantothenic Acid
  • Coenzyme A
  • Acyl Carrier Protein
  • BetaAlanyl Dipeptides.
Beta Alanyl Dipeptides are
  • Carnosine [Histidine + Beta alanine]
  • Anserine [N methyl Carnosine]
    Both present in Skeletal muscle
Uses of Carnosine
  • Activate Myosin ATPase.
  • Chelate Copper
  • Enhance Copper uptake
  • Buffers the pH of anaerobically contracting muscle.
 
DECARBOXYLATION OF AMINO ACID
  • The amino acid undergo alpha decarboxylation to form corresponding Amines
  • PLPQ is the coenzyme for this reaction
 
Examples of Amino Acid Decarboxylation
Amino acid
Biologic Amines
Histidine
Histamine
Tyrosine
Tyramine
Tryptophan
Tryptamine
Lysine
Cadaverine
Glutamic acid Q
Gamma Amino Butyric Acid (GABA)
Serine
Ethanolamine
Cysteine
Betamercapto Ethanolamine
 
COLOUR REACTIONS OF AMINO ACIDS
 
Biuret Test
  • General test for Proteins
  • Cupric ions in alkaline medium forms violet colour with peptide bond nitrogen.
 
Ninhydrin Test
General test for all alpha Amino Acid Amino acid
Amino acid + 2 mols of Ninhydrin → Aldehyde with 1 carbon atom less + CO2 + Purple Complex (Ruhemann's Purple)
Colour Reactions
Test answered by
Xanthoproteic Test (Conc HNO3 is a reagentQ)
Aromatic Amino AcidQ2014 DNB
(Phenyl Alanine, Tyrosine, Tryptophan)
Millon's test
Tyrosine (Phenol)
Aldehyde test can be done in two methods:
  • Acree Rosenheim Test (Formaldehyde and Mercuric Sulphate is used)
  • Hopkin's Cole TestQ
    (Glyoxylic Acid is used)
Tryptophan (Indole group)
Saka Guchi's test
ArGinine (Guanidinium group)
Mnemonic–G is common to all
Sulphur test
Cysteine
Cyanide Nitroprusside Test
Homocysteine
Pauly's Test
Histidine (Imidazole)
Tyrosine (Phenol)
 
Buffering Action of Amino Acids
  • Buffers are solutions which can resist changes when acid or alkali is added.
Maximum buffering capacity is at pH = pKa. So amino acid which has pKa range near physiologic pH can act as an effective buffer.
 
pKa Range of Amino Acid
Dissociating Group
pKa range
Alpha carboxyl group
3.2-4.1
Non alpha COOH of Asp and Glu
4.0-4.8
Imidazole gp of histidine
6.5-7.4
SH gp of Cysteine
8.5-9.0
OH gp of Tyrosine
9.5-10.5
Alpha amino gp
8.0-9.0
Guanidinium gp of Arginine
>12
 
Titration Curve
Titration is done to find out the amount of acid in a given solution. To find out that a measured volume of acid is titrated against a strong alkali. The endpoint of titration is the point at which the pH of solution is 7. A plot called titration curve is obtained.
Definition of titration curve
A plot of (OH-) added (represesented in equivalents) against pH is called Titrarion curve.
  • Lets see the important landmarks in Titration curve of weak acids and certain amino acids in the Image-Based Information boxes.
 
Amino Acids and Amino Acid Derivatives as Neurotransmitters
  • Glycine-Major inhibitory neurotransmitter in brain stem and spinal cord
  • Glutamate Major excitatory neurotransmitter.
 
Amino Acid Derivative as Neurotransmitter
  • Dopamine
  • Epinephrine
  • Norepinephrine
  • Serotonin
  • Gamma Amino Butyric Acid (GABA)
 
DIGESTION OF PROTEINS
Native proteins are resistant to digestion because few peptide bonds are accessible to the proteolytic enzymes without prior denaturation of dietary proteins (by heat in cooking and by the action of gastric acid).
 
Enzymes Catalyze the Digestion of Proteins
There are two main classes of proteolytic digestive enzymes (proteases).
  1. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments.
    • Pepsin in the gastric juice catalyzes hydrolysis of peptide bonds adjacent to amino acids with bulky side-chains (aromatic and branched-chain amino acids and methionine).
    • Trypsin, chymotrypsin, and elastase are secreted into the small intestine by the pancreas.
      • Trypsin catalyzes hydrolysis of lysine and arginine esters.
      • Chymotrypsin catalyzes hydrolysis esters of aromatic amino acids.
      • Elastase catalyzes hydrolysis esters of small neutral aliphatic amino acids.13
  2. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, from the ends of peptides.
    • Carboxypeptidases, secreted in the pancreatic juice, release amino acids from the free carboxyl terminal.
    • Aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal.
    • Dipeptidases and tripeptidases in the brush border of intestinal mucosal cells catalyze the hydrolysis of di-and tripeptides, which are not substrates for amino-and carboxypeptidases.
The proteases are secreted as inactive zymogens; the active site of the enzyme is masked by a small region of the peptide chain that is removed by hydrolysis of a specific peptide bond.
Pepsinogen is activated to pepsin by gastric acid and by activated pepsin.
In the small intestine, trypsinogen, the precursor of trypsin, is activated by enteropeptidaseQ, which is secreted by the duodenal epithelial cells; trypsin can then activate chymotrypsinogen to chymotrypsin, proelastase to elastase, procarboxypeptidase to carboxypeptidase, and proaminopeptidase to aminopeptidase.
 
Absorption of Amino Acid
Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. There are several different amino acid transporters, with specificity for the nature of the amino acid side-chain.
Transporters of Amino Acids
  • For Neutral Amino acids
  • For Basic Amino acids and Cysteine.
  • For Imino Acids and Glycine
  • For Acidic Amino acids
  • For Beta Amino Acids (Beta Alanine)
 
GENERAL AMINO ACID METABOLISM
 
Biosynthesis of Urea
Urea biosynthesis occur in four stages:
  1. Transamination
  2. Oxidative deamination of Glutamate
  3. Ammonia Transport
  4. Disposal of Ammonia
 
I. TRANSAMINATION
Definition
  • Transfer of alpha amino group from one amino acid to a keto acid to form another pair of amino acid and keto acid.
  • Amino group from amino acids are concentrated in the form of Glutamate.
  • Because only Glutamate can undergo oxidative deamination to significant amount thereby releasing ammonia that enter in to urea cycle.
zoom view
Fig. 1.6: Transamination
 
Examples of Transamination
Alanine Amino Transferase (ALT) or Serum Glutamate Pyruvate Transaminase (SGPT)
Aspartate Amino Transferase (AST) or Serum Glutamate Oxaloacetate Transaminase (SGOT)
 
II. OXIDATIVE DEAMINATION
  • The removal of amino group from amino acid is called Deamination.
  • Only Glutamate can undergo significant Oxidative deamination.
zoom view
Fig. 1.7: Oxidative deamination
zoom view
Fig. 1.8: L amino acid oxidase
 
Some Examples of Non Oxidative DeaminationQ
  1. Amino acid Dehydrases for amino acids with hydroxyl group (Serine, Threonine)
  2. Histidase for histidine
  3. Amino acid Desulfhydrases for amino acids with sulfhydryl group, Cysteine and Homocysteine
 
Transdeamination
Conversion of α Amino nitrogen to ammonia is by concerted action of amino transferase and Glutamate Dehydrogenase is often termed as Transdeamination.
 
III. TRANSPORT OF AMMONIA
(Very important Topic PGMEE exams)
Free ammonia is toxic to cells especially to brain. Excess ammonia generated has to be converted to nontoxic form Then transported to liver to enter in to urea cycle.
  1. Transport of Ammonia from most of the tissues including the brain.
  2. Transport of Ammonia from skeletal muscle.
Transport of Ammonia from most of the tissues including the brain.
  • As Glutamine Q with the help of the enzyme Glutamine synthetase.
Glutamine Synthetase
  • Ammonia formed in most tissues including the brain is trapped by Glutamate to form Glutamine.
  • This is called first line trapping of ammonia.
  • ATP is required for this reaction.
Glutaminase
  • In the liver, Glutaminase removes the ammonia from Glutamine.
  • Ammonia enter in to urea cycle in the liver.
zoom view
Fig. 1.9: Glutamine synthetase and glutaminase
 
Transport of Ammonia from Skeletal Muscle
  • From skeletal muscle-as Alanine.Q15
  • In skeletal muscle, excess amino groups are generally transferred to pyruvate to form alanine.
zoom view
Fig 1.10A: Transport of ammonia from different organs
zoom view
Fig. 1.10B: Transport of ammonia from different organs
 
III. DISPOSAL OF AMMONIA
  • The ammonia from all over the body reaches the liver. It is then detoxified to urea by liver cells, then excreted through kidney.
  • UreaQ is the major end product of protein catabolism in the body.
 
Sources of UreaQ
zoom view
Fig. 1.11: Sources of nitrogen and carbon atoms of urea
 
UREA CYCLE
(Very important topic)
The urea cycle is the first metabolic pathway to be elucidated by Sir Hans Krebs and a medical student associate, Kurt Henseleit hence as Krebs Henseleit Cycle.
  • Ornithine consumed in the reaction 2 is regenerated in the reaction 5. Hence called Ornithine Cycle.
 
Site of Urea Cycle
  • Organ-Takes place in liver.
  • Organelle-Partly mitochondrial and partly cytoplasmic.
 
Reactions of Urea Cycle
The first two reaction takes place in the mitochondria. The rest of the reactions takes place in the cytoplasm.
Carbamoyl Phosphate Synthetase –I (CPS-I)
  • Carbamoyl Phosphate is formed from the condensation of CO2, Ammonia and ATP.
  • Takes place in the mitochondria
  • CPS-I is the rate limiting (pacemaker) enzyme in this pathway.
  • Cytosolic CPS-II is involved in pyrimidine synthesis.
  • CPS-I is active only in the presence of N-Acetyl Glutamate, an allosteric activator.Q
  • This step require 2 mols of ATPs.
Ornithine Transcarbamoylase (OTC)
  • Transfer carbamoyl group of carbamoyl phosphate to Ornithine forming Citrulline.
  • Takes place in the mitochondria
  • Subsequent steps takes place in the cytoplasm.
Transporters of Urea Cycle
Ornithine Transporter: For entry of Ornithine
Citrulline Transporter: For exodus of Citrulline
Arginino Succinate Synthetase
  • Links Amino nitrogen of Aspartate to Citrulline
  • Aspartate provides second nitrogen of Urea
  • This enzyme is a Ligase
  • This reaction requires ATP
  • 2 inorganic phosphates are utilized.
Arginino Succinate Lyase
Cleavage of Argino succinate to Arginine and Fumarate.
This enzyme is a Lyase.
ArginaseQ
Hydrolytic cleavage of Arginine, releases Urea and reforms Ornithine which reenter in to mitochondria.
Arginase is a HydrolaseQ16
zoom view
Fig. 1.12: Reactions of urea cycle
 
N-Acetyl Glutamate Synthase
  • Enzyme which catalyses the formation of N Acetyl Glutamate (NAG).
  • Generally considered as the sixth enzyme of Urea Cycle.
  • Because CPS-I is active only in the presence of NAGQ.
    N-Acetyl Glutamate + CoA SH
 
Energetics of Urea Cycle
  • Urea cycle requires 4 high energy phosphates.
  • Urea cycle requires 3 ATPSQ directly17
 
Urea Bicycle
Urea cycle is linked to TCA cycle through Fumarate and Aspartate. Hence this cycle is called urea bicycle.
 
CLINICAL CORRELATIONS-UREA CYCLE DISORDERS
 
Key Points of Urea Cycle Disorders
Characterized by
  • Hyperammonemia
  • Encephalopathy
  • Respiratory alkalosis.
 
Clinical Symptoms Common to all Urea Cycle Disorders
In the neonatal period,
Symptoms and signs are mostly related to brain dysfunction and are similar regardless of the cause of the hyperammonemia.
The affected infant is normal at birth but becomes symptomatic following the introduction of dietary protein.
  • Refusal to eat
  • Vomiting
  • Tachypnea
  • Lethargy
  • Convulsions are common
  • Can quickly progress to a deep coma
In infants and older children
  • Vomiting
  • Neurologic abnormalities (ataxia, mental confusion, agitation, irritability, and combativeness)
 
Ammonia Intoxication is Most Severe in the Deficiency of First Two Enzymes
Because once citrulline synthesized some ammonia is already been covalently linked to an organic metabolite.
 
BIOCHEMICAL DEFECT IN UREA CYCLE DISORDERS
Urea Cycle Disorders due to Enzyme Deficiency
Disorder
Enzyme Defective
Hyperammonemia Type I
Carbamoyl Phosphate Synthetase I (CPS-I)
Hyperammonemia Type II
Ornithine Transcarbamoylase (OTC)
Citrullinemia Type I (Classic Citrullinemia)
Arginosuccinate synthetase
Arginosuccinic aciduria
Arginosuccinate lyase
Hyperargininemia
Arginase
Urea Cycle Disorders due to Transporter Defect
Citrullinemia Type II
Citrin (Transport Aspartate & Glutamate) Defect
Hyperammonemia Hyperornithinemia Homocitrllinuria (HHH) Syndrome
Ornithine Transporter Defect
 
HIGH YIELDING FACTS-UREA CYCLE DISORDERS
 
Hyperammonemia Type II (OTC Deficiency)
  • Most common Urea Cycle disorderQ
  • Disorder with X-linked partially dominant inheritance (All other Urea Cycle Disorders are Autosomal Recessive)
  • Urea cycle disorder with Orotic Aciduria
  • Marked elevations of plasma concentrations of glutamine and alanine with low levels of citrulline and arginine
  • Orotate may precipitate in urine as a pinkcolored gravel or stones.
 
Orotic Aciduria in Hyperammonemia Type II
  • Ornithine Transcarbamoylase defective hence Carbamoyl Phosphate accumulate in the mitochondria.
  • Carbamoyl Phosphate reaches the cytoplasm enter in to Pyrimidine synthesis.
  • Orotic Acid, an intermediate in the Pyrimidine synthesis accumulates which leads to Orotic Aciduria.
 
Argino Succinic Aciduria
  • Trichorrhexis nodosa (dry and brittle hair) is a common finding.18
 
Hyperargininemia (Argininemia), A Distinct Urea Cycle Disorder
  • Hyperargininemia is the urea cycle disorder with least Hyperammonemia. Because by the time Arginine is formed the two nitrogen are already incorporated.
  • There are 2 genetically distinct arginases in humans.
  • One is cytosolic (ARG1) and is expressed in the liver and erythrocytes, and the other (ARG2) is found in renal and brain mitochondria.
  • The gene for ARG1, the enzyme that is deficient in patients with arginase deficiency.
  • The clinical manifestations of this condition are quite different from those of other urea cycle enzyme defects.
  • A progressive spastic diplegia with scissoring of the lower extremities, choreoathetotic movements, and loss of developmental milestones in a previously normal infant.
The compounds excreted in urine in hyper argininemia:
  • Cystine, Ornithine, Lysine, Arginine [COLA]
  • Alpha ketoguanidinovaleric acid
 
N-Acetyl Glutamate Synthase Deficiency
  • The sixth enzyme deficiency which lead to a urea cycle disorder.
  • The condition is almost similar to Hyperammonemia Type I.
  • ArginineQ an allosteric activator of NAG Synthase improves CPS I defect as N-Acetyl Glutamate activates CPS-I
  • But Arginine does not improve N-Acetyl Glutamate deficiency, as the enzyme itself is defective.
 
Hyperammonemia-Hyperornithinemia-Homocitrullinemia (HHH) Syndrome
  • Autosomal recessively inherited disorder.
  • Biochemical Defect is mutation in the ORNT 1 gene that encodes mitochondrial membrane Ornithine Permease.
  • This results in defect in the transport system of ornithine from the cytosol into the mitochondria.
  • This leads to accumulation of ornithine in the cytosol causes hyperornithinemia
  • Deficiency of ornithine in the mitochondria. Results in disruption of the urea cycle and hyperammonemia
  • Homocitrulline is presumably formed from the reaction of mitochondrial carbamoyl phosphate with lysine
 
Citrullinemia Type II
  • The adult form (type II) is caused by the deficiency of a mitochondrial transport protein named citrin.
  • Citrin (aspartate-glutamate carrier protein) is a mitochondrial transporter encoded by a gene (SLC25A13) located on chromosome 7q.
  • One this protein's functions is to transport aspartate from mitochondria into cytoplasm;
  • Aspartate is required for converting citrulline to argininosuccinic acid
  • So Citrulline accumulates.
 
Biochemical Investigation in a Case with Hyperammonemia
Normal Blood Ammonia level-20-40 µg/dl
Methods of estimation of Blood ammonia:
  • Chemical Method-Berthelot Method
  • Enzymatic method-Glutamate Dehydrogenase Method
  • Using Ammonia selective Electrodes Methods of estimation of Urea
Methods of estimation of Urea:
  • Chemical Method-Diacetyl Monoxime–Thiosemicarbazide Method
  • Enzymatic Method-Using Urease
Tandem Mass Spectrometry is the most sensitive tool to detect metabolic Disorders.
 
Biochemical Basis of Treatment of Urea Cycle Disorder
  1. Arginine:
    Essential Amino Acid
    Provide Ornithine
    Arginine is an activator of N AcetylGlutamate Synthase But Contraindicated in Arginase Defect
  2. Acylation therapy:
    The main organic acids used for this purpose are sodium salts of benzoic acid and phenylacetic acid.
    Principle
    Exogenously administered organic acids form acyl adducts with endogenous nonessential amino acids. These adducts are nontoxic compounds with high renal clearances.
  3. Sodium Benzoate
    Benzoate forms hippuric acid with endogenous glycine in the liver. Each mole of benzoate removes 1 mole of ammonia as glycine.
    19
    zoom view
    Fig. 1.13: Biochemical investigation of hyperammonemia
  4. Sodium Phenyl Acetate
    Phenylacetate conjugates with glutamine to form phenylacetylglutamine, which is readily excreted in the urine. One mole of phenylacetate removes 2 moles of ammonia as glutamine from the body
 
INDIVIDUAL AMINO ACIDS
 
Phenyl Alanine and Tyrosine
 
Phenyl Alanine
  • Aromatic Amino Acid
  • Essential Amino Acid
  • Hydrophobic Amino acid
  • Partly glucogenic partly ketogenic
zoom view
Fig. 1.14: Phenyl alanine
 
Tyrosine
  • Aromatic Amino Acid
  • Synthesized from Phenylalanine
  • Nonessential
  • Partly glucogenic and partly ketogenic
zoom view
Fig. 1.15: Tyrosine
 
Synthesis of Tyrosine from Phenylalanine
zoom view
Fig. 1.16: Conversion of phenylalanine to tyrosine
20
Phenylalanine Hydroxylase
  • Enzyme belongs to Mixed Function Oxidase (Monooxygenase)
  • Require coenzymes Tetrahydrobiopterin, NADPH
  • One mol of oxygen is incorporated
  • Irreversible reaction
Tetrahydrobiopterin
  • Resemble folic acid but is not a vitamin.
  • Precursor of Tetrahydrobiopterin is Guanosine Triphosphate (GTP)
  • Rate limiting enzyme in the pathway is GTP Cyclohydrolase
zoom view
 
Enzymes with Tetrahydrobiopterin as Coenzyme
  • Phenyl alanine Hydroxylase
  • Tyrosine Hydroxylase
  • Tryptophan Hydroxylase
  • Nitric Oxide Synthase
 
CATABOLISM OF TYROSINE
zoom view
Fig. 1.17: Catabolism of tyrosine
 
Important Points in the Catabolism of Tyrosine
As phenyl alanine is converted to tyrosine, the degradative pathway is the same for both Phenyl alanine and Tyrosine.
Tyrosine Transaminase
PLP is the coenzyme for this reaction.
Para Hydroxy Phenyl Pyruvate Hydroxylase (4 Hydroxy Phenyl Pyruvate Dioxygenase)
  • This enzyme belongs to Dioxygenase, i.e incorporates both the atoms of oxygen.
  • Cofactor for this enzyme is Copper
  • Ascorbic Acid is also needed for this reaction.
Homogentisate Oxidase
  • Belongs to Dioxygenase
  • Contain Iron at the active site.
Maleyl acetoacetate cis trans isomerase
  • Belongs to Isomerase
  • Need Glutathione (GSH) as cofactor
 
Specialised Products from Tyrosine (Very Important Topic)
  • Melanin
  • Catecholamines
  • Thyroxine
 
SYNTHESIS OF MELANIN
  • Takes place in the melanosome of melanocyte present in the deeper layers of epidermis.
  • Under the influence of MSH
  • Melanin gives pigmentation to the skin and hair.
zoom view
Fig. 1.18: Melanin Synthesis
Tyrosinase
  • Ratelimiting step
  • Monooxygenase
  • Copper is the cofactor for this enzyme.
  • Single enzyme catalyse two reactions.21
 
Synthesis of Catecholamines
Catecholamines are:
  • Dopamine
  • Epinephrine
  • Norepinephrine
Catecholamines are compound which contain Catechol nucleus.
Site of synthesis: Chromaffin cells of Adrenal Medulla and Sympathetic Ganglia.
  • In adrenal medulla major product is Epinephrine (80%).
  • In organs innervated by Sympathetic nerves major product is Norepinephrine (80%).
 
CONVERSION OF TYROSINE TO EPINEPHRINE INVOLVES 4 SEQUENTIAL STEPS
  • Ring hydroxylation
  • Decarboxylation
  • Side chain hydroxylation.
  • N-Methylation
zoom view
Fig. 1.19: Catecholamine
 
Important Points Catecholamine Synthesis
Tyrosine Hydroxylase
  • Rate limiting Step in Catecholamine Synthesis
  • Similar to Phenyl Alanine Hydroxylase.
  • Monoxygenase
  • Require tetrahydrobiopterin
 
Tyrosinase vs Tyrosine Hydroxylase
  • Both the enzymes convert Tyrosine to DOPA
  • Tyrosinase is expressed only in melanocyte where DOPA is used to synthesize Melanin.
  • Tyrosine Hydroxylase is expressed only in the sites where catecholamines are synthesized, where DOPA utilised for Catecholamine synthesis.
  • Tyrosinase is a monooxygenase containing Cu2+ in the active site.
  • Tyrosine hydroxylase is a monoxygenase with Tetrahydrobiopterin as the cofactor.
DOPA Decarboxylase
  • Present in all the tissues.
  • PLP is the coenzyme for this enzymeQ
 
DEGRADATION OF CATECHOLAMINES
  • The half life of catecholamines are very short, only 2-5 minutes.
  • Epinephrine and norepinephrine is catabolised by Catechol O Methyl Transferase (COMT)Q then by Monoamino Oxidase (MAO)Q.
  • The major end product of Epinephrine and norepinephrine is Vanillyl Mandelic Acid (VMA)Q.
  • Normal level of VMA excretion in urine is 2-6 mg/24 hour.
  • The major end product of Dopamine is HomoVanillic Acid (HVA)
zoom view
Fig. 1.20: Degradation of catecholamines
 
Synthesis of Thyroid Hormones
Thyroid hormones are synthesized on thyroglobulin, a large iodinated glycosylated protein.
It contains. 115 tyrosine residues.
Tyrosine residues are iodinated to form Mono-Iodo–Tyrosine (MIT) and Di-iodo Tyrosine (DIT).
Coupling of MIT and DIT on the thyroglobulin produce Thyroxine22
  • MIT + DIT → Tri-iodothyronine (T3)
  • DIT + DIT → Tetra-iodothyronine. (T4) or Thyroxine.
 
CLINICAL CORRELATIONS (PHENYL ALANINE AND TYROSINE METABOLISM)
Metabolic Disorders associated with catabolic pathway of Phenyl alanine and Tyrosine
  • Phenyl ketonuria
  • Alkaptonuria
  • Tyrosinemias
  • Hawkinsinuria
  • Segawa syndrome
Disorders associated with melanin synthesis
  • Albinism
Disorders associated with excess Catecholamines
Pheochromocytoma
 
Metabolic Disorders Associated with Catabolic Pathway of Phenylalanine & Tyrosine
zoom view
Fig. 1.21: Biochemical defects in the metabolism of aromatic amino acids
 
PHENYL KETONURIA
 
Classic Phenyl Ketonuria (Type I PKU)
  • Most Common Metabolic disorder concerned with AminoAcid.
Biochemical Defect
  • Phenylalanine hydroxylase deficiency.
  • Phenyl alanine could not be converted in to tyrosine.
  • Phenyl alanine in the blood rises.
  • Alternate metabolic pathways are opened.
zoom view
Fig. 1.22: Alternate metabolic pathways in PKU
Clinical Presentation of Phenyl Ketonuria
  • The affected infant is normal at birth.
  • Profound intellectual disability develops gradually if the infant remains untreated.
  • Vomiting, sometimes severe enough to be misdiagnosed as pyloric stenosis
  • Older untreated children become hyperactive with autistic behaviors, including purposeless hand movements, rhythmic rocking, and athetosis.
  • The infants are lighter in their complexion than unaffected siblings. (Phenyl alanine not converted to Tyrosine, so decreased melanin synthesis).
  • These children have an unpleasant mousey or musty odor of phenylacetic acid.Q JIPMER 2015
The brain is the main organ affected by hyperphenylalaninemia.
Lab Diagnosis of PKUQ
Guthries Test (Bacterial Inhibition Assay of Guthrie)
  • Rapid screening Test in the blood sample.
  • First method used for this purpose.
  • Certain strains of Bacillus Subtilis need Phenyl Alanine as an essential growth factor.
  • Bacterial growth is proportional to blood phenyl alanine.23
Ferric Chloride Test
  • Screening test in urine sample.
  • Identifies Phenyl Ketones in urine sample.
  • A simple test for diagnosis of infants with developmental and neurologic abnormalities
  • Nowadays it has no place in any screening program especially in developed countries.
  • These tests have been replaced by more precise and quantitative methods (fluorometric and tandem mass spectrometry).
 
Tandem Mass Spectrometry
  • The method of choice is tandem mass spectrometry, which identifies all forms of hyperphenylalaninemia
Other methods
  • Molecular Biology Techniques like Phenyl Alanine Hydroxylase specific probes.
  • Quantitative measurement of Blood Phenyl Alanine. (Blood level >20 mg/dl in PKU)
  • Enzyme assay in dry blood spot also done
 
Treatment of Classical PKU
  • A low-phenylalanine diet
  • Administration of large neutral amino acids (LNAAs) is another approach to diet therapy.
  • Sapropterin dihydrochloride (Kuvan), a synthetic form of BH4, which acts as a cofactor in patients with residual PAH activity, is approved by the FDA to reduce phenylalanine levels in PKU.
  • Preliminary trials with recombinant phenylalanine ammonia lyase have been encouraging and demonstrated reduced blood levels of phenylalanine during treatment.
 
Rationale for using Large Neutral Amino Acids (LNAAs) as Treatment for PKU
  • LNAAs (tyrosine, tryptophan, arginine, leucine, isoleucine, valine, methionine, histidine, lysine, threonine and phenylalanine) share the same transporter protein (LNAA type 1, LAT-1) for transit through the intestinal cell membrane and blood-brain barrier.
  • Binding of LNAA to the transporter protein is a competitive process.
  • The rationale for use of LNAA is that these molecules compete with phenylalanine for transport across the blood- brain barrier; therefore, large concentrations of other LNAAs in the intestinal lumen and in the blood reduce the uptake of phenylalanine into bloodstream and the brain.
 
Non Classical Phenyl Ketonuria
Biochemical Defect
Hyperphenylalaninemia due to Tetrahydrobiopterin defect
  1. Due to Dihydrobiopterin ReductaseQ Defect (Type II and Type III PKU)
  2. Due to defect in the enzymes that synthesize Tetrahydrobiopterin (Type IV and Type V PKU)
    1. 6-pyruvoyltetrahydropterin synthase, (Most Common)
    2. Guanosine Triphosphate (GTP) Cyclohydrolase
Lab Diagnosis of Non classical PKU
  • Meausurement of Neopterin and Biopterin [Oxidative Product of Dihydrobiopterin and Tetrahydrobiopterin] level in urine
  • Tetrahydrobiopterin (BH4) loading test normalises Plasma Phenyl Alanine level.
  • Enzyme Assay in dry blood spots on filter paper.
  • Genetic test. Mutation analysis and deletion/duplication studies are clinically available for all these enzyme defects and help to confirm the diagnosis.
 
Segawa Syndrome (Hereditary Progressive Dystonia)
  • Tetrahydrobiopterin deficiency due to defect in the enzyme GTP Cyclohydrolase.
  • But interestingly no Hyperphenylalaninemia.
  • Autosomal Dominant Inheritance
  • Dystonia with diurnal variation.
  • Females are affected more than males.24
 
Alkaptonuria
  • Autosomal recessive disorder
  • 1st inborn error detected
  • Belongs to Garrod's Tetrad (Alkaptonuria, Albinis, Pentosuria, Cystinuria).
Biochemical Defect
  • Homogentisate Oxidase Deficiency
  • Accumulation of Homogentisic Acid (Homogentisate)which polymerises to form Alkaptone bodies.
zoom view
Fig. 1.23: Biochemical defect alkaptonuria
Clinical Presentation of Alkaptonuria
  • Normal Life till 3rd or 4th decade.
  • Urine Darkens on standing is the only manifestation in children.
  • In adults Ochronosis ie Alkaptone Bodies deposited in Intervertebral Disc, cartilage of nose, pinna, etc leading to pigmentation.
  • Arthritis
  • No mental retardation.Q
Laboratory DiagnosisQ
  • Alkalanisation increase darkening of urine.
  • Benedicts test positive in urine because homogentisic acid is reducing agent.
  • Ferric Chloride test positive
  • Silver Nitrate Test positive.
Treatment
  • New Drug is Nitisinone [NTBC] which inhibit para Hydroxyl Phenyl Pyruvate hydroxylase which leads to the decreased accumulation of homogentisic acid
  • Symptomatic Treatment
 
Tyrosinemia
There are three types of Tyrosinemias
Type I, Type II and Type III
 
Type I (Hepatorenal Tyrosinemia, Hereditary Tyrosinemia)
Biochemical defect
  • Fumaryl Aceto acetate Hydrolase deficiency
  • Organs affected are liver, kidney, and peripheral nerves.
  • Organ damage is believed to result from accumulation of metabolites of tyrosine degradation, especially fumarylacetoacetate and succinylacetone.
  • Cabbage like odour due to Succinyl Acetone
Diagnosis
  • Plasma Tyrosine value less diagnostic value. Because it is dependent on diet
  • Elevated Succinyl Acetone in urine and blood is more diagnostic
Treatment
  • Diet low in Phenylalanine and Tyrosine
  • New Drug is Nitisinone [NTBC]
 
Type II Tyrosinemia (Oculocutaneous Tyrosinemia, Richner-Hanhart Syndrome)
Biochemical Defect
  • Tyrosine Transaminase deficiency
Clinical manifestations
  • Palmar and plantar hyperkeratosis,
  • Herpetiform corneal ulcers
  • Intellectual disability25
 
Type III (Neonatal Tyrosinemia)
Biochemical Defect
Parahydroxyl Phenyl Pyruvate Hydroxylase (4 Para Hydro- xyPhenyl Pyruvate Dioxygenase 4-HPPD deficiency
 
Hawkinsinuria
  • Hawkinsinuria is inherited as an autosomal dominant trait
  • Certain missense mutations in the gene for Para hydroxyl Phenyl Pyruvate Hydroxylase) (4 Para Hydroxy Phenyl Pyruvate Dioxygenase)
  • This result in an abnormal enzyme activity.
  • The mutant enzyme, incapable of normally oxidizing 4-hydroxyphenylpyruvate to homogentisic acid
  • Instead it forms an intermediate that reacts with cysteine to form the unusual organic acid hawkinsin ([2-l-cystein-S-yl-1,4-dihydroxycyclohex-5-en-1-yl]acetic acid,
  • Hawkinsin named after the first affected family
  • Secondary glutathione deficiency may occur.
  • An unusual odor (described as like that of a swimming pool)
 
Disorders Associated with Excess Catecholamines
  • Pheochromocytoma
  • Paraganglioma
  • Pheochromocytoma-Associated Syndromes
  • Neurofibromatosis type 1 (NF 1)
  • Multiple endocrine neoplasia type 2A and type 2B (MEN 2A, MEN 2B)
 
Pheochromocytoma
Symptomatic catecholamine-producing tumors, in adrenal and extraadrenal retroperitoneal, pelvic, and thoracic sites.
Clinical Presentation
The classic triad of Pheochromocytoma:
  • Episodes of palpitations
  • Headaches
  • Profuse sweating are typical and constitute a classic triad.
  • All three symptoms are associated with hypertension.
Biochemical Testing of Pheochromocytoma and paraganglioma
Elevated plasma and urinary levels of:
  • Catecholamines
  • Metanephrines
  • Vanillyl Mandelic acid
Biochemical Methods Used for Pheochromocytoma and Paraganglioma Diagnosis
Diagnostic Method
Sensitivity
Specificity
24 hour Urinary Testing
1. Vanillylmandelic acid
++
++++
2. Catecholamines
+++
+++
3. Fractionated metanephrinesQ
++++
++
4. Total metanephrines
+++
++++
Plasma Tests
1. Catecholamines
+++
++
2. Free metanephrines
++++
+++
 
Albinism
Disorder associated with deficiency of Tyrosinase enzyme which synthesize Melanin, the pigment of skin and eye.
 
Classification of Albinism
Generalised Albinism or Oculocutaneous Albinism (OCA)
  • OCA-1: Tyrosinase deficient
  • OCA-2: Tyrosinase positive (Most common Albinism)
  • OCA-3: (Rufous, red OCA)
Syndromes associated with Oculocutaneous Syndrome
  • Prader willi and Angelman syndrome
  • Hermansky-Pudlak syndrome
  • Chédiak-Higashi syndrome
Ocular Albinism
  • Ocular albinism (Nettleship falls type)
Localised Albinism:
Seen in:
  • Piebaldism
  • Waardenberg syndrome26
 
TRYPTOPHAN
  • Aromatic Amino Acid
  • Essential Amino Acid
  • Special group present is Indole group
  • Glucogenic and Ketogenic
 
Catabolic Pathway Tryptophan (Kynurenine-Anthranilate Pathway)
  • Major metabolic fate of Tryptophan is to be oxidized by Tryptophan Pyrrolase
Tryptophan Pyrrolase (Tryptophan Oxygenase)
  • Dioxygenase
  • Iron Porphyrin Metalloprotein (i.e. it is a heme containing Protein)
Kynureninase
  • Coenzyme is PLP.
 
Clinical Correlation-Kynurenine Anthranilate Pathway
Pellagra like symptoms in PLP deficiency
  • Decreased Kynureninase leads to decreased NAD+ pathway.
  • Hence Niacin deficiency, which lead to Pellagra like symptoms.
Xanthurenate is excreted in urine in PLP deficiency
  • This is because PLP deficieny leads to decreased Kynureninase activity.
  • Hence Kynurenine accumulate which is converted to Xanthurenate.
zoom view
Fig. 1.24: Metabolic pathways of tryptophan
 
Specialised Products from Tryptophan
  • Niacin (Nicotinic Acid)
  • Serotonin
  • Melatonin
 
Nicotinic Acid Pathway of Tryptophan
  • 3% of Trptophan enter this pathway
  • 60 mg Tryptophan is converted to 1 mg of Niacin.Q
  • Quinolinate Phosphoribosyl Transferase is the rate limiting step in this pathway.
 
Serotonin (5 Hydroxy Tryptamine)
Synthesized in the Argentaffin cells in the intestine, mast cells, platelets and in the brain.
Functions of Serotonin
  • Neurotransmitter in the Brain
  • Mood Elevation
  • GI Motility
  • Temp Regulation
  • Potent Vasoconstrictor
 
Melatonin
Synthesized in the Pineal gland.27
Functions of Melatonin
  • Diurnal Variation
  • Biological Rhythm
  • Sleep wake Cycle
zoom view
Fig. 1.25: Specialised products from tryptophan
 
Important Enzymes in the Synthesis of Serotonin
Tryptophan Hydroxylase
  • Rate limiting step in the Serotonin and Melatonin Synthesis.
  • By this enzyme Tryptophan is converted to 5 HydroxyTryptophan.
  • Tetrahydrobiopterin is the coenzyme for this enzyme
  • Monoxygenase.
 
Hydroxylases Dependent on Tetrahydrobiopterin
  • Phenyl Alanine Hydroxylase
  • Tyrosine Hydroxylase
  • Tryptophan Hydroxylase
Remember all are Aromatic Amino Acids
Amino Acid Decarboxylase
  • 5 OH Trytophan is decarboxylated to 5 OH Try-ptamine, or Serotonin.
 
CATABOLISM OF SEROTONIN
  • Mono Amino Oxidase is the enzyme.
  • 5 Hydroxy Indole Acetic Acid (5HIAA) is the degradatory product of Serotonin.
  • Normal urinary Excretion of 5 HIAA is <5 mg/day.
 
SYNTHESIS OF MELATONIN
  • N Acetylation of serotonin followed by N-methylation in the pineal body forms Melatonin.
  • Methyl donor is S-Adenosyl Methionine (SAM)
 
Excretory Product of Tryptophan
Normal excretory product of Tryptophan in Urine is 5 Hydroxy Indole Acetate and Indole 3 Acetate.
 
METABOLIC DISORDERS ASSOCIATED WITH TRYPTOPHAN METABOLISM
  • Carcinoid syndrome
  • Hartnup's disease
  • Blue Diaper syndrome
 
Carcinoid Tumour (Argentaffinoma)
  • Belongs to Gastrointestinal Neuroendocrine Tumours
  • Tumour of Argentaffin Cells that secrete Serotonin.
  • Increased Synthesis of Serotonin
Clinical Symptoms
  • Most common symtoms are Intermittent Diarhoea (32-84%) and Flushing (63-75%)
  • Sweating
  • Fluctuating Hypertension
  • Pellagra like Symtoms
Diagnosis
  • Serum Serotonin increased
  • Urinary 5 HIAA increased
  • Neuroendocrine markers used for diagnosis are:
    • Chromogranin A
    • Neuron Specific Enolase
    • Synaptophysin
 
Typical and Atypical Carcinoid Typical Carcinoid
  • Is caused by a midgutcarcinoid tumor
  • Increased synthesis of Serotonin
  • Expanded serotonin pool size, increased blood and platelet serotonin
  • Increasedurinary5-hydroxyindolaceticacid (5-HIAA).28
 
Atypical Carcinoid
  • Foregutcarcinoids are the most likely to cause an atypical carcinoid syndrome.
Biochemical Defect
  • Due to a deficiency in the enzyme aromatic amino acid decarboxylase.
  • 5-Hydroxy Tryptophan (5 HTP) cannot be converted to 5-Hydroxytryptamine (5HT) (serotonin)
  • 5-HydroxyTryptophan is secreted into the bloodstream.
  • Plasma serotonin levels are normal.
  • Characteristically, urinary 5-Hydroxy Tryptophan and 5-Hydroxy Tryptamine (5-HTP is converted to 5-HT in the kidney) are increased.
  • But urinary 5-HIAA levels are only slightly elevated
zoom view
 
Hartnup Disorder
  • Autosomal recessive condition.
  • Named after first family in which the disorder identified.
Biochemical Defect
  • Defective absorption of Tryptophan and other Neutral Amino Acid from Intestine and Renal Tubules
  • The transporter protein for these amino acids (B0AT1) is encoded by the SLC6A19gene
  • Two chemically close transcription factors, angiotensin-converting enzyme (ACE2) in the intestine and renal tubules, and collectrin in the renal tubules, are required for expression of B0AT1 transporter protein by the SLC6A19gene.
  • The mutated gene in patients with Hartnup disorder, unable to interact with the above transcription factors, results in deficiency of B0AT1 protein either in the intestine or in the renal tubules or in both
Clinical Features
  • Asymptomatic
  • Cutaneous Photosensitivityis the most common presenting complaint.
  • Intermittent Ataxia manifested as unsteady wide based gait.
  • Pellagra like symptoms
 
Pellagra Like Symptoms in Hartnup's Disorder
Decreased absorption of Tryptophan from intestine.
Decreased availability of Trp for NAD+ pathway leading to Niacin deficiency.
Laboratory Diagnosis of Hartnup Disease
  • Obermeyer Test (Test for indole compounds in the urine) positive.
Treatment
  • Lipid-soluble esters of amino acids and tryptophan ethyl ester.
  • Treatment with nicotinic acid or nicotinamide (50-300 mg/24 hr) and a high-protein diet
 
Blue Diaper Syndrome (Drummond Syndrome)
  • Tryptophan is specifically malabsorbed.
  • The defect is expressed only in the intestine(unlike Hartnup Disease) and not in the kidney.
Blue Staining of the Diaper in Drummond Syndrome
This is due to bacterial breakdown of unabsorbed Tryptophan to Indican and IndigoBlue.
 
SIMPLE AMINO ACIDS
 
Glycine
  • Simplest Amino Acid
  • Nonessential
  • Glucogenic
  • Optically inactive Amino acid.Q
 
Biosynthesis of GlycineQ
  • Glycine Amino Transferase catalyse the synthesis of Glycine from Glyoxylate, Glutamate and AlanineQ
  • From Serine by Serine Hydroxy Methyl Transferase. This is a reversible reaction
zoom view
Fig. 1.26: Conversion of Serine to Glycine
  • By Glycine Synthase System in Invertebrates
  • From Threonine by Threonine Aldolase29
 
Serine Hydroxy Methyl Transferase
  • Belongs to class II enzyme.
  • Freely reversible
  • Vitamins involved in conversion of Serine to Glycine are PLP and Folic Acid.QDNB
  • When Serine is converted to Glycine, the β carbon atom of Serine is donated to THFA.
 
Catabolism of Glycine
By Glycine Cleavage system Q
Present in Liver mitochondria.
Glycine Cleavage system consists of three enzymes and an H Protein that has covalently attached Dihyrolipoyl moiety. The three enzymes are:
  1. Glycine dehydrogenase
  2. Amino methyl Transferase
  3. Dihydrolipomide Dehydrogenase
The overall reaction is
 
Specialised Products from Glycine
  • Creatine, Creatine Phosphate and Creatinine
  • Heme
  • Purine nucleotide
  • Glutathione
Glycine as conjugating agent
  • Conjugation of Bile acid (Glycocholic Acid, Glyco-chenodeoxy Cholic Acid)
  • Conjugation of Benzoic Acid
Glycine as Neurotransmitter
  • Both excitatory and inhibitory Neurotransmitter
Glycine is the recurring Amino acid present in the Collagen
  • Every third amino acid in Collagen is Glycine.
 
CREATININE
  • Synthesized from 3 Amino Acids (Glycine, Arginine & MethionineQ)
 
Steps of Synthesis of Creatinine
Step I Glycine Arginine Amido Transferase
  • First step in the Kidney.
  • Guanidino group of Arginine is transferred to Glycine to form Guanidino Acetic Acid.
Step II Guanidino Acetate Methyl Transferase
  • Second step in the Liver
  • Creatine is formed
  • S Adenosyl Methionine is the methyl donor
Step III Creatine Kinase
  • Third step in the Muscle
  • Creatine Phosphate is formed
Step IV
  • Occur spontaneously
  • Creatinine is formed.
zoom view
Fig. 1.27: Synthesis of creatinine
 
Heme
  • In the Liver and Erythroid Precursor cells
 
Formation of Purine Ring
  • C4, C5, N7 of Purine ring is contributed by Glycine.
Remember Glycine do not contribute to Pyrimidine Ring.Q
 
GLUTATHIONEQ
  • Is otherwise called Gamma Glutamyl Cysteinyl Glycine
  • TripeptideQ from three Amino Acids-Glutamic Acid, Cysteine & Glycine.
  • Pseudopeptide.Q
  • Abbrevated as GSH.
  • Business part of Glutathione is Sulfhydryl group of Cysteine30
 
Functions of GlutathioneQ
  1. Meister'sCycle or Gamma Glutamyl Cycle
    • Absorption of neutral Amino acids in the Intestine, Kidney tubules and brain.
    • 3 mols of ATP utilised for the transport of Amino acid.
  2. Free Radical Scavenging
    • Especially in the RBC, hence responsible for RBC membrane integrity.Q
      zoom view
      Fig. 1.28: Free radical scavenging
  3. Reduction of MetHemoglobinQ
    • Keep the iron in the Heme in the Ferrous state by Reduced Glutathione
  4. Conjugation reactions in Phase II Xenobiotic reactions
    • Glutathione S Transferase is the enzyme.
  5. Acts as Coenzyme for Some Reactions
 
METABOLIC DISORDERS ASSOCIATED WITH GLYCINE
 
Primary Hyperoxaluria Type I
  • The most common form of Primary hyperoxaluria.
  • It is due to a deficiency of the peroxisomal enzyme alanine-glyoxylate aminotransferase, (expressed only in the liver peroxisomes and requires pyridoxine (vitamin B6)as its cofactor)
  • Protein targeting defect.
zoom view
Fig. 1.29: Biochemical defect in hyperoxaluria
 
Primary Hyperoxaluria Type II (Glyceric Aciduria)
  • Due to a deficiency of D-glycerate dehydrogenase(glyoxylate reductase enzyme complex)
 
Secondary Hyperoxaluria
  • Pyridoxine deficiency(cofactor for alanine-glyoxylate aminotransferase)
  • After ingestion of ethylene glycol
  • High doses of vitamin C,
  • After administration of the anesthetic agent methoxyflurane (which oxidizes directly to oxalic acid)
  • In patients with inflammatory bowel disease or extensive resection of the bowel (enteric hyperoxaluria).
 
Nonketotic Hyper Glycinemia
  • Due to a defect in the Glycine cleavage system
 
ALANINE
  • Simple Amino Acid
  • Nonessential Amino Acid
  • Principal Glucogenic Amino Acid.
  • Transports amino group from Skeletal Muscle.Q
  • Participate in Glucose-Alanine Cycle (Cahill Cycle)
 
Biosynthesis of Alanine
  • From Pyruvate by Transamination:
 
SERINE
  • Hydroxyl Group containing Amino Acid
  • Glucogenic Amino acid
  • Nonessential Amino Acid
  • Polar Amino Acid31
 
Biosynthesis of Serine
  • From Glycine by Serine Hydroxymethyl Transferase. PLP is a coenzyme in this reaction.
  • From Glycolytic intermediate 3 Phospho Glycerate.
 
Metabolic Functions of Serine
  1. Primary donor of one carbon group.
    zoom view
    Fig. 1.30: Conversion of serine to glycine
  2. Serine is used for formation of Cysteine
    Serine + HomocysteineCysteine + Homoserine
  3. For Phospholipid synthesis
    Phosphatidyl Serine
  4. Serine analogs as drugs
    Cycloserine-Antituberculous drug
    Azaserine-Anticancer drug
  5. Serine is used for:
    • Ethanolamine Synthesis
    • Choline (Trimethyl Ethanolamine) synthesis
    • Betaine (Trimethyl Glycine) synthesis
    zoom view
    Fig. 1.31: Synthesis of Betaine
  6. Serine is the precursor of SelenocysteineQ DNB/AIPGMEE
  7. Serine used for Glycoprotein synthesis
    O Glycosylation takes place at Serine and Threonine residues.
  8. Most common sites for Phosphorylation are Serine and Threonine.
  9. Serine and Palmitoyl CoA are the starting material for the synthesis of Sphingosine, thereby Ceramide.
 
SULPHUR CONTAINING AMINO ACIDS
They are Methionine and Cysteine
 
Methionine
  • Sulphur Containing Amino Acid
  • Essential Amino Acid
  • Glucogenic Amino Acid
 
Cysteine
  • Nonessential Amino Acid
  • Glucogenic Amino Acid
 
Specialized Products Derived from CysteineQDNB
  • Cysteine on decarboxylation gives Betamercaptoethanolamine.
  • Coenzyme A
  • Taurine
  • Glutathione
  • Cystine-Condensation product of two Cysteine.
 
The Amino Acids that Decreases Ageing
  • Cysteine, hence ageing is otherwise called Cysteine deficiency syndrome.
  • Taurine
 
The Amino Acid that Accelerate Ageing
  • Homocysteine.
 
METABOLISM OF SULPHUR CONTAINING AMINO ACIDS
 
Steps of Methionine Metabolism
  1. Conversion of Methionine to S-Adenosyl Methionine (SAM) & Transmethylation reactions
  2. S Adenosyl Methionine to Homocysteine
  3. Two fates of Homocysteine:
    1. Synthesis of Cysteine
    2. Resynthesis of Methionine
  4. Degradation of Cysteine32
zoom view
Fig. 1.32: Metabolism of sulphur containing amino acid
 
Step I: Methionine Adenosyl Transferase (MAT)
  • Methionine converted to S-adenosyl Methionine, the principal methyl donor of the body.
  • ATP donates the Adenosyl group to methionine.
  • 3 IsoenzymeQDNB forms for MAT are MAT-I, MAT-II & MAT-III
  • MAT-I & MAT-III in the liver. MAT-II in the extrahepatic tissue.
 
Methionine has to be Activated to S-Adenosyl Methionine (SAM)
In methionine, the thioether linkage (C-S-C) is very stable. Adenosyl group is transferred to sulphur atom makes the methyl group labile. Hence methyl group can be easily transferred to acceptors.
 
Step II: Fates of Homocysteine
  1. Resynthesis of Methionine
    • By transferring a methyl group to Homocysteine, Methionine is resynthesized.
    • N5 Methyl THFA and Vitamin B12 is involved.
    Folate trap is discussed below.
  2. Synthesis of Cysteine (Trans sulfuration reactions) Cystathionine Beta synthase
    • Homocysteine condenses with Serine to form Cystathionine by removing a H2O by the enzyme Cystathionine Beta Synthase.
    • PLP is the coenzyme.
Cystathionase
  • Cystathionine to Cysteine and Homoserine by Cystathionase.
  • PLP is the coenzyme
  • By further reactions Homoserine is converted to Propionyl CoA then to Succinyl CoA
 
Functions of S-Adenosyl Methionine
  • Transmethylation reactions
  • DNA Methylation
  • Polyamine Synthesis
 
Transmethylation Reactions
Acceptor of Methyl group
Methylated Compound
Guanidinoacetate
Creatine
Norepinephrine
Epinephrine
Epinephrine
Metanephrine
Ethanolamine
Choline
Carnosine
Anserine
Acetyl Serotonin
Melatonin
 
POLYAMINE SYNTHESIS
Polyamines are organic compounds having multiple amino groups. They are:
  • Cadaverinederived from decarboxylation of Lysine
  • Putrescinederived from decarboxylation of Ornithine.
  • Spermidinederived from Ornithine and Methionine
  • Sperminederived from Ornithine and Methionine
zoom view
Fig. 1.33: Synthesis of polyamines
 
Steps of Polyamine Synthesis
  • Ornithine is decarboxylated to form Putrescine, by the enzyme ornithine Decarboxylase.
  • S adenosyl Methionine is decarboxylated to form Decarboxylated SAM.33
  • Decarboxylated SAM donates 3 carbon atom and 1 α amino group to Putrescine to form Spermidine.
  • Decarboxylated SAM donates 3 carbon atom and 1 α amino group to Spermidine to form Spermine.
 
Significance of Polyamines
  • They bear multiple positive charges, they associate readily with DNA and RNA.
  • Function in cell proliferation and growth.
  • Act as growth factors for cultured mammalian cells.
  • Stabilize intact cells and membranes and orgenelles.
  • Role in carcinogenesis.
Methionine can be synthesized from Homocysteine but it is an essential Amino Acid.
This is because Homocysteine is derived from Methionine.
 
Vitamins in the Metabolism of Sulphur Containing Amino Acids
  • Three vitamins needed are Vitamin B12, Folic Acid and Vitamin B6.
  • Vitamin B12 and Folic Acid for Methionine Synthase reaction.
  • Vitamin B6for Cystathionine Beta Synthase (Transsulfuration reaction) and Cystathionase.
zoom view
Fig. 1.34: Folate trap (THFA starvation)
 
METABOLIC DISORDERS ASSOCIATED WITH SULPHUR CONTAINING AMINO ACIDS
 
Classic Homocystinuria
Most common inborn error of methionine metabolism.
Biochemical defect
  • Due to deficiency of Cystathionine Beta Synthase.
  • Homocysteine is not converted to Cysteine, so there is cysteine deficiency
  • More homocysteine is available for methionine synthesis, so there is hyper methioninemia.
Clinical Features
  • Normal at birth
  • Symtoms during infancy are nonspecific and may include failure to thrive and developmental delay.
  • The diagnosis is usually made after 3 years of age, when subluxation of the ocular lens (ectopia lentis)occurs. This causes severe myopia and iridodonesis (quivering of the iris).
  • Progressive intellectual disability is common
  • Skeletal abnormalities resembling those of Marfan syndrome tall and thin, with elongated limbs and arachnodactyly scoliosis, pectus excavatum or carinatum, genu valgum, pes cavus, high-arched palate, and crowding of the teeth are commonly seen.
  • These children usually have fair complexions, blue eyes, and a peculiar malar flush
  • Thromboembolic episodes involving both large and small vessels, especially those of the brain, are
  • Common and may occur at any age
Diagnosis
  • Elevations of both methionine and homocystine (or homocysteine) in body fluids are the diagnostic
  • Cystine level is low in the plasma
  • Screening test for Homocystinuria-Cyanide Nitroprusside Test.Q in freshly voided urine as homocystine is highly unstable.
  • Enzyme analysis in liver biopsy specimen or cultured fibroblasts.
  • DNA mutation analysis can be done.
  • Prenatal diagnosis by Enzyme assay of cultured amniotic cells or chorionic villi or by DNA analysis.34
Treatment
  • High doses of vitamin B6 (200-1,000 mg/24 hr) causes dramatic improvement in most patients
  • Some patients do not respond to Vitamin B6, may be due to Folate depletion. For them folic acid (1-5 mg/24 hr) has been added to the treatment regimen
  • Restriction of methionine intake in conjunction with cysteine supplementation is recommended for patients who are unresponsive to vitamin B6
  • Betaine (trimethylglycine) lowers homocysteine levels in body fluids by remethylating homocysteine to methionine.
  • Administration of large doses of vitamin C(1 g/day) has improved endothelial function
 
Non Classic Homocystinuria
Can be due to
  • Due to defects in Methylcobalamin formation
  • Due to Deficiency of Methylene tetra hydrofolate Reductase (MTHFR)
  1. Homocystinuria due to defect in Methyl Cobalamin formation
    • Methylcobalamin is the cofactor for the enzyme methionine synthase, which catalyzes remethylation of homocysteine to methionine.
    • Homocysteine cannot be remethylated to Methionine.
    • Homocysteine accumulate.
    • Methionine level decreases.
Laboratory findings
  • Megaloblastic anemia - The presence of megaloblastic anemia differentiates Methyl Cobalamin formation defects from homocystinuria due to methylenetetrahydrofolate reductase deficiency.
  • Homocystinuria
  • Hypomethioninemia
Treatment
Vitamin B12 in the form of hydroxycobalamin (1-2 mg/24 hr) is used to correct the clinical and biochemical findings
  1. Homocystinuria Caused by Deficiency of Methylenetetrahydrofolate Reductase
Due to Deficiency of Methylene tetrahydrofolate Reductase (MTHFR)
  • This enzyme reduces N5, N10-methylenetetrahydrofolate to form 5-methyltetrahydrofolate,
  • N5 Methyl THFA provides the methyl group needed for remethylation of homocysteine to methionine
  • Hence Hypomethionemia
  • Homocystinemia and Homocystinuria
  • Absence of megaloblastic anaemia (Unlike Methyl Cobalamin formation defect)
Treatment
  • Combination of folic acid, vitamin B6, vitamin B12.
  • Methionine supplementation (Because Methionine is not resynthesized)
  • Betaine (early treatment with betaine seems to have the most beneficial effect)
 
Cystathioninuria
Cystathionase Deficiency
Mental Retardation, Anaemia, Thrombocytopenia
Cyanide Nitroprusside Test Negative.
 
Cystinuria
  • Included in Garrod's Tetrad (Cystinuria, Albinism, Pentosuria, Alkaptonuria)
  • Defect in Dibasic Amino Acid Transporter.
  • Defective reabsorption of Cystine, Ornthine, Lysine and Arginine (Remember-COLA)
  • Cystine, Ornithine, Lysine and Arginine(COLAQ in urine.
  • Cystine Stones in urine.
  • Cyanide Nitroprusside Test Positive.
  • Treated with ample hydration and alkalinisation of urine.
 
Oasthouse Syndrome
  • Malabsorption of Methionine and other Neutral Amino Acid
 
Primary Hypermethioninemia
  • Due to deficiency of hepatic Methionine Adenosyl Transferase (MAT I and III)
  • MAT II present in other tissues are not defective.
  • Peculiar smell of Boiled Cabbage.
 
Cystinosis
  • Lysosomal Storage Disorder.
  • Systemic disease caused by a defect in the metabolism of cystine.35
  • Caused by mutations in the CTNS gene, which encodes a novel protein, cystinosin.
  • Cystinosin is a H+-driven lysosomal cystine transporter.
  • Results in accumulation of cystine crystals in most of the major organs of the body:
    • Kidney,
    • Liver,
    • Eye,
    • Brain.
Diagnosis
  • Detection of cystine crystals in the cornea
  • Confirmed by measurement of increased leukocyte cystine content.
Treatment
  • Specific therapy is available with cysteamine,which binds to cystine and converts it to cysteine.
  • This facilitates lysosomal transport and decreases tissue cystine.
  • Kidney transplantation is a viable option in patients with renal failure.
 
BRANCHED CHAIN AMINO ACIDS
Branched Chain Amino acid
Metabolic fate
Valine
Glucogenic
Leucine
Ketogenic
Isoleucine
Both Ketogenic and Glucogenic
 
Three Common Steps in the Metabolism of Branched Chain Amino Acids
Reaction
Enzyme
Coenzyme
1. Transamination
Branched Chain Amino Acid Transaminase
PLP
2. Oxidative Decarboxylation
Branched Chain Keto Acid Dehydrogenase
Thiamine Pyrophosphate, FAD, NAD+, Lipomide and CoA
3. Dehydrogenation
Acyl CoA Dehydrogenase
FAD
 
After the First Three Common Steps
zoom view
 
Main Metabolic Disorders Associated with Branched Chain Amino Acid
  • Maple Syrup Urine Disease
  • Isovaleric Aciduria
 
Maple Syrup Urine Disease
Biochemical Defect
  • Deficiency of the enzyme Branched Chain Ketoacid Dehydrogenase.
  • Defective reaction is-Defective Decarboxylation.36
 
Components of Branched Chain Keto Acid Dehydrogenase Complex and Defective Components in MSUDQ
Gene
Component
MSUD Types
E1α
Branched Chain α Keto acid decarboxylase (contains TPP)
Type IA MSUD
E1β
Branched Chain α Keto acid decarboxylase
Type IB MSUD
E2
Dihydrolipoyl Transacylase (contains Lipomide)
Type II MSUD
E3
Dihydrolipomide Dehydrogenase (Contains FAD)
Type III MSUD
Clinical Features
  • Affected infants who are normal at birth develop poor feeding and vomiting in the 1st week of life.
  • Lethargy and coma, convulsions may ensue within a few days
  • Metabolic Acidosis
  • Physical examination reveals hypertonicity and muscular rigidity with severe opisthotonos.
  • Periods of hypertonicity may alternate with bouts of flaccidity manifested as repetitive movements of the extremities (boxing and bicycling)
  • The peculiar odor of maple syrup (burnt sugar) found in urine, sweat, and cerumen.
  • Mental Retardation
Lab Diagnosis
  • Plasma shows marked elevation of leucine, isoleucine, valine, and alloisoleucine (a stereoisomer of isoleucine not normally found in blood)
  • Urine contains high levels of leucine, isoleucine, and valine and their respective ketoacids
  • Ketoacids are detected by Di Nitro Phenyl Hydrazine (DNPH) Test,
  • Rothera's Test
  • Enzyme Analysis in leukocytes and cultured fibroblast
  • Tandem Mass Spectrometry.
Treatment
Restrict Branched ChainAminoAcid
Give high doses Thiamine.
 
Isovaleric Aciduria
Biochemical Defect
  • Defective Leucine metabolism.
  • Defective Enzyme is Isovaleryl CoA Dehydrogenase
  • Characteristic odor of Sweaty FeetQ is present.
 
Intermittent Branched Chain Ketonuria
  • Retains some activity of Branched Chain α Keto acid decarboxylase
 
BASIC AMINO ACID
 
Lysine
  • Represented by the letter K
  • Essential Amino Acid
  • Saccharopine is an intermediate in the Lysine Catabolic pathway.
  • Amino Acid deficient in Cereals
  • Predominantly Ketogenic
 
Functions of Lysine
  • Hydroxy Lysine is important in Covalent Cross links in Collagen and Desmosine crosslinks in Elastin.
  • ε Amino group of Lysine forms Schiff's bases.
  • Lysine along with Methionine (SAM is the methyl donor) are the precursors of Carnitine.
  • Bacterial Putrefaction (Decarboxylation) of Lysine forms Cadaverine.
  • Histone Proteins are Lysine rich.37
zoom view
Fig. 1.35: Metabolism of arginine
 
Arginine
  • Glucogenic
  • Semiessential Amino Acid
  • L Glutamate Semi aldehyde to α Keto Glutarate to Glucogenic pathway.
 
Metabolic Functions of Arginine
  • Nitric Oxide Synthesis.
  • Agmantine
  • Arginine splits to Ornithine and Urea (Terminal step in Urea Cycle)
  • Creatine
 
Nitric Oxide
  • Uncharged molecule having an unpaired electron, so it is highly reactive, free radical.
  • Very short half life (0.1 seconds)
  • Formerly called Endothelium Derived Relaxing Factor.
  • Gaseous molecule.
  • Second messenger is cGMP.
 
Functions of Nitric Oxide
  • Potent Vasodilator.
  • Involved in Penile erction
  • Neurotransmitter in brain and Peripheral Nervous System.
  • Low level of NO involved in Pylorospasm in Congenital Hypertrophic Pyloric Stenosis.
  • Inhibit adhesion, activation and aggregation of Platelets.
Therapeutic Uses of Nitric Oxide
  • Inhalation of Nitric Oxide in the treatment of Pulmonary Hypertension.
  • Treatment of Impotence (Sildenafil inhibit cGMP Phosphodiesterase)
  • Glyceryl Nitrite which is converted to Nitric Oxide is used in Angina Pectoris.
Synthesis of Nitric Oxide
zoom view
Fig. 1.36: Synthesis of nitric oxide
Nitric Oxide synthase
  • Cytosolic enzyme
  • Mono oxygenase
 
Five Redox Cofactors are
  1. NADPH
  2. FAD
  3. FMN
  4. Heme
  5. Tetrahydrobiopterin
 
Three Major Isoforms of Nitric Oxide Synthase
Subtype
Name
Characteristics
Deficiency leads to
1.
nNOS
First identified in the neurons
Activated by increase in Ca2+
Pyloric stenosis
Aggressive sexual behaviour
2.
iNOS
Prominent in macrophages
Independent of elevated Ca2+
More susceptible to certain types of infection
3.
eNOS
First identified in endothelial cells.
Activated by Ca2+
Elevated mean blood pressure.
 
Mechanism of Action of Nitric Oxide
zoom view
 
Agmatine
  • Derived from Arginine by decarboxylation
  • Properties of Neurotransmitter.
  • May have Antihypertensive Properties.38
 
HISTIDINE
  • Semi essential Amino Acid
  • Contains Imidazole ring.
  • Maximum buffering capacity at physiological pH.
 
Metabolism of Histidine
zoom view
Fig. 1.37: Metabolism of histidine
 
Important Points of the Histidine Metabolism Pathway
  • Urocanate is a derivative of Histidine.
  • FIGLU is Formimino Glutamic Acid
  • FIGLUis derived from Histidine.
  • In Folic Acid deficiency FIGLU is excreted in Urine.
 
Histidine Load Test
  • To identify Folic Acid Deficiency.
  • FIGLU excreted in urine is measured following a Histidine load.
 
Biologically important compounds derived from Histidine:
  • Histamine from histidine by decarboxylation. PLP is a coenzyme.
  • Carnosine (Beta Alanyl Histidine)
  • Anserine (Methyl Carnosine)
  • Homocarnosine (Gamma Amino Butyryl Histidine)
  • Ergothioneine
 
Function of Histamine and Receptor Responsible for its Action
Type of Receptor
Effect
H1
Smooth muscle contraction. Increased vascular permeability.
H2
Gastric HCl secretion.
H3
Synthesis and release of histamine in the brain.
Metabolic error due to deficiency of Histidase is histidinemia
 
ACIDIC AMINO ACIDS
 
Glutamic Acid (Glutamate)
  • Nonessential Amino Acid
  • Glucogenic Amino Acid
  • Central role in metabolism of Amino Acid.
  • Amino group of all Amino Acids are concentrated as GlutamateQ by transamination.
 
Biosynthesis of Glutamate
By reductive amidation of α Keto Glutarate catalysed by Glutamate dehydrogenase.
zoom view
Fig. 1.38: Biosynthesis of Glutamate
 
Metabolic Functions of Glutamic Acid
  1. Synthesis of N-Acetyl Glutamate
    Positive regulator of Carbamoyl Phosphate synthetase-1 of urea cycle
  2. Synthesis of Glutathoine (Gamma Glutamyl Cysteinyl Glycine)
  3. Synthesis of Gamma Amino Butyric Acid (GABA)
    • Glutamic Acid on decarboxylation gives GABA.
    • PLP is the coenzyme.39
 
Glutamine
Biosynthesis of Glutamine
Glutamine synthesized from Glutamic Acid by Glutamine Synthetase.
zoom view
Fig. 1.39: Biosynthesis of Glutamine
 
Metabolic Functions of Glutamine
  • Converts inorganic ammonium ions in to the α amino nitrogen of amino acid. This reaction is called first line trapping of Ammonia.
  • Carry amino group from brain and most other tissues.
  • N3 and N9 of Purine ring derived from Glutamine.
  • N3 of Pyrimidine is derived from glutamine.
  • Source of NH2 group of Guanine and Cytosine.
  • Glutamine is a conjugating agent.
  • Source of Ammonia excretion for Kidney, which has a role in renal regulation of acid base balance.
 
Aspartic Acid (Aspartate)
  • Nonesssential
  • Glucogenic Amino Acid
Synthesis of Aspartate
Transamination of Oxaloacetate forms Aspartate
 
Functions of Aspartate
  • Contribute its alpha amino group for urea synthesis.
  • Contributesto purine synthesis
  • Contributes to pyrimidine synthesis.
 
Canavan Disease
  • Autosomal recessive disorder
  • More prevalent in individuals of Ashkenazi Jewish descent than in other ethnic groups
Biochemical defect
  • Deficiency of aspartoacylase,leads to Canavan disease,
  • N-Acetylaspartic acid, a derivative of aspartic acid, is synthesized in the brain.
  • The exact function of N-acetylaspartic acid is unknown, but it may serve as a reservoir for acetate, which is needed for myelin synthesis.
  • Aspartoacylase, cleaves the N-acetyl group from N-acetylaspartic acid.
Characterised by
  • Leukodystrophy
  • Excessive excretion of N-acetylaspartic acid.
Diagnosis
  • Aspartoacylase deficiency can be determined in skin fibroblasts
  • Increased excretion of N-acetylaspartic acid in the urine.
 
ASPARAGINE
Synthesis of Asparagine
Aspartate is converted to Asparagine by Asparagine Synthetase.
zoom view
Fig. 1.40: Biosynthesis of asparagine
Asparagine Synthetase
  • Asparagine synthetase is analogous to Glutamine synthetase
  • In Asparagine synthetase, Glutamine rather than ammonium ions, provides nitrogen.
  • Hence cannot fix ammonia like Glutamine synthetase.
  • Bacterial Asparagine synthetase can however, also use ammonium ion.
 
Catabolism of Glutamate, Glutamine, Aspartate and Asparagine
  • Glutamine and Glutamate forms Alpha Keto Glutarate.Q
  • Asparagine and Aspartate forms Oxaloacetate.Q
zoom view
Fig. 1.41: Catabolism of Asparagine
40
 
AMINO ACIDS ENTER INTO TCA CYCLE AT DIFFERENT LEVELS
  1. As Pyruvate to Oxaloacetate:
    • Hydroxy Proline, Serine, Cysteine, Threonine, Glycine to Pyruvate
    • Lactate to Pyruvate
  1. As Alanine to Pyruvate to Oxaloacetate
    • Tryptophan to Alanine to Pyruvate
  2. Directly to Oxaloacetate
    • Aspartate
  3. As Aspartate to Oxaloacetate
    • Asparagine
  4. As Glutamate to Alpha Keto Glutarate (5C)
    • Histidine, Proline, Glutamine and Arginine
    • Glutamine and Glutamate are the major anaplerotic substrates of Alpha Keto Glutarate
  5. At the level of Succinyl CoA (4C)
    • Valine, Isoleucine, Methionine and Threonine.
    • These are the compounds that form Propionyl CoA.
  6. At the level of Fumarate (4C)
    • Tyrosine and Phenyl Alanine
zoom view
Fig. 1.42: Entry of amino acids to TCA cycle
 
Quick Review Points for National Board Pattern of Exams
  • Amino acid that absorbs UV light-Tryptophan, Phenyl Alanine, Tyrosine
  • Amino acid with no asymmetric carbon atom-Glycine
  • Beta Alanine is derived from Uracil and Cytosine
  • Amino acid at isoelectric pH has no net charge
  • Most common amino acid that undergo oxidative deamination is Glutamate
  • Coenzyme for transamination reaction is Pyridoxal Phosphate (PLP)
  • Amino acid that transport ammonia from most organs including the brain is Glutamine
  • Amino acid that transport ammonia from skeletal muscle is Alanine.
  • The nitrogen atoms of Urea are contributed by Ammonia and Aspartate.
  • The rate limiting step of Urea Cycle is Carbamoyl Phosphate Synthetase I
  • Most common urea cycle disorder is Hyperammonemia Type II (Ornithine Transcarbamoylase Defect)
  • Polyamines are derived from Ornithine, Methionine and Lysine
  • Amino acid involved in Cahill Cycle is Alanine
  • Amino acid that play an important role during Starvation as a gluconeogenic AA is Alanine
  • Transamination concentrate amino group as Glutamate.
  • Precursor of carnitine is Lysine and Methionine
  • Selenocysteine is derived from Serine
  • Glutamic acid is decarboxylated to GABA
  • Glutamic acid is deaminated to Alpha Keto Glutarate
  • Folate trap traps the THFA as its Methyl Derivative.
  • Amino acids that enter TCA Cycle as Succinyl Choline is Valine, Isoleucine and Methionine
 
Metabolic Disorder and Biochemical Defect
Metabolic Disorder
Biochemical defect
Hyperammonemia Type I
Carbamoyl Phosphate Synthetase I
Hyperammonemia Type II
Ornithine Transcarbamoylase
Citrullinemia Type I
Arginino Succinate Synthetase
Citrullinemia Type II
Citrin (Aspartate- Glutamate) Transporter
Arginino Succinic Aciduria
Arginino Succinate Lyase
Argininemia
Argininase
HHH syndrome
ORNT-I defect (Ornithine Permease)
Classic Phenyl Ketonuria
Phenyl Alanine Hydroxylase41
Alkaptonuria
Homogentisate Oxidase
Tyrosinemia Type I
Fumaryl Aceto Acetate Hydrolase
Tyrosinemia Type II
Tyrosine Transaminase
Tyrosinemia Type III
Para Hydroxy Phenyl Pyruvate hydroxylase/Para hydroxyl Phenyl Pyruvate Dioxygenase
Hawkinsinuria
Para Hydroxy Phenyl Pyruvate hydroxylase/Para hydroxyl Phenyl Pyruvate Dioxygenase is mutant, so that it catalyse only partial reaction.
Segawa Syndrome
GTP Cyclohydrolase
Albinism
Tyrosinase
Pheochromocytoma
Excess production of Catecholamines
Carcinoid Syndrome
Excess production of Serotonin
Hartnup's Disease
Defective absorption of Tryptophan and other neutral amino acids from renal tubules and intestines
Primary Hyperoxaluria Type I
Alanine-Glyoxylate Amino Transferase
Primary Hyperoxaluria Type II
D-Glycerate Dehydrogenase/Glyoxylate reductase Enzyme Complex
NonKetotic HyperGlycinemia
Glycine Cleavage System
Classic Homocystinuria
Cystathionine Beta Synthase
Non Classic Homocystinuria
I. Methyl Cobalamin formation defect
II. Methylene THFA Reductase
Cystathioninuria
Cystathionase
Cystinuria
Defective reabsorption of Cystine, Ornithine, Lysine and Arginine
Oasthouse Syndrome
Malabsorption of Methionine and other neutral amino acids
Type I A MSUD
E gene that codes for Branched Chain Keto acid Decarboxylase component of Branched Chain Keto acid Dehydrogenase Complex
Type I B MSUD
E gene that codes for Branched Chain Keto acid Decarboxylase component of Branched Chain Keto acid Dehydrogenase Complex
Type II MSUD
E2 gene that codes for Dihydrolipoyl Transacylase component of Branched Chain Keto acid Dehydrogenase Complex
Type III MSUD
E3 gene that codes Dihydrolipomide Dehydrogenase component of Branched Chain Keto acid Dehydrogenase Complex
Isovaleric Aciduria
Iso valeryl CoA Dehydrogenase
Canavan Disease
N Asparto Acylase
 
Specialised Products from Amino Acids
Amino Acid
Metabolic products
Tyrosine
Melanin
Catecholamines (Epinephrine, Norepinephrine, Dopamine)
Thyroxine
Tryptophan
Serotonin
Melatonin
Niacin
Cysteine
Cystine
Taurine
Glutathione
Betamercaptoethanolamine
Glycine
Purine
Heme
Glutathione
Creatinine
Arginine
Nitric oxide
Arginine
Arginine splits to Ornithine and Urea
Creatine
Histidine
FIGLU
Histamine
Glutamate
N acetyl Glutamate
Glutathione
Gamma Amino Butyric Acid
Glutamine
N3 & N9 of Purine
N3 of Pyrimidine
Aspartate
Purine
Pyrimidine
Urea Synthesis
 
Peculiar Odours in Different Amino Acidurias
Inborn Error of Metabolism
Urine Odor
Glutaricacidemia (type II)
Sweaty feet, acrid
Hawkinsinuria
Swimming Pool
Isovaleric Acidemia
Sweaty feet, Acrid
3-Hydroxy-3-methylglutaric aciduria
Cat urine
Maple syrup urine disease
Maple syrup
Hypermethioninemia
Boiled cabbage
Multiple carboxylase deficiency
Tomcat urine
Oasthouse urine disease
Hops-like
Phenylketonuria
Mousey or musty
Trimethylaminuria
Rotting fish
Tyrosinemia
Boiled cabbage, rancid butter42
// Review Questions //
 
 
Classification of Amino Acids
1. Which of the following have a positive charge in physiological pH?
(AIIMS Nov 2016)
  1. Arginine
  2. Aspartic acid
  3. Isoleucine
  4. Valine
2. What is the pH of the solution, if the hydrogen ion concentration is 5 millimoles/L?
(AIIMS Nov 2016)
  1. 2.3
  2. 3.7
  3. 6.6
  4. 3.5
3. Selenocysteine is coded by:
(AIIMS Nov 2015)
  1. UAG
  2. UGA
  3. UAA
  4. GUA
4. All of the following are essential aminoacids except:
(AIIMS May 2006)
  1. Methionine
  2. Lysine
  3. Alanine
  4. Leucine
5. Polar amino acids is/are:
(PGI May 2012)
  1. Serine
  2. Tryptophan
  3. Tyrosine
  4. Valine
  5. Lysine
6. Nonpolar amino acid are:
(PGI Nov 2010)
  1. Alanine
  2. Tryptophan
  3. Isoleucine
  4. Lysine
  5. Tyrosine
7. Hydrophobic amino acids are:
(PGI May 2010)
  1. Methionine
  2. Isoleucine
  3. Tyrosine
  4. Alanine
  5. Asparagine
8. Basic amino acids is/are:
(PGI Dec 2013)
  1. Leucine
  2. Arginine
  3. Lysine
  4. Histidine
9. Guanidinium group is associated with
(PGI June 2009)
  1. Tyrosine
  2. Arginine
  3. Histidine
  4. Lysine
  5. Tryptophan
10. Amino acid produced by adding hydroxyl group to benzene ring chain of phenylalanine:
(Kerala 2011)
  1. Threonine
  2. Histidine
  3. Tyrosine
  4. Serine
11. Sulphur containing amino acid is:
(Kerala 2009)
  1. Cysteine
  2. Leucine
  3. Arginine
  4. Threonine
12. Which of the following is a non-aromatic amino acid with a hydroxyl R-group?
(Kerala 2012)
  1. Phenylalanine
  2. Lysine
  3. Threonine
  4. Methionine
13. Which is not an essential amino acid?
(Kerala 2006)
  1. Tryptophan
  2. Threonine
  3. Histidine
  4. Cysteine
14. Which of the following is not an aromatic amino acid?
  1. Phenylalanine
  2. Tyrosine
  3. Tryptophan
  4. Valine
15. Which of the following group contains only nonessential amino acid?
(Recent Question 2016)
  1. Acidic amino acid
  2. Basic amino acid
  3. Aromatic amino acid
  4. Branched chain amino acid
16. Amide group containing amino acid is:
(Recent Question 2016)
  1. Glutamate
  2. Glutamic acid
  3. Glutamine
  4. Aspartate
17. Semi essential amino acids are:
(PGI 94)
  1. Arginine
  2. Histidine
  3. Glycine
  4. Phenylalanine
18. Which of the following is semiessential amino acid?
(Recent Question 2016)
  1. Arginine
  2. Histidine
  3. Glycine
  4. Phenylalanine
19. Aminoacyl t-RNA is required for all except:
(AI 2000)
  1. Hydroxyproline
  2. Methionine
  3. Cysteine
  4. Lysine
 
Properties of Amino Acids
20. HCO3-/H2CO3 is considered most effective buffer at physiological pH because:
(AIIMS Nov 2016)
  1. It has pKa close to physiological pH
  2. It is formed from a weak acid and base
  3. Its components can be increased or decreased by the body
  4. It can donate and accept H+
21. The graph shown below is the titration curve of a biochemical compound. Which of the following statement is true?
(AIIMS May 2016)
zoom view
43
  1. The maximum buffering capacity of the compound is represented by points A and B
  2. The points A and B represent the range of maximum ionisation of the amine and carboxyl group
  3. The compound has three ionisable side chains
  4. The compound has one ionisable group
22. Replacing alanine by which amino acid will increase UV absorbance of protein at 280 nm wavelength?
(AIIMS Nov 2008)
  1. Leucine
  2. Proline
  3. Arginine
  4. Tryptophan
23. Which of the following proteins cannot be phosphorylated using protein kinase in prokaryotic organisms?
(AI 2012)
  1. Threonine
  2. Tyrosine
  3. Serine
  4. Asparagine
24. Carboxylation of clotting factors by vitamin K is required to be biologically active. Which of the following amino acid is carboxylated?
(AIIMS Nov 2008)
  1. Histidine
  2. Histamine
  3. Glutamate
  4. Aspartate
25. Which of the following is/are not optically inactive amino acids?
(PGI May 2014)
  1. Threonine
  2. Tyrosine
  3. Valine
  4. Glycine
  5. Serine
26. Property of photochromisity is seen amongst the following aminoacids:
(AI 1997)
  1. Unsaturated amino acid
  2. Aromatic amino acid
  3. Monocarboxylic acid
  4. Dicarboxylic acid
27. The property of proteins to absorb ultraviolet rays of light is due to:
(AIIMS June 99)
  1. Peptide bond
  2. Imino group
  3. Di sulphide bond
  4. Aromatic amino acid
28. All biologically active amino acids are:
(AIIMS Nov 93)
  1. L-forms
  2. D-forms
  3. Mostly D-forms
  4. D-and L-forms
29. Optically inactive amino acid is:
(AI 99)
  1. Proline
  2. Glycine
  3. Lysine
  4. Leucine
30. Flexibility of protein depends on:
(AI 1994)
  1. Glycine
  2. Tryptophan
  3. Phenylalanine
  4. Histidine
31. Which aminoacid can protonate and deprotonate at neutral pH?
(AIIMS May 95)
  1. Histidine
  2. Leucine
  3. Glycine
  4. Arginine
32. Phosphorylation of aminoacid by:
(PGI June 98)
  1. Serine
  2. Tyrosine
  3. Leucine
  4. Tryptophan
33. Which of the following amino acid is purely Glucogenic?
(Recent Question 2016)
  1. Valine
  2. Lysine
  3. Alanine
  4. Glycine
 
GENERAL AMINO ACID METABOLISM
 
Digestion and Absorption of Proteins, Transamination and Transport of Amino Acids
34. Ammonia from brain is detoxified as:
(AIIMS Nov 2016)
  1. Glutamate
  2. Glutamine
  3. Alanine
  4. Urea
35. True about glutamate dehydrogenase
  1. Can use NADH or NADPH (Recent Question 2016)
  2. PLP is the coenzyme
  3. Enzyme of transamination
  4. Ammonium ion is not released in the free form
36. Increased alanine during prolonged fasting represents:
(AIIMS Nov 2011)
  1. Increased breakdown of muscle proteins
  2. Impaired renal function
  3. Decreased utilization of amino acid from Gluconeogenesis
  4. Leakage of amino acids from cells due to plasma membrane damage
37. Transfer of an amino group from an amino acid to an alpha keto acid is done by:
(AI 2011)
  1. Transaminases
  2. Aminases
  3. Transketolases
  4. Deaminases
38. The amino acid which serves as a carrier of ammonia from skeletal muscle to liver is:
(AI 2006)
  1. Alanine
  2. Methionine
  3. Arginine
  4. Glutamine
39. Glutamine in blood acts as:
(PGI Dec 98)
  1. NH3 transporter
  2. Toxic element
  3. Stored energy
  4. Abnormal metabolite
40. Amino acid absorbtion is by:
(Recent Question 2016)
  1. Facilitated transport
  2. Passive transport
  3. Active transport
  4. Pinocytosis
41. The transporter gene defective in Hartnup's disease:
(JIPMER Dec 2016)
  1. SLC 6A 19
  2. SLC 6A 18
  3. SLC 36 A2
  4. SLC 7A7
42. Nontoxic form of storage and transportation of ammonia:
(Recent Question 2016)
  1. Aspartic acid
  2. Glutamic acid
  3. Glutamine
  4. Glutamate
 
Urea Cycle
43. CPS-I used in which pathway
(Recent Question 2016)
  1. Pyrimidine synthesis
  2. Purine synthesis
  3. Urea cycle
  4. TCA cycle
44. Urea cycle enzymes are:
(PGI May 2010)
  1. Glutaminase
  2. Asparginase
  3. Arginosuccinate synthetase
  4. Ornithine transcarbmoylase
  5. Glutamate dehydrogenase44
45. Which enzymes are part of urea cycle?
(PGI 2012)
  1. Ornithine transcarbamoylase.
  2. Asparaginase
  3. Glutamate synthase
  4. Arginosuccinase
46. Urea cycle occurs in:
(AI 2011)
  1. Liver
  2. GIT
  3. Spleen
  4. Kidney
47. In which of the following condition there is increased level of ammonia in blood?
(Kerala 2008)
  1. Ornithine transcarbamoylase deficiency
  2. Galactosaemia
  3. Histidinaemia
  4. Phenyl ketonuria
48. Urea cycle occurs in:
(Kerala 2008)
  1. Cytoplasm
  2. Mitochondria
  3. Both of the above
  4. Endoplasmic reticulum
The pathways that take place in two compartements are
  • Heme synthesis
  • Urea cycle
  • Gluconeogenesis
49. Which of the following enzymes(s) is/are not involved in urea cycle?
(PGI May 2012)
  1. Glutamate dehydrogenase
  2. Argino succinate synthetase
  3. α Ketoglutarate dehydrogenase
  4. Isocitrate dehydrogenase
  5. Fumarase
50. Glutamate dehydrogenase in mitochondria is activated by:
(Recent Question 2016)
  1. ATP
  2. GTP
  3. NADH
  4. ADP
51. Nitrogen atoms of urea contributed by:
(Recent Question 2016)
  1. Ammonium and aspartate
  2. Ammonium and glutamate
  3. Ammonium and glycine
  4. Ammonium and asparagine
52. A 6-month-old boy admitted with failure to thrive with high glutamine and Uracil in urine Hypoglycemia, high blood ammonia. Treatment given for 2 months. At 8 months again admitted for failure to gain weight. Gastric tube feeding was not tolerated child became comatose. Parenteral dextrose given. Child recovered from coma within 24 hours. What is the enzyme defect?
(AIIMS May 2015)
  1. CPS1
  2. Ornithine transcarbamoylase
  3. Arginase
  4. Argininosuccinate synthetase
53. Which of the following is true in relation of urea cycle:
(PGI Dec 05)
  1. First 2 steps in cytoplasm
  2. First 2 steps in mitochondria
  3. Defect of enzyme of any step can cause deficiency disease
  4. Urea is formed by NH3, glutamic acid and CO2
  5. Citruline is formed by combination of carbomoyl phosphate and Lornithine
54. A baby presents with refusal to feed, skin lesions, seizures, ketosis, organic acids in urine with normal ammonia; likely diagnosis:
(AI 2001)
  1. Proprionic aciduria
  2. Multiple carboxylase deficiency
  3. Maple syrup urine disease
  4. Urea cycle enzyme deficiency
55. True about urea cycle:
(PGI May 2015)
  1. Nitrogen of the urea comes from alanine & ammonia
  2. Uses ATP during conversion of argininosuccinate to arginine
  3. On consumption of high amount of protein, excess urea is formed.
  4. Occur mainly in cytoplasm
  5. Synthesis of argininosuccinate consumes energy
56. All are true regarding Urea cycle except:
(PGI Nov 2014)
  1. Urea is formed from ammonia
  2. Rate limiting enzyme is Ornthine transcarbamoylase
  3. Require energy expenditure
  4. Malate is a byproduct of urea cycle
  5. One nitrogen of urea comes from Aspartate
57. Enzyme involved in nonoxidative deamination is:
(Recent Question 2016)
  1. L-amino acid oxidase
  2. Glutamate dehydrogenase
  3. Glutaminase
  4. Amino acid dehydrases
58. Which of these is a conservative mutation:
(AIIMS Dec 98)
  1. Glutamic acid-glutamine
  2. Histidine-glycine
  3. Alanine-leucine
  4. Arginine-aspartic acid
 
INDIVIDUAL AMINO ACID METABOLISM
 
Aromatic Amino Acids
59. Melanin derived from
(Recent Question 2016)
  1. Tryptophan
  2. Tyrosine
  3. Methionine
  4. Alanine
60. Melatonin derived from
(Recent Question 2016)
  1. Tryptophan
  2. Tyrosine
  3. Methionine
  4. Alanine
61. Treatment of tyrosinemia type 1 is:
(Recent Question 2016)
  1. NTBC
  2. Vitamin B6
  3. Large neutral amino acids
  4. Tyrosine restricted diet
62. Which is elevated in PLP deficiency?
(Recent Question 2016)
  1. FIGLU
  2. Xanthurenic acid
  3. Methyl malonic acid
  4. Homocystine
63. Dopamine is synthesized from:
(Recent Question 2016)
  1. Tryptophan
  2. Threonine
  3. Tyrosine
  4. Lysine45
64. In phenylketonuria the main aim of first line therapy is:
(AIIMS Nov 2010)
  1. Replacement of the defective enzyme
  2. Replacement of the deficient product
  3. Limiting the substrate for deficient enzyme
  4. Giving the missing amino acid by diet
65. A 40-year-old woman presents with progressive palmoplantar pigmentation X-ray spine shows calcification of IV disc. On adding benedicts reagent to urine, it gives greenish brown precipitate and blue-black supernatant fluid. What is the diagnosis?
(AIIMS Nov 2008)
  1. Phenylketonuria
  2. Alkaptonuria
  3. Tyrosinemia type2
  4. Argininosuccinicaciduria
66. Dopamine hydroxylase catalyse:
(Kerala 2007)
  1. Dopamine → norepinephrine
  2. Dopa to dopamine
  3. Nor epinephrine to epinephrine
  4. Tyrosine to dopa
67. Type I tyrosinemia is caused by:
(Recent Question 2016)
  1. Tyrosine transaminase
  2. Fumaryl Aceto acetate hydrolase
  3. 4 Hydroxy phenyl pyruvate hydroxylase
  4. Maleyl acetoacetate isomerase
68. Terminal product of phenyl alanine metabolism is:
(PGI May 2014)
  1. Fumarate
  2. Acetyl CoA
  3. Oxaloacetate
69. Enzyme deficiency in albinism is:
(Recent Question 2016)
  1. Tyrosinase
  2. Tyrosine hydroxylase
  3. Phenylalanine hydroxylase
  4. Homogentisate oxidase
70. Mousy body odour is due to:
(JIPMER May 2015)
  1. Phenyl alanine
  2. Phenyl acetate
  3. Phenyl butazone
  4. Phenyl acetyl glutamine
71. The aminoacid that can be converted into a vitamin:
(Kerala 91)
  1. Glycine
  2. Tryptophan
  3. Phenyalalanine
  4. Lysine
72. Which of the following amioacids is involved in the synthesis of thyroxine:
(Karnat 97)
  1. Glycine
  2. Methionine
  3. Threonine
  4. Tyrosine
73. Tyrosinemics are more susceptible to develop:
(AIIMS Feb 97)
  1. Adenocarcinoma colon
  2. Melanoma
  3. Retinoblastoma
  4. Hepatic carcinoma
74. Metabolites of tryptophan can give rise to:
(PGI June 02)
  1. Diarrhoea
  2. Vasoconstriction
  3. Flushing
  4. Can predispose to albinism
  5. Phenylketonuria
75. Correct combination of urine odour in various metabolic disorder:
(PGI Nov 2013)
  1. Phenyl ketonuria-mousy body odour
  2. Tyrosinemia- rotten cabbage
  3. Hawkinsinuria-potato smell
  4. Maple syrup disease-rotten tomato
  5. Alkaptonuria-rotten egg
76. Which of the following is true regarding phenyl ketonuria?
(PGI Nov 2014)
  1. Dietary phenyl alanine restriction is used as a treatment
  2. Occur due to deficiency of Phenyl Alanine Hydroxylase
  3. Occur due to increase activity of phenyl alanine hydroxylase
  4. Diet should contain high phenyl alanine containing food items
  5. Tyrosine should be supplied in the diet
 
Simple Amino Acids
77. Hyperoxaluria associated with which amino acid?
(Recent Question 2016)
  1. Glycine
  2. Serine
  3. Threonine
  4. Lysine
78. Which of the following is true about glycine?
(Kerala 2008)
  1. Glycine is an essential amino acid
  2. Sulphur containing at 4th position
  3. Has a guanidine group
  4. Optically inactive
79. Which of the following would not act as source of glycine by transamination?
(Recent Question 2016)
  1. Alanine
  2. Aspartate
  3. Glutamate
  4. Glyoxylate
80. Glycine cleavage system in liver mitochondria is associated with which enzyme?
(Recent Question 2016)
  1. Glycine dehydrogenase
  2. Glycine transaminase
  3. Glycine decarboxylase
  4. Glycine dehydratase
81. Guanido acetic acid is formed in ………. from ……….
(JIPMER 2000, DNB 98)
  1. Kidney; Arginine + Glycine
  2. Liver; Methionine + Glycine
  3. Liver; Cysteine + Arginine
  4. Muscle; Citrulline + Aspartate
82. Conversion of glycine to serine requires:
(PGI Dec 02)
  1. Folic acid
  2. Thiamine
  3. Vitamin C
  4. Fe2+
  5. Pyridoxal phosphate
83. N Methyl glycine is known as:
(Recent Question 2016)
  1. Ergothionine
  2. Sarcosine
  3. Carnosine
  4. Betaine46
84. What is the metabolic defect in primary oxaluria Type II?
(Recent Question 2016)
  1. Glycine cleavage system
  2. Alanine glyoxalate amino transferase
  3. D glycerate dehydrogenase
  4. Excess vitamin C
85. All are true about glutathione except:
(AIIMS Nov 2008)
  1. It is a tripeptide
  2. It converts hemoglobin to methemoglobin.
  3. It conjugates xenobiotics
  4. It is cofactor of various enzymes
 
Sulphur Containing Amino Acids
86. Tripeptide is:
(Recent Question 2016)
  1. Glutathione
  2. Anserine
  3. Carnosine
  4. Homocarnosine
87. In a case of classic homocystinuria what should be supplemented in the diet to prevent heart attacks:
(JIPMER May 2016)
  1. Pyridoxine
  2. Methionine
  3. Methyl cobalmine
  4. Niacin
88. Sulphur of cysteine are not used /utilised in the body for the following process/product:
(PGI May 2015)
  1. Help in the conversion of cyanide to thiocyanate
  2. Thiosulphate formation
  3. Introduction of sulphur in methionine
  4. Disulphide bond formation between two adjacent peptide
89. N acetyl cysteine replenishes:
(JIPMER 2012)
  1. Glutathione
  2. Glycine
  3. Glutamate
  4. GABA
90. Which of the following is true about glutathione?
(PGI 2000)
  1. Contain sulfhydral group
  2. Forms met Hb from Hb
  3. It does not detoxify superoxide radicals
  4. Transport amino acid across cell membrane
  5. Part of enzymes
91. In glutathione which amino acid is reducing agent:
(AIIMS June 1997)
  1. Glutamic acid
  2. Glycine
  3. Cysteine
  4. Alanine
 
Acidic and Basic Amino Acids
92. Vasodilator produced by decarboxylation of:
(Recent Question 2016)
  1. Histidine
  2. Glutamic acid
  3. Aspartic acid
  4. Lysine
93. Nitric oxide synthesised from:
(Recent Question 2016)
  1. Arginine
  2. Citrulline
  3. Alanine
  4. Cysteine
94. Histidine load test is used for:
(Recent Question 2016)
  1. Folate deficiency
  2. Histidine deficiency
  3. Histamine deficiency
95. True about nitric oxide are all except:
(Recent Question 2016)
  1. Produced from arginine
  2. Nitric oxide synthase has three isoforms
  3. Otherwise called endothelium derived relaxing factor
  4. Acts through cAMP
96. Creatinine is formed from:
(PGI June 06)
  1. Arginine
  2. Lysine
  3. Leucine
  4. Histamine
97. Histidine is converted to histamine by which reaction?
(Recent Question 2016)
  1. Carboxylation
  2. Oxidation
  3. Decarboxylation
  4. Amination
 
Branched Chain Amino Acid
98. In MSUD amino acid that is excreted:
(Recent Question 2016)
  1. Histidine
  2. Methionine
  3. Leucine
  4. Lysine
99. Branched chain ketoacid decarboxylation is defective in:
(AI 2010)
  1. Maple syrup urine disease
  2. Hartnup disease
  3. Alkaptonuria
  4. GMI gangliosidosis
100. MSUD type I A is due to mutation of:
(Recent Question 2016)
  1. E1 α
  2. E1 β
  3. E2
  4. E3
101. Which is not formed from branched chain amino acid?
(Recent Question 2016)
  1. Xanthurenate
  2. Tiglyl CoA
  3. Acetoacetyl CoA and acetyl coA
  4. Acetyl coA and succinyl CoA
102. Treatment used in Isovaleric aciduria:
(Latest Question)
  1. Arginine
  2. Lysine
  3. Glycine
  4. Methionine
103. Which of the following aminoacid is excreted in urine in maple syrup urine disease:
(AI 1999)
  1. Tryptophan
  2. Phenyl alanine
  3. Leucine
  4. Arginine
104. Diseases of branched chain aminoacid includes:
(PGI Nov 2013)
  1. Phenyl ketonuria
  2. Maple syrup urine disease
  3. Taysachs disease
  4. Isovaleric acidemia
  5. Niemann pick disease
 
Other Amino Acids and Entry of Amino Acid to TCA Cycle
105. Proline is formed from:
(Recent Question 2016)
  1. Alpha ketoglutarate
  2. Glutamate
  3. Pyruvate
  4. Alanine47
106. The nitrogen atom of aspartate formed from asparagines using enzyme asparaginase is from:
(Recent Question 2016)
  1. Ammonium
  2. Glutamate
  3. Glutamine
  4. Alpha ketoglutarate
107. Oxaloacetate is formed from:
(Recent Question 2016)
  1. Proline
  2. Histidine and arginine
  3. Glutamate and glutamine
  4. Aspartate and asparagine
108. Amino acid responsible for thioredoxin reductase activation:
(Recent Question 2016)
  1. Serine
  2. Selenocysteine
  3. Cysteine
  4. Alanine
109. Oxaloacetate is derived from which amino acids
(Recent Question 2016)
  1. Glutamine and glutamate
  2. Asparagine and aspartate
  3. Histidine and arginine
  4. Glutamine and proline
110. Smell of sweaty feet is seen in:
(Recent Question 2016)
  1. MSUD
  2. Phenyl Ketonuria
  3. Homocystinuria
  4. Glutaric Acidemia
111. During the formation of hydroxyl proline and hydro-xyl lysine, the essential factors required is/are:
(PGI Dec 2003)
  1. Pyridoxal phosphate
  2. Ascorbic acid
  3. Thiamine pyrophosphate
  4. Methylcobalamine
  5. Biotin
112. Succinyl CoA is formed by:
(PGI June 1998)
  1. Histidine
  2. Leucine
  3. Valine
  4. Lysine
113. In one carbon metabolism serine loses which carbon atom?
(Recent Question 2016)
  1. Alpha
  2. Beta
  3. Gamma
  4. Delta
48// Answers to Review Questions //
 
 
Classification of Amino Acids
1. a. Arginine
(Ref: Harper 30/e p18 Table 3-2)
At physiological pH positive charge is for Histidine, Arginine and Lysine.
At physiological pH negative charge is for Aspartic acid and Glutamic acid.
Charge of an amino acid depends on its isolelectric pH
If pH of medium > pI, the amino acid is negatively charged
If pH of medium < pI, the amino acid is positively charged.
2. a. 2.3
pH = –log [H+] = log 1/[H+]
[H+] = 5 millimoles/L = 5 × 10–3 moles/L
pH = log 1/5 × 10-3 moles/L = log 0.2 + log 1/103 = −0.6987 + 3 = 3 − 0.6987 = 2.3
3. b. UGA
  • Stop codon UGA codes Selenocysteine
  • Stop codon UAG codes Pyrrolysine
4. c. Alanine
(Ref: Harper 30/e p282 Table 27-1)
Essential (MettVilPhly Read As Met Will Fly)
All the other amino acid
Nonessential
Me thionine
Arginine
All the other amino acid
Threonine
Tryptophan
Valine
Isoleucine
Leucine
Phenyl Alanine
Lysine
5. a, e. Serine, Lysine
(Ref: Harper 30/e p18 Table 3-2)
Classification of Amino Acids Based on Side Chain Characteristics (Polarity)
Polar Amino Acids (Hydrophilic):
Uncharged Amino acids are Serine, Threonine, Glutamine, Asparagine, Cysteine, Glycine
Charged Amino acids are Aspartic Acid, Glutamic Acid, Histidine, Arginine, Lysine.
Nonpolar Amino Acid (Hydrophobic)
Alanine, Leucine, Isoleucine, Valine, Phenyl Alanine, Tyrosine, Tryptophan, Proline, Methionine.
6. a, b, c, e. Alanine, Tryptophan, Isoleucine, Tyrosine
(Ref: Harper 30/e p18 3-2)
7. a, b, c, d. Methionine, Isoleucine, Tyrosine, Alanine
(Ref: Harper 30/e p18 3-2)
8. b, c, d. Arginine, Lysine, Histidine
(Ref: Harper 30/e p17, Table 3-1)
  • Basic aminon acids are Histidine, Arginine and Lysine
  • Acidic amino acids are Aspartic Acid (Aspartate), Glutamic Acid (Glutamate)
9. b. Arginine
(Ref: Harper 30/e p18, Table 3-2)
Special Groups Present In Amino Acids
Amino Acid
Special Group
Arginine
GuanidiniumQ
Phenyl Alanine
Benzene
Tyrosine
Phenol
Histidine
ImidazoleQ
Proline
Pyrrolidine
Methionine
Thioether Linkage
Tryptophan
Indole
Cysteine
Thioalcohol (SH)
10. c. Tyrosine
(Ref: Harper 29th, Table 3.1)
  • Special group in phenyl alanine is benzene ring
  • Special group in Tyrosine is Phenol ring
  • The enzyme that hydroxylate Phenyl Alanine to Tyrosine is Phenyl Alanine Hydroxylase
11. a. Cysteine
  • Sulphur containing amino acids are Cysteine and Methionine.
  • The Sulphur of Cysteine is provided by Methionine.
  • Special group in Cysteine is Sulfhydryl group Thioalcohol (-SH)
  • Special group in Methionine is Thioether (C-S-C)
12. c. Threonine
(Ref: Harper 30/e p17 Table 3.1)
  • Aromatic amino acid with hydroxyl group-Tyrosine
Non aromatic amino acid with hydroxyl group are Serine and Threonine
13. d. Cysteine
(Ref: Harper 30/e p282)
Essential Amino acids are Methionine, Threonine, Tryptophan, Valine, Isoleucine, Leucine, Phenyl Alanine, Lysine (Mnemonic MeTT VIL PhLy) and Histidine
Semiessential Amino acids is Arginine
14. d. Valine
(Ref: Harper 30/e p17, Table 3-1)
Aromatic amino acids are
  • Histidine (with Imidazole ring)
  • Phenyl Alanine (Benzene ring)
  • Tyrosine (Phenol ring)
  • Tryptophan (Indole ring)49
15. a. Acidic amino acid
(Ref: Harper 30/e p282)
  • Group of amino acid that contain only essential amino acid is Branched chain amino acids (Leucine, Isoleucine, Valine)
  • Group of amino acid that contain only nonessential amino acid is Acidic Amino acids, Amide group containing amino acids, Imino acid, Simple amino acids
16. c. Glutamine
  • Glutamine and Asparagine are Amide group containing amino acids.
  • Aspartate and Glutamate are Acidic amino acid.
17. a, b. Arginine, Histidine
(Ref: Harper 30/e p282, Table 27-1)
18. a. Arginine
(Ref: Harper 30/e p282, Table 27-1)
19. a. Hydroxyproline
  • Derived amino acids do not require Aminoacyl tRNA.
  • Among the above options Hydroxyproline is a derived amino acid
 
Derived Amino Acid seen in ProteinQ
4-Hydroxy Proline
  • Found in Collagen
  • Vitamin C is needed for hydroxylation.
5-Hydroxy Lysine
Methyl lysine
  • Found in Myosin
Gamma carboxy glutamate
  • Found in clotting factors, like Prothrombin that bind Ca2+
  • Vitamin K is needed for Gammacarboxylation.
Cystine
  • Found in proteins with disulphide bond.Q 2014 DNB
  • Two cysteine molecules join to form cystine.
  • e.g. Insulin, Immunoglobulin
Desmosine
  • Found in ElastinQ AIIMS Nov 2014
 
Properties of Amino Acids
20. c. Its components can be increased or decreased by the body.
According tp Henderson Hasselbalch equation
pH = pKa + log [HCO3–]/[H2CO3]
The physiological pH is 7.4
In case of bicarbonate buffer
pH = 6.1 + log 24/1.2 = 6.1 + log 20 = 6.1 + 1.3 = 7.4, which is exactly equal to physiological pH.
Hence bicarbonate buffer is most effective buffer due to two reasons
  1. The concentration of [HCO3] is very high, i.e. 24 mmol/l hence the ratio is maintained at 20, hence pH can be maintained.
  2. The components is under physiological control ie
    • [HCO3] can be regulated by kidneys
    • CO2, hence Carbonic acid [H2CO3] is regulated by lungs. So any imbalance in [HCO3]/[H2CO3] can regulated by renal or respiratory method and ratio 20 can be maintained
21. a. The maximum buffering capacity is represented by the points A and B
zoom view
22. d. Tryptophan
(Ref: Harper 30/e p21,22)
  • Amino Acid Absorb UV Light
  • Amino Acids which absorb 250-290 nm (Maximum at 280 nm) uv light are tryptophan, phenylalanine, tyrosine.
  • Maximum absorption of UV light by tryptophan.
  • Remember-aromatic amino acids absorb uv light
23. d. Asparagine
(Ref: Harper 30/e Chapter 9, p93)
Protein kinases phosphorylate proteins by catalyzing transfer of the terminal phosphoryl group of ATP to the hydroxyl groups of seryl, threonyl, or tyrosyl residues, forming O-phosphoseryl, O-phosphothreonyl, or O-phosphotyrosyl residues, respectively
  • Commonest site of phosphorylation is Serine and Threonine followed by Tyrosine.
24. c. Glutamate
(Ref: Harper 30/e p717)
The Vitamin that act as coenzyme for carboxylation is Biotin
The vitamin that act as coenzyme for gamma carboxylation is vitamin K
The Proteins that are gamma carboxylated by vitamin K are:50
  • Factor II (Prothrombin),
  • Factor VII (Proconvertin or Serum Prothrombin conversion Accelerator, SPCA),
  • Factor IX (Antihemophilic factor or Christmas factor),
  • Factor X (Stuart prower factor),
  • Protein C, Protein S,
  • Osteocalcin, Nephrocalcin
  • Product of gene gas 6.
25. a, b, c, e. Threonine, Tyrosine, Valine, Serine.
(Ref: Harper 30/e p19)
  • Glycine is the only optically inactive amino acid
26. b. Aromatic amino acid
(Ref: Harper 30/e p21,22)
Amino Acid Absorb UV Light
  • Amino Acids which absorb 250-290 nm (Maximum at 280 nm) UV light are tryptophan, phenylalanine, tyrosine.
  • Maximum absorption of UV light by tryptophan.
  • Remember-aromatic amino acids absorb UV light
27. d. Aromatic amino acid
(Ref: Harper 30/e p21,22)
28. a. L-forms
(Ref: Harper 30/e p18)
  • Amino acids mostly exists in L-forms
  • Carbohydrates exists in D-forms
29. b. Glycine
  • Only optically inactive amino acid is Glycine
30. a. Glycine
(Ref: Harper 30/e p39)
  • Glycine having the smallest R group fit in to small spaces and induces bends in the alpha helix.
  • Glycine is usually present in beta turns.
31. a. Histidine
  • Amino acid which can protonate and deprotonate means those which can act as buffer.
  • Amino acid whose pKa = pH of the medium has maximum buffering capacity.
  • pKa of imidazole group of histidine is 6.5-7.4.
  • At pH = 7, Imidazole group of histidine can act as buffer.
32. a, b. Serine, Tyrosine
(Ref: Harper 30/e Chapter 9, p93)
33. c. Alanine > Valine/Glycine
  • Lysine is both ketogenic and gluco (glyco) genic
  • Glycine, Alanine and Valine are purely Glucogenic.
  • But Alanine is the principal Glucogenic amino acid.
  • Remember-Glucose Alanine cycle in starvation for provision of substrate for gluconeogenesis.
 
GENERAL AMINO ACID METABOLISM
 
Digestion and Absorption of Proteins, Transamination and Transport of Amino Acids
34. b. Glutamine
Example:
  • Ammonia from the brain and most other tissue is detoxified as Glutamine. The enzyme is Glutamine Synthestase. This is called as first line trapping of Ammonia.
  • Ammonia is transported as Alanine from muscle.
Urea cycle does not operate in the brain
35. a. Can use NADH or NADPH
  • Other options PLP is not thecoenzyme of Oxidative deamination by GDH.
  • GDH is the enzyme of Oxidative deamination.
  • Ammonium ion is released in free form.
36. a. Increased breakdown of muscle protein
During prolonged fasting, there is increased gluconeogenesis. Alanine provided by muscle is one of the substrates for gluconeogenesis.
This is called Glucose Alanine Cycle or Cahill Cycle.
So plasma level of Alanine rises in prolonged starvation.
Remember:
  • In prolonged fasting plasma level of Alanine rises.
  • In hyperammonemia plasma level of Glutamine rises.
37. a. Transaminases
(Ref: Harper 30/e p290)
Key points transmination
  • Interconvert pair of α amino acids and α Ketoacid.
    • Ketoacid formed by transamination from Alanine is Pyruvate
    • Ketoacid formed by transamination from Aspartate is Oxaloacetate.
    • Ketoacid formed by transamination from Glutamate is α Keto Glutarate
  • Freely reversible.
  • Transmination concentrate α amino group of nitrogen as L-Glutamate.
  • L-Glutamate is the only enzyme that undergo significant amount of oxidative deamination in mammals.
  • Takes place via ping pong mechanism.
  • Takes an important role in biosynthesis of nutritionally nonessential amino acids.
  • Specific for one pair of substrate but nonspecific for other pair of substrates.
  • Pyridoxal Phosphate is the coenzyme
38. a. Alanine
(Ref: Lippincott 6/e p253)
  • Transport form of Ammonia from most tissues including brain is Glutamine
  • Transport form of Ammonia from skeletal muscle is Alanine.
39. a. Ammonia transporter
Transport form of ammonia from brain and most other tissues
40. c. Active transport
(Ref: Harper 30/e p539)
Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. There are several different amino acid transporters, with specificity for the nature of the amino acid side-chain.
Transporters of Amino Acids51
  • For Neutral Amino acids
  • For Basic Amino acids and Cysteine.
  • For Imino Acids and Glycine
  • For Acidic Amino acids
  • For Beta Amino Acids (Beta Alanine)
Meisters cycle
  • For absorption of Neutral Amino acids from Intestines, Kidney tubules and brain.
  • The main role is played by Glutathione. (GSH)
  • For transport of 1 amino acid and regeneration of GSH 3 ATPs are required.
Disorders associated with Meister's Cycle
Oxoprolinuria
  • 5 Oxoprolinase deficiency leads to oxoprolinuria
Disorders Associated with Absorption of Amino acids
Hartnup's Disease
Malabsorption of neutral amino acids, including the essential amino acid tryptophan
SLC6A19, which is the major luminal sodium-dependent neutral amino acid transporter of small intestine and renal tubules, has been identified as the defective protein
Blue Diaper Syndrome or Drummond Syndrome Indicanuria
Tryptophan is specifically malabsorbed and the defect is expressed only in the intestine and not in the kidney. Intestinal bacteria convert the unabsorbed tryptophan to indican, which is responsible for the bluish discoloration of the urine after its hydrolysis and oxidation
Cystinuria
Dibasic amino acids, including cystine, ornithine, lysine, and arginine are taken up by the Na-independent SLC3A1/SLC7A9, in the apical membrane which is defective in cystinuri (a)
Most common disorder associated with Amino acid malabsorption.
Lysinuric Protein Intolerance
(SLC7A7) carrier at the basolateral membrane of the intestinal and renal epithelium is affected, with failure to deliver cytosolic dibasic cationic amino acids into the paracellular space in exchange for Na+ and neutral amino acids.
Oasthouse Urine Disease
(Smith Strang Disease)
A methionine-preferring transporter in the small intestine was suggested to be affected. Cabbage-like odor, containing 2-hydroxybutyric acid, valine, and leucine.
IminoGlycinuria
Malabsorption of proline, hydroxyproline, and glycine due to the proton amino acid transporter SLC36A2 defect
Dicarboxylic Aciduria
Excitatory amino acid carrier SLC1A1 is affected.
Associated with neurologic symptoms such as POLIP (polyneuropathy, ophthalmoplegia, leukoencephalopathy, intestinal pseudo-obstruction
41. a. SLC 6A19
Hartnup's Disease
Malabsorption of neutral amino acids, including the essential amino acid tryptophan
SLC6A19, which is the major luminal sodium-dependent neutral amino acid transporter of small intestine and renal tubules, has been identified as the defective protein
42. c. Glutamine.
  • Transport form of Ammonia from most tissues is Glutamine.
  • The enzyme responsible is called Glutamine Synthetase.
  • Belong to Ligase class.
  • Require ATP.
 
Urea Cycle
43. c. Urea cycle
44. c. Argininosuccinate synthetase, d. Ornithine transcarbamoylase
(Ref: Harper 30/e p293)
Reactions of Urea Cycle
The first two reaction takes place in the mitochondria. The rest of the reactions takes place in the cytoplasm.
Carbamoyl Phosphate Synthetase –I (CPS-I)
  • Carbamoyl Phosphate is formed from the condensation of CO2, Ammonia and ATP.
  • CPS-I is the rate limiting (pacemaker) enzyme in this pathway
  • CPS-I is active only in the presence of N-Acetyl Glutamate, an allosteric activator.
  • This step require 2 mols of ATPs.
Ornithine Transcarbamoylase (OTC)
Transfer carbamoyl group of Carbamoyl Phosphate to Ornithine forming Citrulline.
Subsequent steps takes place in the cytoplasm.
Arginino Succinate Synthetase
  • Links Amino nitrogen of Aspartate to Citrulline and provides second nitrogen of Urea
  • This reaction requires ATP.
  • 2 inorganic phosphates are utilized.
Arginino Succinate Lyase
Cleavage of Argino succinate to Arginine and Fumarate.
ArginaseQ
Hydrolytic cleavage of Arginine, releases Urea and reforms Ornithine which reenter in to mitochondria
45. a, d. Ornithine transcarbamoylase, Arginosuccinase
(Ref: Harper 30/e p293)52
Enzymes of urea Cycle and its classes
Enzymes name
Class of enzyme it belongs
Carbamoyl-phosphate synthase I
Class 6 (Ligase)
Ornithine carbamoyl transferase
Class 2 (Transferase)
Argininosuccinate synthase
Class 6 (Ligase)
Argininosuccinate lyase (ArgininoSuccinase)
Class 4 (Lyase)
Arginase
Class 3 (Hydrolase)
46. a. Liver
  • Site of urea synthesis in liver mitochondria and cytosol.
  • Derived amino acids which has almost exclusive role in urea cycle are Ornithine, Citrulline, Arginino Succinate.
  • Four amino acids which has no net loss or gain in urea cycle is ornithine, citrulline, arginino succinate, arginine.
47. a. Ornithine transcarbamoylase deficiency
Increased ammonia in blood is suggestive of a urea cycle disorder. So answer is an enzyme of urea cycle.
Hyperammonemia Type II (OTC Deficiency)
  • Most common Urea Cycle disorderQ
  • Disorder with X-linked partially dominant inheritance (All other Urea Cycle Disorders are Autosomal Recessive)
  • Urea cycle disorder with Orotic Aciduria
  • Marked elevations of plasma concentrations of glutamine and alanine with low levels of citrulline and arginine
  • Orotate may precipitate in urine as a pink colored gravel or stones.
48. c. Both
The pathways that take place in two compartements are
  • Heme synthesis
  • Urea cycle
  • Gluconeogenesis
49. a. c, d, e. Glutamate dehydrogenase, α Ketoglutarate dehydrogenase, Isocitrate dehydrogenase, Fumarase
(Ref: Harper 30/e p276,277)
  • Glutamate Dehydrogenase-Oxidative deamination
  • Arginino Succinate Synthetase-Urea Cycle
  • Alpha Keto Glutarate Dehydrogenase & Isocitrtate Dehydrogenase-TCA Cycle
  • Fumarase-TCA Cycle
50. d. ADP
(Ref: Harper 30/e p291)
Glutamate Dehydrogenase (GDH)
  • Liver Glutamate Dehydrogenase (GDH) is allosterically inhibited by ATP, GTP, NADH.
  • Liver Glutamate Dehydrogenase (GDH) is allosterically activated by ADP
  • Reversible reaction but strongly favour Glutamate formation
  • Can use either NAD+ or NADP+.
51. a. Ammonium and aspartate
(Ref: Harper 30/e p293)
  • First nitrogen by Ammonium ion-by thr reaction CPS-I
  • Second nitrogen by Aspartate-by the reaction ArgininoSuccinate Synthetase
52. b. Ornithine transcarbamoylase
(Ref: Nelson 20/e Defects in Metabolism of Amino Acids ‘p672′)
Flow chart to diagnose Urea Cycle disorders
zoom view
53
In the given case clue to diagnosis are
  1. High Glutamine- Usually seen in hyperammonemia. Because glutamine is the transport form of ammonia from brain and most other tissues. So in hyperammonemia Glutamine level is elevated.
  2. Increased uracil in urine can be seen in Ornithine Transcarbamoylase defect because as OTC defective, carbamoyl phosphate in mitochondria spills to cytoplasm. Then it enter in to Pyrimidine synthesis. Pyrimidine intermediates and pyrimidines can accumulate. Hence Uracil in urine.
53. b, c, e. First 2 steps in mitochondria, Defect of enzyme of any step can cause deficiency disease, Citruline is formed by combination of carbomoyl phosphate and L ornithine
Urea Cycle
  • First two steps in mitochondria, rest three steps in the cytoplasm.
  • Ornthine condenses with Carbamoyl Phosphate to form citrulline by the action of the enzyme OTC.
  • Disorder is associated with all the steps of urea cycle disorders
Disorder
Enzyme Defective
Urea Cycle Disorders Due to Enzyme Deficiency
Disorder
Enzyme Defective
Hyper ammonemia type I
Carbamoyl Phosphate Synthetase I (CPS-I)
Hyperammonemia type-II
Ornithine Transcarbamoylase (OTC)
Citrullinemia type I (Classic Citrullinemia)
Argino succinate synthetase
Arginosuccinic aciduria
Arginosuccinate lyase
Hyperargininemia
Arginase
Urea Cycle Disorders due to Transporter Defect
Citrullinemia type II
Citrin (Transport Aspartate & Glutamate) Defect
Hyperammonemia Hyperornithinemia Homocitrllinuria (HHH) Syndrome
Ornithine Transporter Defect
54. b. Multiple carboxylase deficiency
(Ref: Nelson 20/e Defects in Amino Acid Metabolism)
zoom view
55. c, d, e. On consumption of high amount of protein, excess urea is formed, Occur mainly in cytoplasm, Synthesis of argininosuccinate consumes energy
  • Nitrogen of urea comes from Ammonia and Aspartate
  • ATP is required for CPS-I and ArgininoSuccinate Synthetase
  • Out of the 5 reactions, 3 reactions occur in cytoplasm. So occur mainly in the cytoplasm.
  • On consumption of high protein, urea synthesis is increased.
56. b, d. Rate limiting enzyme is ornithine transcarbamoylase, Malate is a byproduct of urea cycle
  • Nitrogen of Urea are contributed by Ammonia and Aspartate.
  • 3ATPs are directly required for urea cycle
  • Rate limiting step is Carbamoyl Phosphate Synthase-I
  • Fumarate is a byproduct of urea cycle.
57. d. Amino acid dehydrases54
 
Some Examples of Nonoxidative DeaminationQ Recent Question
  1. Amino acid Dehydrases for amino acids with hydroxyl group (Serine, Threonine)
  2. Histidase for histidine
  3. Amino acid Desulfhydrases for amino acids with sulfhydryl group, Cysteine & Homocysteine
58. c. Alanine-leucine
Conservative mutation means an amino acid replaced by another amino acid of same characteristics.
Glutamic acid-negatively Charged Polar
Glutamine-Uncharged Polar
Histidine-Positively Charged Polar
Glycine-Uncharged, nonpolar
Alanine-Uncharged Nonpolar
Leucine-Uncharged Nonpolar
Arginine-Positively Charged Polar
Aspartic Acid-Negatively charged Polar
 
INDIVIDUAL AMINO ACID METABOLISM
 
Aromatic Amino Acids
59. b. Tyrosine
60. a. Tryptophan
61. a. NTBC
62. b. Xanthurenic acid
  • Urinary metabolite in Vitamin B6 deficiency-Xanthurenic acid
  • Urinary Metabolite in Folic Acid Deficiency-Formimino Glutamic acid, Homocystine
  • Urinary metabolite in Vitamin B12 deficiency-Homocystine, Methyl Malonic Acid
63. c. Tyrosine
Metabolic products formed from Tyrosine are
  • Melanin
  • Thyroxine
  • Catecholamines (Dopamine, Epinephrine, Norepinephrine)
Albinism is due to-Defect in Tyrosinase
64. c. Limiting the substrate for deficient enzyme
(Ref: Nelson 20/e p638, Defects inMetabolism of Amino Acids)
  • The primary goal of therapy is to reduce phenylalanine levels in the plasma and brain.
Treatment of Classical PKU
  • A low-phenylalanine diet
  • Administration of large neutral amino acids (LNAAs) is another approach to diet therapy.
  • Sapropterin dihydrochloride (Kuvan), a synthetic form of BH4, which acts as a cofactor in patients with residual PAH activity, is approved by the FDA to reduce phenylalanine levels in PKU.
Preliminary trials with recombinant phenylalanine ammonia lyase have been encouraging and demonstrated reduced blood levels of phenylalanine during treatment.
65. b. Alkaptonuria
(Ref: Nelson 20/e p 642, Defects inMetabolism of Amino Acids)
Alkaptonuria
  • Autosomal Recessive Disorder Is due to a deficiency of Homogentisic Acid Oxidase
  • 1st inborn error detected.
  • Belongs to Garrod'sTetrad [Alkaptonuria, Albinism, Pentosuria, Cystinuria]
Biochemical Defect
Homogentisate Oxidase deficiency leads to accumulation of Homogentisic Acid (Homogentisate) which polymerises to form Alkaptone bodies.
Clinical Presentation
  • Normal Life till 3rd or 4th decade.
  • Urine Darkens on standing is the only manifestation in children.
  • In adults Ochronosis-Alkaptone Bodies in Intervertebral Disc, cartilage of nose, pinna, etc.
Laboratory Diagnosis
  • Alkalanisation increase darkening of urine.
  • Benedicts test positive in urine because homogentisic acid is reducing agent.
  • Ferric Chloride test positive
  • Silver Nitrate Test positive.
  • No Mental Retardation
Treatment
  • New Drug is Nitisinone [NTBC] which inhibit para Hydroxyl Phenyl Pyruvate hydroxylase which prevent the accumulation of homogentisic acid.
  • Symptomatic Treatment
66. a. Dopamine → norepinephrine
(Ref: Harper 30/e p320)
Conversion of Tyrosine to Epinephrine involves 4 sequential steps
  1. Ring hydroxylation
  2. Decarboxylation
  3. Side chain hydroxylation.
  4. N-Methylation
67. b. Fumaryl aceto acetate hydrolase
(Ref: Nelson 20/e Defects in Metabolism of Amino Acids)
Amino acidurias and enzyme defect
Classic Phenyl Ketonuria
Phenyl Alanine Hydroxylase
Alkaptonuria
Homogentisate Oxidase
Tyrosinemia Type I
Fumaryl Aceto Acetate Hydrolase
Tyrosinemia Type II
Tyrosine Transaminase
Tyrosinemia Type III
Para Hydroxy Phenyl Pyruvate hydroxylase/Para hydroxyl Phenyl Pyruvate Dioxygenase55
Hawkinsinuria
Para Hydroxy Phenyl Pyruvate hydroxylase/Para hydroxyl Phenyl Pyruvate Dioxygenase is mutant, so that it catalyse only partial reaction.
Segawa Syndrome
GTP Cyclohydrolase
Albinism
Tyrosinase
68. a, b. Fumarate, Acetyl CoA
(Ref: Harper 30/e p304)
  • Terminal end products of Phenyl Alanine and Tyrosine metabolism is Fumarate, acetate and Acetyl CoA
Amino Acid
Terminal end products
Asparagine, Aspartate
Oxaloacetate
Glutamine, Glutamate
α KetoGlutarate
Proline
α KetoGlutarate
Arginine, Ornithine
α KetoGlutarate
Histidine
α KetoGlutarate
Glycine, Serine
CO2, NH3, N5N10 Methylene THFA or Pyruvate
Alanine
Pyruvate
Threonine
Glycine, Acetaldehyde
Methionine
Cysteine, Succinyl CoA
Cysteine
Pyruvate, 3 Mercaptolactate
Phenyl Alanine, Tyrosine
Fumarate, Acetyl CoA, Acetate
Tryptophan
Acetyl CoA
Leucine
Acetoacetate, Acetyl - CoA
Isoleucine
Acetyl CoA, Succinyl CoA
Valine
Succinyl CoA, β Aminoisobutyrate
69. a. Tyrosinase
(Ref: Nelson 20/e Defects inMetabolism of Amino Acids)
Aminoaciduria
Enzyme deficiency
Albinism
Tyrosinase
Phenyl Ketonuria
Phenyl alanine hydroxylase
Alkaptonuria
Homogentisate 0xidase
Homocystinuria
Cystathionine Beta Synthase
Maple syrup Urine Disease
Branched Chain ketoacid Dehydrogenase
70. b. Phenyl acetate
(Ref: Nelson 20/e p 637, Defects in Metabolism of Amino Acids)
Clinical Manifestations of PKU
  • The affected infant is normal at birth. Profound mental retardation develops gradually if the infant remains untreated. Cognitive delay may not be evident for the 1st few months.
  • Vomiting, sometimes severe enough to be misdiagnosed as pyloric stenosis, may be an early symptom.
  • The infants are lighter in their complexion than unaffected siblings.
  • Some may have a seborrheic or eczematoid rash, which is usually mild and disappears as the child grows older.
  • These children have an unpleasant odor of phenylacetic acid, which has been described as musty or mousey.
  • Neurologic signs include seizures (≈25%), spasticity, hyperreflexia, and tremors; more than 50% have electroencephalographic abnormalities.
  • Microcephaly, prominent maxillae with widely spaced teeth, enamel hypoplasia, and growth retardation are other common findings in untreated children.
71. b. Tryptophan
  • Tryptophan can be converted to Niacin.
  • The rate limiting enzyme in Niacin synthesis is Quinolinate Phosphoribosyl Transferase (QPRTase)
Specialised products of Tryptophan
  • Serotonin (5 Hydroxy Tryptamine)
  • Melatonin
  • Niacin
72. d. Tyrosine
Specialised products of Tyrosine are
Melanin, Thyroxine, Catecholamines (Dopamine, Epinephrine, Norepinephrine)
73. d. Hepatic carcinoma
(Ref: Nelson 20/e p 640,genetic disorders of metabolism)
Tyrosinemia Type I (Tyrosinosis, Hereditary Tyrosinemia, Hepatorenal Tyrosinemia)
Clinical Manifestations of Tyrosinemia Type I
  • Untreated, the affected infant appears normal at birth and typically presents between 2 and 6 month of age
  • An acute hepatic crisis commonly heralds the onset of the disease and is usually precipitated by an intercurrent illness that produces a catabolic state. Fever, irritability, vomiting, hemorrhage, hepatomegaly, jaundice, elevated levels of serum transaminases, and hypoglycemia are common. An odor resembling boiled cabbage may be present, due to increased methionine metabolites. Cirrhosis and eventually hepatocellular carcinoma occur with increasing age. Carcinoma is unusual before 2 years of age.
  • Episodes of acute peripheral neuropathy resembling acute porphyria occur in ≈40% of affected children. These crises, often triggered by a minor infection, are characterized by severe pain, often in the legs, associated with hypertonic posturing of the head and trunk, vomiting, paralytic ileus, and, occasionally, self-induced injuries of the tongue or buccal mucosa.
  • Renal involvement is manifested as a Fanconi-like syndrome with normal anion gap metabolic acidosis, 56hyperphosphaturia, hypophosphatemia, and vitamin D-resistant rickets. Nephromegaly and nephrocalcinosis may be present on ultrasound examination.
  • Hypertrophic cardiomyopathy and hyperinsulinism are seen in some infants.
74. a, b, c. Diarrhoea, Vasoconstriction, Flushing
Actions of Serotonin (Metabolite of Tryptophan) are
  • Neurotransmitter in the Brain
  • Mood Elevation
  • GI Motility
  • Temp Regulation
  • Cutaneous flushing
75. a, b. Phenyl ketonuria-Mousy body odour, Tyrosinemia- rotten cabbage
Peculiar odours in different Amino acidurias
Inborn Error of Metabolism
Urine Odor
Glutaricacidemia (type II)
Sweaty feet, acrid
Hawkinsinuria
Swimming Pool
Isovaleric Acidemia
Sweaty feet, Acrid
3-Hydroxy-3-methylglutaric aciduria
Cat urine
Maple syrup urine disease
Maple syrup
Hypermethioninemia
Boiled cabbage
Multiple carboxylase deficiency
Tomcat urine
Oasthouse urine disease
Hops-like
Phenylketonuria
Mousey or musty
Trimethylaminuria
Rotting fish
Tyrosinemia
Boiled cabbage, rancid butter
76. a, b, e. Dietary phenyl alanine restriction is used as a treatment, Occur due to deficiency of Phenyl Alanine Hydroxylase, Tyrosine should be supplied in the diet.
  • Phenyl Ketonuria is due to deficiency of Phenyl Alanine Hydroxylase
  • Dietary restriction of Phenyl Alanine with supplementation of Tyrosine is needed as Tyrosine is a nonessential amino acid synthesized from Phenyl Alanine by the action of Phenyl Alanine Hydroxylase.
 
Simple Amino Acids
77. a. Glycine
78. d. Optically inactive
(Ref Harper 30/e p19)
  • Glycine is the only optically inactive amino acid.
  • Sulphur containing amino acids are Cysteine and Methionine
  • Guanidinium group is present in Arginine.
  • Glycine is a nonessential amino acid
79. b. Aspartate
(Ref: Harper 30/e p283)
Biosynthesis of Glycine
  • Glycine Amino Transferase catalyse the synthesis of Glycine from Glyoxylate, Glutamate and AlanineQ
  • From Serine by Serine Hydroxy Methyl Transferase. This is a reversible reaction
    zoom view
  • By Glycine Synthase System in Invertebrates
  • From Threonine by Threonine Aldolase
80. a. Glycine dehydrogenase
(Ref: Harper 30/e p302)
Glycine Cleavage system consists of three enzymes and an H protein that has covalently attached Dihyrolipoyl moiety. The three enzymes are:
  1. Glycine dehydrogenase
  2. Amino methyl transferase
  3. Dihydrolipomide dehydrogenase
81. a. Kidney; Arginine + Glycine
Steps of synthesis of Creatinine
Step I Glycine Arginine Amido Transferase
  • First step in the Kidney.
  • Guanidino group of Arginine is transferred to Glycine to form Guanidino Acetic Acid.
Step II Guanidino Acetate Methyl Transferase
  • Second step in the Liver
  • Creatine is formed
  • S Adenosyl Methionine is the methyl donor
Step III Creatine Kinase
  • Third step in the muscle
  • Creatine phosphate is formed
Step IV
  • Occur spontaneously
  • Creatinine is formed.
82. a, e. Folic acid, Pyridoxal phosphate
  • Glycine is converted to Serine by Serine Hydroxy methyl Transferase
  • Coenzymes required are Folic acid, Pyridoxal Phosphate
83. b. Sarcosine
Sarcosine
N Methyl Glycine
Betaine
Trimethyl Glycine
Choline
Trimethyl Ethanolamine
Ethanolamine
Serine on decarboxylation
Ergothionine
Derivative of Histidine
Betamercaptoethanolamine
Cysteine on decarboxylation
Carnosine
Beta Alanyl Histidine57
Anserine
Carnosine on Methylation
Homo Carnosine
GABA + Histidine
Serotonin
5 Hydroxy Tryptamine
84. c. D Glycerate dehydrogenase
Primary Hyperoxaluria Type I
  • The most common form of Primary hyperoxaluria.
  • It is due to a deficiency of the peroxisomal enzyme alanine-glyoxylate aminotransferase, (expressed only in the liver peroxisomes and requires pyridoxine (vitamin B6) as its cofactor)
  • Protein targeting defect.
Primary Hyperoxaluria Type II (Glyceric Aciduria)
  • Due to a deficiency of D-glycerate dehydrogenase (glyoxylate reductase enzyme complex)
Secondary Hyperoxaluria
  • Pyridoxine deficiency (cofactor for alanine-glyoxylate aminotransferase)
  • After ingestion of ethylene glycol
  • High doses of vitamin C
  • After administration of the anesthetic agent methoxyflurane (which oxidizes directly to oxalic acid)
  • In patients with inflammatory bowel disease or extensive resection of the bowel (enteric hyperoxaluria).
Nonketotic Hyper Glycinemia
  1. Due to a defect in the Glycine Cleavage System
85. b. It converts hemoglobin to methemoglobin
(Ref: Harper 30/e 23)
Functions of glutathione
Glutathione is a tripeptide
  • Free radical scavenging
  • Transport of Amino acid across cell membrane
  • Keep iron in the ferrous sate, so prevent methHb formation.
  • Act as coenzyme for certain enzymes.
Phase II Xenobiotic reaction in Conjugation
 
Sulphur Containing Amino Acids
86. a. Glutathione
(Ref: Harper 30/e p23)
87. c. Methyl cobalmine
88. c. Introduction of sulphur in methionine
Methionine is an essential amino acid, so it cannot be synthesised from Cysteine.
  • But sulphur of cysteine is donated by sulphur of methionine.
  • This is called transsulfuration reaction.
  • PLP is the coenzyme of transulfuration.
  • The reaction is catalysed by cystathionine beta Synthase and cystathionase enzyme.
89. a. Glutathione
  • The active part of both Glutathione and N Acetyl Cysteine is Sulfhydryl group of Cysteine. So N-Acetyl Cysteine replenishes Glutathione.
90. a, d, e. Contain sulfhydral group, Transport amino acid across cell membrane, Part of enzymes
Functions of glutathione
  • Free radical scavenging
  • Transport of Amino acid across cell membrane
  • Keep iron in the ferrous sate, so prevent methHb formation.
  • Act as coenzyme for certain enzymes.
91. c. Cysteine
(Ref: Harper 30/e p23)
  • Glutathione is a tripeptide (Gamma Glutamic acid + Cysteine + Glycine
  • Gamma glutamyl Cysteinyl Glycine
  • Atypical peptide bond is present between Gamma Glutamic acid and cysteine.
  • -SH (Sulhydryl) group of cysteine is the active part of glutathione.
 
Acidic and Basic Amino Acids
92. a. Histidine
Histidine is decarboxylated to Histamine, which is a vasodilator
93. a. Arginine
(Ref: Harper 30/e p661)
Synthesis of Nitric Oxide
zoom view
94. a. Folate deficiency
(Ref: Harper 30/e p299)
Important Points of the histidine metabolism pathway
  • Urocanate is a derivative of Histidine.
  • FIGLU is Formimino Glutamic Acid
  • FIGLU is derived from Histidine.
  • In Folic Acid deficiency FIGLU is excreted in Urine.
Histidine Load Test
  • To identify Folic Acid Deficiency.
  • FIGLU excreted in urine is measured following a Histidine load.
95. d. Acts through cAMP
(Ref: Harper 30/e p661)
  • Nitric Oxide acts through cGMP
  • Formed from Arginine
  • iNOS, eNOS, nNOS are three isoforms of Nitric Oxide Synthase58
Nitric Oxide
Uncharged molecule having an unpaired electron, so it is highly reactive, free radical.
  • Very short half life (0.1 seconds)
  • Formerly called Endothelium Derived Relaxing Factor.
  • Gaseous molecule.
  • Second messenger is cGMP.
Functions of Nitric Oxide
  • Potent Vasodilator.
  • Involved in Penile erction
  • Neurotransmitter in brain and Peripheral Nervous System.
  • Low level of NO involved in Pylorospasm in Congenital Hypertrophic Pyloric Stenosis.
  • Inhibit adhesion, activation and aggregation of Platelets.
96. a. Arginine
(Ref: Harper 30/e p320)
Three amino acids from which Creatine and creatinine is synthesized are
  1. Glycine
  2. Arginine
  3. Methionine
97. c. Decarboxylation
  • Amino acid is converted to ketoacid by Deamination or transamination.
  • Amino acid converted to biological amines by decarboxylation.
Amino acid
Biologic Amines
Histidine
Histamine
Tyrosine
Tyramine
Tryptophan
Tryptamine
Lysine
Cadaverine
Glutamic AcidQ
Gamma Amino Butyric Acid (GABA)
Serine
Ethanolamine
Cysteine
Betamercapto Ethanolamine
 
Branched Chain Amino Acid
98. a, c. Histidine, Leucine
99. a. Maple syrup urine disease
(Ref Harper 30th 276-278)
Maple Syrup Urine Disease
Biochemical Defect
  • Deficiency of the enzyme Branched Chain Ketoacid Dehydrogenase.
  • Defective reaction is Defective Decarboxylation.
Clinical Features
  • Mental Retardation
  • Convulsion
  • Acidosis, Coma
  • Smell of Burnt Sugar [Maple Syrup]
Tests for MSUD
  • Di Nitro Phenyl Hydrazine Test (DNPH Test)
  • Rothera's Test
  • Enzyme Analysis
Treatment
  • Restrict Branched Chain Amino Acid
  • Give high doses Thiamine.
100. a. E1 α
(Ref: Nelson 20/e Defects in Metabolism of Amino Acids, Harper 30/e p311)
  • Types of MSUDQ
Gene
Component
MSUD Types
E1α
Branched Chain α Keto acid decarboxylase (contains TPP)
Type IA MSUD
E1β
Branched Chain α Keto acid decarboxylase
Type IB MSUD
E2
Dihydrolipoyl Transacylase (contains Lipomide)
Type II MSUD
E3
Dihydrolipomide Dehydrogenase (Contains FAD)
Type III MSUD
101. a. Xanthurenate
(Ref: Harper 30/e p309)
  • Xanthurenate is formed from Tryptophan if Kynureninase enzyme is defective.
102. c. Glycine
(Ref: Nelson's Text Book of Pediatrics 20/e Chapter 79.6)
Treatment of IsovalericAcidemia
Hydration
Reversal of the catabolic state (by providing adequate calories orally or intravenously), correction of metabolic acidosis (by infusing sodium bicarbonate)
Removal of the excess isovaleric acid.
By Administering Glycine
Because isovalerylglycine has a high urinary clearance, administration of glycine (250 mg/kg/24 hr) is recommended to enhance formation of isovalerylglycine.
By administering L-Carnitine
L-carnitine (100 mg/kg/24 hr orally) also increases removal of isovaleric acid by forming is ovalerylcarnitine, which is excreted in the urine.
103. c. Leucine
Lab Diagnosis of MSUD
  • Plasma shows marked elevation of leucine, isoleucine, valine, and alloisoleucine (a stereoisomer of isoleucine not normally found in blood)
  • Urine contains high levels of leucine, isoleucine, and valine and their respective ketoacids
104. b, d. Maple syrup urine disease, Isovaleric acidemia
  • Phenyl Ketonuria associated with Aromatic Amino acid59
  • Taysach's Disease and Niemann Pick disease are Sphingolipidoses
 
Other Amino Acids and Entry of Amino Acid to TCA Cycle
105. b. Glutamate
(Ref: Harper 30/e p284)
Proline
The initial reaction of proline biosynthesis converts glutamate to the mixed acid anhydride of glutamate phosphate). Subsequent reduction forms glutamate semialdehyde, which following spontaneous cyclization is reduced to L-proline
106. c. Glutamine
(Ref: Harper 30/e p267)
Asparagine Synthetase
  • Asparagine Synthetase is analogous to Glutamine Synthetase
  • In Asparagine Synthetase, Glutamine rather than ammonium ions, provides nitrogen.
  • Hence cannot fix ammonia like Glutamine Synthetase.
  • Bacterial Asparagine Synthetase can however, also use ammonium ion
107. d. Aspartate and asparagine
(Ref: Harper 30/e p298)
(See Figure Next Page)
108. b. Selenocysteine
(Ref: Harper 30/e p16)
Selenocysteine is seen in the active site of following Enzymes and ProteinsQ
  • Thioredoxin reductase
  • Glutathione peroxidase
  • Iodothyronine deiodinase
  • Selenoprotein P
109. b. Asparagine and aspartate
(Ref: Harper 30/e p299)
  • Asparagine and Aspartate forms Oxaloacetate
  • Glutamine and Glutamate forms alpha Ketoglutarate.
110. d. Glutaric acidemia
(Ref: Nelson's Textbook of Pediatrics 20/e p635 Table 84.3)
Peculiar odours in different Amino acidurias
Inborn Error of Metabolism
Urine Odor
Glutaricacidemia (type II)
Sweaty feet, acrid
Hawkinsinuria
Swimming Pool
Isovaleric Acidemia
Sweaty feet, Acrid
3-Hydroxy-3-methylglutaric aciduria
Cat urine
Maple syrup urine disease
Maple syrup
Hypermethioninemia
Boiled cabbage
Multiple carboxylase deficiency
Tomcat urine
Oasthouse urine disease
Hops-like
Phenylketonuria
Mousey or musty
Trimethylaminuria
Rotting fish
Tyrosinemia
Boiled cabbage, rancid butter
111. b. Ascorbic acid
  • Hdroxylation of Proline and Lysine
  • Enzyme: Prolyl and Lysyl Hydroxylase
  • Coenzyme is Vitamin C
112. c. Valine
  • Succinyl CoA is formed by Valine, Leucine, Methionine
113. b. Beta carbon atom
(Ref: Harper 30/e p284)
zoom view
Enzyme is Serine Hydroxy Methyl Transferase
Serine loses the beta Carbon atom to form Glycine and Methylene THFA.60
zoom view
// Answers to Image-Based Questions //
1. c. Proline
Identifying feature alpha NH2 group is incorporated in the pyrrolidine ring.
2. c. Nonpolar aromatic
The given amino acid is tryptophan. So its nonpolar and Aromatic.
3. c. Phenyl ketonuria
Identifying features:
  • Blue eyes, blonde hair and fair skin is a classical desctription of a case of PKU. The child is intellectually disabled and they have mousy body odour.
4. c. Ochronosis
Identifying features are:
  • Age of presentation
  • No intellectual disability
  • Back ache because of deposition of alkaptone bodies in IV disc
  • Blackish discoloration in eyes, pinna and other cartilaginous tissues
  • Blackish discoloration of urine.
5. c. Tyrosinase
The case given in picture is Albinism. The enzyme defect is Tyrosinase.
6. b. Homocystinuria
The given picture shows pectus carinatum. Elongated limbs and dislocated lens. All these features fit in to diagnosis of Homocystinuria.
7. c. Branched chain keto acid dehydrogenase
Identifying features are early age of ptesentation with failure to thrive, convulsion, hypotonia.
Smell of burnt sugar or caramel odour.
Positive DNPH test.