Test: Chemistry and Metabolism of Amino Acids- 4 - NEET PG MCQ

• Functions of glutathione include:
• Scavenging free radicals
• Facilitating the transport of amino acids across cell membranes
• Maintaining iron in the ferrous state, thereby preventing methaemoglobin formation
• Acting as a coenzyme for specific enzymes

Glutathione is a tripeptide composed of gamma-glutamic acid, cysteine, and glycine.

An unusual peptide bond exists between gamma-glutamic acid and cysteine.

Synthesis of Nitric Oxide I

• Urocanate is a metabolite of histidine.
• FIGLU, or Formiminoglutamic acid, is produced from histidine.
• In cases of folic acid deficiency, FIGLU is expelled in urine.
• The histidine load test is used to detect folic acid deficiency.
• Levels of FIGLU excreted in urine are assessed after administering a histidine load.

Nitric Oxide operates via cGMP, which is synthesised from Arginine. The three isoforms of Nitric Oxide Synthase include iNOS, eNOS, and nNOS.

Nitric Oxide is a neutral molecule characterised by an unpaired electron, making it a highly reactive free radical. It has an extremely short half-life of approximately 0.1 seconds and was previously referred to as Endothelium Derived Relaxing Factor. This gaseous molecule acts as a second messenger through cGMP.

• Functions of Nitric Oxide:
• Powerful vasodilator.
• Plays a role in penile erection.
• Acts as a neurotransmitter in both the brain and peripheral nervous system.
• Low levels of NO are associated with pylorospasm in congenital hypertrophic pyloric stenosis.
• Inhibits the adhesion, activation, and aggregation of platelets.

Three amino acids involved in the synthesis of creatine and creatinine are:

• Glycine
• Arginine
• Methionine

Amino acids are transformed into ketoacids through either deamination or transamination. Amino acids are also converted into biological amines via decarboxylation.

Maple Syrup Urine Disease is characterised by a biochemical defect resulting from a deficiency in the enzyme Branched Chain Ketoacid Dehydrogenase. This leads to a malfunction in the decarboxylation process.

• Clinical features include:
• Mental retardation
• Convulsions
• Acidosis and coma
• Distinctive smell of burnt sugar (maple syrup)

Tests for MSUD encompass:

• Di Nitro Phenyl Hydrazine Test (DNPH Test)
• Rothera's Test
• Enzyme analysis

Treatment involves:

• Restricting branched chain amino acids
• Administering high doses of thiamine

Xanthurenate is produced from Tryptophan when the Kynureninase enzyme is impaired.

Treatment of Isovaleric Acidemia involves several steps:

• Reversing the catabolic state by supplying sufficient calories either orally or via intravenous methods.
• Correcting metabolic acidosis through the infusion of sodium bicarbonate.
• Eliminating the excess isovaleric acid.

To achieve this:

• Administering Glycine: Since isovalerylglycine is rapidly cleared in urine, it is advisable to provide glycine at a dosage of 250 mg/kg/24 hr to promote the synthesis of isovalerylglycine.
• Administering L-Carnitine: L-carnitine at a dosage of 100 mg/kg/24 hr, taken orally, further aids in the removal of isovaleric acid by converting it into isovalerylcarnitine, which is then excreted through urine.

Lab diagnosis of MSUD reveals a significant increase in:

• Leucine
• Isoleucine
• Valine
• Alloisoleucine (a stereoisomer of isoleucine not typically present in blood)

The urine also exhibits elevated levels of:

• Leucine
• Isoleucine
• Valine
• And their corresponding ketoacids

Phenylketonuria linked to aromatic amino acid, along with Tay-Sachs disease and Niemann-Pick disease, falls under the category of sphingolipidoses.

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

Selenocysteine is present in the active sites of the following enzymes and proteins:

• Thioredoxin reductase
• Glutathione peroxidase
• Iodothyronine deiodinase
• Selenoprotein P

Asparagine and aspartate convert into oxaloacetate. Glutamine and glutamate transform into alpha-ketoglutarate.

Peculiar odors in different Amino acidurias

The hydroxylation of proline and lysine is facilitated by specific enzymes known as prolyl and lysyl hydroxylases. These enzymes require vitamin C as a cofactor for their activity.