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Fundamentals intermediate

Amino Acid Metabolism

Amino acid metabolism encompasses the pathways by which the body processes amino acids for energy, biosynthesis, and nitrogen disposal. This article covers transamination, deamination, and the urea cycle.

By Wikipept Community | 2 min read
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Essential vs. Non-Essential Amino Acids

The human body requires 20 standard amino acids for protein synthesis. These are classified based on whether the body can synthesize them:

  • Essential amino acids (9): Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine. These must come from the diet.

Mnemonic: “PVT TIM HALL” — Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine (semi-essential), Leucine, Lysine.

  • Non-essential amino acids (11): Can be synthesized endogenously from metabolic intermediates.
  • Conditionally essential: Arginine, cysteine, glutamine, tyrosine, glycine become essential during growth, illness, or stress.

Transamination

Transamination is the first major step in amino acid catabolism. Aminotransferases (transaminases) transfer the alpha-amino group from an amino acid to alpha-ketoglutarate, producing glutamate and a new alpha-keto acid.

The cofactor pyridoxal phosphate (PLP), derived from vitamin B6, is essential for this reaction. PLP acts as a temporary amino group carrier, alternating between its aldehyde form (PLP) and amine form (PMP).

Oxidative Deamination

Glutamate undergoes oxidative deamination by glutamate dehydrogenase in the mitochondrial matrix. This reaction releases free ammonia (NH4+) and regenerates alpha-ketoglutarate. The ammonia enters the urea cycle for safe disposal.

The Urea Cycle

The urea cycle converts toxic ammonia into urea, which is excreted in urine. It spans two cellular compartments:

  1. Mitochondrial matrix: Carbamoyl phosphate synthetase I combines ammonia and CO2 to form carbamoyl phosphate. Ornithine transcarbamylase combines carbamoyl phosphate with ornithine to form citrulline.
  2. Cytosol: Citrulline is converted to argininosuccinate, then to arginine, which is cleaved by arginase to yield urea and regenerate ornithine.

The cycle consumes four high-energy phosphate equivalents per urea molecule: two from ATP in carbamoyl phosphate synthesis and one from ATP to AMP (equivalent to two high-energy bonds) in argininosuccinate synthesis.

Clinical Significance

Defects in urea cycle enzymes cause hyperammonemia, a medical emergency that can lead to brain damage. Understanding amino acid metabolism is also critical for managing phenylketonuria (PKU), maple syrup urine disease, and homocystinuria.