Histidine: Functions and Metabolism

Amino Acid Metabolism

Histidine

• Essential, glucogenic amino acid
• Functions:
  • Can be decarboxylated to form histamine, a vasodilator and a key substance in allergic diseases
  • Combines with β-alanine to form carnosine and anserine, which have antioxidant properties and are present in vertebrate muscles
  • Can give glutamic acid through the formation of urocanic acid and formimino glutamic acid as intermediate compounds
  • Present in blood as ergothionine, a reducing substance found in RBCs

Proline

• Non-essential, glucogenic amino acid
• Functions:
  • Can be hydroxylated to form hydroxyproline, a key component of collagen synthesis, with the help of vitamin C
  • Formed from glutamic acid through the formation of glutamic semialdehyde and pyroline carboxylic acid

Glutamic Acid

• Non-essential, glucogenic amino acid
• Functions:
  • Enters into the formation of γ-Aminobutyric acid (GABA), a key compound in the brain, through decarboxylation
  • Synthesizes glutathione, a tripeptide formed with cysteine and glycine, which is a tissue respiratory enzyme
  • Involved in the synthesis of folic acid (B10) and arginine
  • Used in detoxication of ammonia, forming glutamine which is then converted back to glutamic acid and ammonia

Glutamine

• Non-essential amino acid
• Functions:
  • Used to detoxicate phenyl acetic acid
  • Involved in the synthesis of purines and pyrimidines, donating nitrogen atoms to the ring structures
  • Used in detoxication of ammonia, forming ammonia and glutamic acid again

Aspartic Acid and Asparagine

• Non-essential, glucogenic amino acid
• Functions:
  • Decarboxylated to form β-alanine, used in the synthesis of carnosine and anserine
  • Involved in the synthesis of purines and pyrimidines, donating nitrogen atoms to the ring structures
  • Can be converted to asparagine by adding ammonia, which can then be removed through deamidation

Arginine

• Semi-essential, glucogenic amino acid
• Functions:
  • Involved in the synthesis of urea in the liver
  • Used in the synthesis of creatine and creatine phosphate, along with glycine and methionine
  • Can give glutamic acid and proline through the formation of glutamic semialdehyde

Lysine

• Essential amino acid
• Functions:
  • Hydroxylated to form hydroxylysine in collagen synthesis
  • Can give ammonia, but cannot be reanimated by ammonia
  • Combines with biotin to form biocytin, a coenzyme for certain carboxylases