USMLE

Histidine

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Amino Acids
  1. Serine
  2. Asparagine
  3. Alanine
  4. Valine
  5. Leucine
  6. Isoleucine
  7. Phenylalanine
  8. Tryptophan
  9. Methionine
  10. Proline
  11. Glycine
  12. Threonine
  13. Cysteine
  14. Tyrosine
  15. Glutamine
  16. Aspartate
  17. Glutamate
  18. Histidine
  19. Lysine
  20. Arginine

Summary

Histidine, which abbreviates to the 3-letter His or single-letter H, is one of the 20 amino acids that make up proteins in our body. Histidine’s R-group is an imidazole group; that is, a 5-membered pentagon ring containing two nitrogens. Due to those two nitrogens, histidine is a polar, hydrophilic amino acid. The nitrogens can also act as proton acceptors, making Histidine a basic, positively charged amino acid at physiological pH. 

Key Points

  • Histidine
    • Abbreviations
      • His, H
    • Chemical Structure
    • R-group: -CH2-Imidazole
      • Aromatic 5-membered imidazole ring
    • Polarity
      • Polar (water soluble/hydrophilic)
    • Acidity/Basicity
      • Basic
      • pKa = 6.5 (rounded)
    • Charge at pH 7
      • Positive (+)
    • Other Information
      • May act as both proton donor and acceptor at physiological pH (~7)
        • pKa is relatively close to physiological pH, with more deprotonated at pH 7
        • Proton donor/acceptor role may be utilized at active sites of enzymes