Histidine

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