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