Histidine (commonly abbreviated to His or H)  is one of the twenty naturally-occurring amino acids. Of these amino acids it is one of ten polar and one of three positively-charged amino acids . The codons of His are CAU or CAC.
Histidine is an aromatic amino acid, which acts as an acid when protonated. When unprotonated it has a positively charged imidazole functional group and acts as a general base. It plays an important role in stabilising the folding structures of proteins and active sites of enzymes  . This ability is due to its binding properties with metals and the basic properties of the nitrogen atoms in the ring. It is only found as an L- isomer and it plays an important role in growth and repair of tissues in the body.
Berg, J. M., Tymoczko, J. L., & Stryer, L. (2002). Biochemistry (5th ed.). New York: W.H. Freeman.
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