Histidine is an amino acid found in proteins. In mammals, histidine is an essential amino acid, meaning it must be present in the diet. Histidine's nonpolar side chain classifies it as a basic amino acid.

Histidine is the least basic of the three basic amino acids. The imidazole ring in the side chain of the free amino acid loses its proton at about pH 6. When histidine is incorporated into proteins, the pKa is raised to about 7 (Table 5.3). Becauase the histidine side chain can exchange protons near physiological pH, it often plays a role in enzymatic catalysis involving proton transfer.

The basic amino acids are strongly polar, and as a consequence, they are usually found on the exterior surfaces of proteins, where they can be hydrated by the surrounding aqueous environment.

Histidine is an allosteric inhibitor of glutamine synthetase, an enzyme with a central role in nitrogen metabolism in the cell.

One Letter Code	Three Letter Code	Molecular Wt. (Daltons)	Genetic Code Codons
H	HIS	137.15	CAU, CAC