Histidine

(CC) Image: David E. Volk
Histidine, a common amino acid.

Histidine, abbreviated His or H, is one of the twenty common $\alpha$-amino acids used by living organisms to build proteins. It is one of the four aromatic amino acids along with phenylalanine, tyrosine and tryptophan. At physioligical pH, histidine is usually protonated and is found mostly on the surface of proteins.