histidine

A basic amino acid with the formula C₃H₃N₂CH₂CH(NH₂)COOH. The occurrence of histidine in proteins is rare. Synthesis is via a complex pathway, starting from the condensation of ATP and 5-phosphoribosyl pyrophosphate, while breakdown is via glutamic acid. In bacteria, the synthesis of histidine has been found to be under genetic control, with histidine acting as its own repressor.