Biochemical Physiological Actions
: Because the pKa of the imidazole group is close to physiological pH, the imidazole moiety can be either uncharged or positively charged, depending on the local environment. This property makes histidine residues of exceptional interest in the active sites of many proteins, such as hemoglobin and myoglobin, where the imidazole ring can readily alternate between the charged (imidazolium) and uncharged (imidazole) states to participate in bond formation and breakage. Histidine is biosynthesized from ATP, 5-phosphoribosyl-1-pyrophosphate (PRPP), and glutamine. In turn, histidine is degraded to glutamate by histidase, urocanase, and imidazolonepropionase via the formation of urocanate, 4-imidazolone 5-propionate, and N-formiminoglutamate.