histidines

Histidines refer to the amino acid histidine and the histidine residues found within proteins. Histidine is distinguished by an imidazole-containing side chain, which can act as both a hydrogen donor and acceptor. The imidazole group has a pKa near 6.0, allowing the side chain to be neutral or positively charged near physiological pH; this dual ability makes histidines useful in enzymatic catalysis, proton transfer, and pH sensing, as well as in coordinating metal ions.

Biologically, histidine is one of the 20 standard amino acids incorporated into proteins during translation. In humans, histidine is considered essential for infants but not strictly essential in adult diets, though adequate intake is important. Animals lacking the necessary biosynthetic pathways must obtain histidine from food. Plants, bacteria, and fungi synthesize histidine through the histidine biosynthesis pathway, a multistep process starting from phosphoribosyl pyrophosphate.

Functions of histidines in proteins are diverse. Histidine residues often participate in acid–base catalysis at enzyme active sites, serve as ligands for metal ions in metalloproteins, and contribute to structural stabilization through hydrogen bonding. Histidine kinases in two-component signaling systems use phosphorylatable histidine residues as part of signal transduction. The versatility of the imidazole ring also underpins buffering capacity in proteins, contributing to local pH regulation.

Clinically, a rare condition called histidinemia arises from decreased activity of histidine metabolism, leading to elevated histidine levels in blood and urine. Dietary sources of histidine include meat, fish, dairy products, and certain grains and legumes.