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Histidine is one of the 20 most common natural amino acids, coded for in DNA. Nutritionally, in humans, histidine is considered an essential amino acid, but mostly only in children. The imidazole side chains of histidine and the relatively neutral pK (ca 6.0) mean that relatively small shifts in cellular pH will change its charge. For this reason, this amino acid side chain finds its way into considerable use as a co-ordinating ligand in metalloproteins, and also as a catalytic site in certain enzymes. The imidazole side chain has two nitrogens with different properties, one is bound to hydrogen and donates its lone pair to the aromatic ring and as such is slighty acidic, while the other one donates only one electron to the ring so it has a free lone pair and is basic. These properties are exploited in different ways in proteins. In catalytic triad s, the basic nitrogen of histidine is used to abstract a proton from serine, threonine or cysteineCysteine is a naturally occurring hydrophilic ("water loving") amino acid which has a sulfhydryl ( S H) group and is found in most proteins. When it is exposed to air it oxidizes to form cystine which is two cysteine molecules joined by a disulfide bond. to activate it as a nucleophileIn chemistry, a nucleophile (literally nucleus lover is a reagent which is attracted to centres of positive charge. A nucleophile participates in a chemical reaction by donating electrons to a species known as an electrophile in order to form a chemical b. In a histidine proton shuttle, histidine is used to quickly shuttle protons, it can do this by abstracting a proton with its basic nitrogen to make a positively charged intermediate and then use another molecule, a buffer, to extract the proton from its acidic nitrogen. In carbonic anhydraseCarbonic anhydrase (EC 4. 1) is an enzyme that assists the rapid interconversion of carbon dioxide and water into carbonic acid, protons, and bicarbonate ions. It increases the efficiency of the reaction about a million-fold. Its active site contains a zis, a histidine proton shuttle is utilized to rapidly shuttle protons away from a zinc-bound water molecule to quickly regenerate the active form of the enzyme.

The amino acid is a precursor for histamineHistamine is a monoamine chemical involved in local immune responses. Chemical properties Chemically, histamine is 2-(4-imidazolyl)ethylamine and has the formula C H N. Histamine is synthesized by the decarboxylation of the amino acid histidine, a reactio biosynthesis.