Histidine (English: Histidine, abbreviated as His or H) is one of 20 kinds of amino acids present in the protein being the most common. In the areas of nutrition, the histidine began to be considered an essential amino acid for children, but long-term studies have shown that for adults, histidine is still an essential amino acid. An isoelectric point of 7.59, is a basic amino acid with a positive charge under physiological conditions. His synthesis of the codon for CAU and CAC.
Histidine residues - (imidazole group) have specific pK value of about 6.5, close to neutral than other protein functional groups. Histidine residues in enzyme catalysis often act on proton transfer; and demand in many enzyme reactions around neutral pH, and histamine related side chains shrink. For a link to the location of two protons and hydrogen bonding, with the imidazole ring, the contribution of its strong acid-base catalysis group, serine proteases is the most typical example. An acid group and cysteine side chains like please like and metal bonding, such as Zn2 +, Cu2 + and Fe2 +. Its nucleophilicity Imidazole group can play with carboxylic anhydride and other chemical reactions, while light oxidation and iodination sensitive, this is the modification of the body; in vitro reaction, histidine residues in the protein adsorption ability to provide protons. Imidazole group by nucleophilic attack will produce acetylimidazole. Histidine imidazole chain pKa of about 6, but overall, this amino acid has a pKa of 7.6. This represents, pH value in the physiologically relevant, its relatively small changes will change the average charge. When pH is less than 6, the chain will be protonated imidazole, like Henderson - Hassell Barr He described equation. When protonated, imidazole chain having two NH bonds and positively charged. This positive charge is equally dispersed between the two nitrogen atoms, and form two important resonance structure.