Histidiini

Histidiini, known in English as histidine, is one of the 20 standard amino acids used to build proteins. It is classified as an essential amino acid in humans, meaning it must be obtained from the diet. It features an imidazole side chain that can accept or donate a proton near physiological pH, giving histidine a distinctive buffering capacity and an important role in enzyme catalysis and metal ion binding. The imidazole group has a pKa around 6.0, allowing histidine to participate in proton transfer within protein active sites.

Histidine exists both free in body fluids and as a component of proteins. It is relatively well represented in various enzymes and structural proteins and often participates in catalytic functions, ligand coordination, and stabilization of protein structures. In physiology, histidine is also the immediate precursor to histamine, a signaling molecule involved in immune responses, gastric acid secretion, and neurotransmission.

Humans cannot synthesize histidine, so dietary intake is essential for growth and tissue repair. Typical dietary sources include meat, fish, eggs, dairy products, legumes, nuts, and whole grains, providing histidine alongside other essential amino acids in balanced amounts.

Biochemically, histidine can be decarboxylated by histidine decarboxylase to form histamine. Histamine is then degraded by histamine-N-methyltransferase and diamine oxidase. In proteins, histidine residues contribute to catalytic activity and can coordinate metal ions, highlighting the versatile functional roles of this amino acid in biology.