histidiner

Histidine is one of the twenty standard amino acids used by cells to build proteins. In humans, L-histidine is considered essential, particularly for infants and during periods of rapid growth; adults are often described as having a conditional requirement. As a proteinogenic amino acid, histidine is incorporated into proteins during translation according to the genetic code.

Its side chain is an imidazole ring, giving histidine the chemical name 2-amino-3-(1H-imidazol-4-yl)propanoic acid. The imidazole group has a pKa near 6.0, so it can be protonated or unprotonated near physiological pH. This makes histidine a versatile acid–base catalyst and a flexible metal-binding ligand in many enzymes. Only the L enantiomer of histidine is used in human biology.

Biologically, histidine serves several roles. It is a precursor to histamine, a signaling molecule involved in immune responses, gastric acid secretion, and neurotransmission. In metabolism, histidine is degraded via pathways that convert it to urocanate and eventually to glutamate. Disorders of histidine metabolism, such as histidinemia, can arise from enzyme deficiencies and lead to elevated histidine levels in the blood.

Dietary sources include meat, fish, eggs, dairy products, and certain cereals and legumes. Daily intake recommendations vary, but histidine is regarded as essential for growth in infants and may be needed under specific physiological conditions in adults. Within proteins, histidine residues frequently participate in enzyme active sites and in binding to metal ions, contributing to catalytic activity and structural stability.