Histidine

Histidine (abbreviated as His or H; encoded by the codons CAU and CAC) is an ɑ-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –⁺NH₃ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO^- form under biological conditions), and a side chain imidazole, classifying it as a positively charged (at physiological pH), aromatic amino acid. Initially thought essential only for infants, longer-term studies shown it is essential for adults also.

Histidine was first isolated by German physician Albrecht Kossel and Sven Hedin in 1896. It is also a precursor to histamine, a vital inflammatory agent in immune responses.

Chemical properties

The conjugate acid (protonated form) of the imidazole side chain in histidine has a pK_a of approximately 6.0. This means that, at physiologically relevant pH values, relatively small shifts in pH will change its average charge. Below a pH of 6, the imidazole ring is mostly protonated as described by the Henderson–Hasselbalch equation. When protonated, the imidazole ring bears two NH bonds and has a positive charge. The positive charge is equally distributed between both nitrogens and can be represented with two equally important resonance structures.