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Concept
Histidine
Summary
Histidine (symbol His or H) is an essential 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 an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, it has now been shown in longer-term studies to be essential for adults also. It is encoded by the codons CAU and CAC.
Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896. It is also a precursor to histamine, a vital inflammatory agent in immune responses. The acyl radical is histidyl.
The conjugate acid (protonated form) of the imidazole side chain in histidine has a pKa of approximately 6.0. Thus, below a pH of 6, the imidazole ring is mostly protonated (as described by the Henderson–Hasselbalch equation). The resulting imidazolium 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. Sometimes, the symbol Hip is used for this protonated form instead of the usual His. Above pH 6, one of the two protons is lost. The remaining proton of the imidazole ring can reside on either nitrogen, giving rise to what are known as the N1-H or N3-H tautomers. The N3-H tautomer is shown in the figure above. In the N1-H tautomer, the NH is nearer the backbone. These neutral tautomers, also referred to as Nδ and Nε, are sometimes referred to with symbols Hid and Hie, respectively. The imidazole/imidazolium ring of histidine is aromatic at all pH values.
The acid-base properties of the imidazole side chain are relevant to the catalytic mechanism of many enzymes. In catalytic triads, the basic nitrogen of histidine abstracts a proton from serine, threonine, or cysteine to activate it as a nucleophile.
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