Histidine (abbreviated as His or H)[2] is an α-amino acid with an imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans and other mammals. It was initially thought that it was only essential for infants, but longer-term studies established that it is also essential for adult humans.[3]
Chemical properties
The imidazolesidechain of histidine has a pKa of approximately 6.0, and, overall, the amino acid has a pKa of 6.5. 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.
Aromaticity
The imidazole ring of histidine is aromatic at all pH values.[4] It contains six pi electrons: four from two double bonds and two from a nitrogen lone pair. It can form pi stacking interactions,[5] but is complicated by the positive charge.[6] It does not absorb at 280 nm in either state, but does in the lower UV range more than some amino acids.[7][8]
Biochemistry
The imidazole sidechain of histidine is a common coordinating ligand in metalloproteins and is a part of catalytic sites in certain enzymes. In catalytic triads, the basic nitrogen of histidine is used to abstract a proton from serine, threonine, or cysteine to activate it as a nucleophile. In a histidine proton shuttle, histidine is used to quickly shuttle protons, it can do this by abstracting a proton with its basic nitrogen to make a positively-charged intermediate and then use another molecule, a buffer, to extract the proton from its acidic nitrogen. In carbonic anhydrases, a histidine proton shuttle is utilized to rapidly shuttle protons away from a zinc-bound water molecule to quickly regenerate the active form of the enzyme.
NMR
As expected, the 15N chemical shifts of these nitrogens are indistinguishable (about 200 ppm, relative to nitric acid on the sigma scale, on which increased shielding corresponds to increased chemical shift). As the pH increases to approximately 8, the protonation of the imidazole ring is lost. The remaining proton of the now-neutral imidazole can exist on either nitrogen, giving rise to what is known as the N-1 or N-3 tautomers. NMR shows that the chemical shift of N-1 drops slightly, whereas the chemical shift of N-3 drops considerably (about 190 vs. 145 ppm). This indicates that the N-1-H tautomer is preferred, it is presumed due to hydrogen bonding to the neighboring ammonium. The shielding at N-3 is substantially reduced due to the second-order paramagnetic effect, which involves a symmetry-allowed interaction between the nitrogen lone pair and the excited pi* states of the aromatic ring. As the pH rises above 9, the chemical shifts of N-1 and N-3 become approximately 185 and 170 ppm. It is worth noting that the deprotonated form of imidazole, imidazolate ion, would be formed only above a pH of 14, and is therefore not physiologically relevant. This change in chemical shifts can be explained by the presumably decreased hydrogen bonding of an amine over an ammonium ion, and the favorable hydrogen bonding between a carboxylate and an NH. This should act to decrease the N-1-H tautomer preference.[9]
^MROZEK Agnieszka, KAROLAK-WOJCIECHOWSKA Janina, KIEC-KONONOWICZ Katarzyna (2003). "Five-membered heterocycles. Part III. Aromaticity of 1,3-imidazole in 5+n hetero-bicyclic molecules". Journal of Molecular Structure. 655 (3): 397–403. doi:10.1016/S0022-2860(03)00282-5. ((cite journal)): Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
^Lijun Wang,, Na Sun,, Simon Terzyan,, Xuejun Zhang, and, David R. Benson. A Histidine/Tryptophan π-Stacking Interaction Stabilizes the Heme-Independent Folding Core of Microsomal Apocytochrome b5 Relative to that of Mitochondrial Apocytochrome b5. Biochemistry 2006 45 (46), 13750-13759
^Robert H. Blessing, Edward L. McGandy. Base stacking and hydrogen bonding in crystals of imidazolium dihydrogen orthophosphate. Journal of the American Chemical Society 1972 94 (11), 4034-4035.
^Katoh R. Absorption Spectra of Imidazolium Ionic Liquids. Chemistry Letters. Vol. 36 (2007), No. 10 p.1256.
^AR Goldfarb, LJ Saidel, E Mosovich. THE ULTRAVIOLET ABSORPTION SPECTRA OF PROTEINS. Journal of Biological Chemistry, 1951, p.397-404.
^Wensink, J; Van, Den, Hamer, Cj (1988). "Effect of excess dietary histidine on rate of turnover of 65Zn in brain of rat". Biological trace element research. 16 (2): 137–50. doi:10.1007/BF02797098. PMID2484542. ((cite journal)): Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
