Histidine residues chemical modification

Prior to these structural studies, work using chemically modified proteins had shown the requirement for at least four histidine ligands per subunit together with a glutamic acid residue and Tyr-109 (Tyr-114 in met). The most recent X-ray work has eliminated the possibility that Tyr-109 is a ligand, a possibility proposed in earlier X-ray work. Other work on chemical modification of residues has been reviewed.1277... 

There is only one tyrosine residue in some sea snake neurotoxins. This residue is usually quite difficult to modify, but once it is modified, the toxicity is lost (9). Histidine seems not to be essential as the chemical modification of this residue does not affect the toxicity 10). 

Chemical modifications of proteins (enzymes) by reacting them with iV-acylimidazoles are a way of studying active sites. By this means the amino acid residues (e.g., tyrosine, lysine, histidine) essential for catalytic activity are established on the basis of acylation with the azolides and deacylation with other appropriate reagents (e.g., hydroxylamine). 

The mechanism by which serine peptidases, particularly serine endopep-tidases (EC 3.4.21), hydrolyze peptide bonds in peptides and proteins has been extensively investigated by X-ray crystallography, site-directed mutagenesis, detection of intermediates, chemical modification, H-NMR spectroscopy, and neutron diffraction [2-14], These studies revealed that all serine peptidases possess a catalytic triad, composed of a serine, a histidine, and an aspartate residue, and a so-called oxyanion hole formed by backbone NH groups. 

Oxidation of two out of 13 tryptophan residues in a cellulase from Penicillium notatum resulted in a complete loss of enzymic activity (59). There was an interaction between cellobiose and tryptophan residues in the enzyme. Participation of histidine residues is also suspected in the catalytic mechanism since diazonium-l-H-tetrazole inactivated the enzyme. A xylanase from Trametes hirsuta was inactivated by N-bromosuc-cinimide and partially inactivated by N-acetylimidazole (60), indicating the possible involvement of tryptophan and tyrosine residues in the active site. As with many chemical modification experiments, it is not possible to state definitively that certain residues are involved in the active site since inactivation might be caused by conformational changes in the enzyme molecule produced by the change in properties of residues distant from the active site. However, from a summary of the available evidence it appears that, for many / -(l- 4) glycoside hydrolases, acidic and aromatic amino acid residues are involved in the catalytic site, probably at the active and binding sites, respectively. 

The presence of imidazole groups in the active site region of human carbonic anhydrase B has, in fact, been demonstrated by chemical modification. Thus, bromoacetate reacts specifically with the 3 -N of a histidine residue to give a partially active monocarboxymethyl enzyme (65). The reaction depends on the initial combination of the bromoacetate ion with the anion binding site (65,83). In a detailed study, Bradbury (83) has shown that the irreversible reaction at saturation with iodoacetate... 

Flavin-containing mitochondrial MAO-A and MAO-B catalyze the oxidative deamination of neurotransmitters, such as dopamine, serotonin, and norepinephrine in the central nervous system and peripheral tissues. The enzymes share 73% sequence homology and follow the same kinetic and chemical mechanism but have different substrate and inhibitor specificities. Chemical modification experiments provide evidence that a histidine residue is essential for the catalysis. There is also strong evidence that two cysteine residues are present in the active site of MAO. 

MF Roberts, RA Deems, TC Mincey, EA Dennis. Chemical modification of the histidine residue in phospholipase A2 (Naja naja naja). A case of half-site reactivity. J Biol Chem 252 2405-2411, 1977. 

Modified histidine residues of HRP elucidated by the specific chemical modification of histidine residues of CPO with diethylpyrocarbonate The catalytic activity of HRP was not [60]... 

Blanke SR, Hager LP (1990) Chemical modification of chloroperoxidase with diethyl-pyrocarbonate. Evidence for the presence of an essential histidine residue. J Biol Chem 265 12454-12461... 

Urrutigoity M, Baboulene M, Lattes A (1991) Use of pyrocarbonates for chemical modification of histidine residues of horseradish peroxidase. Bioorg Chem 19 66-76... 

In contrast to albumin, hemoglobin and metallothionein, in which gold(l) coordination to cysteine thiol residues has been clearly established by physicochemical methods, the binding of gold to cyclophilin is unusual in that the Et3PAu+ moiety binds to a histidine residue in preference to the two free thiols present in the protein structure. When cysteine-34 of albumin is blocked by chemical modification, EtsPAuCl, but not auranofin, wiU bind to the numerous histidine residues present. 

Residues with ionizable side chains other than carboxyl groups could in principle be responsible for the pH dependence of the catalytic reaction. The group with pKo about 4.5 could be an a-amino group or a histidine, but there is evidence from chemical modification studies which makes this highly unlikely. Knowles (108) has discussed this in detail. 

Asp-49/35 is essential to optimal calcium binding and catalysis. Replacement of Asp-49 with other amino acids or chemical modification of the side-chain carboxylate reduces Acat to less than 5% of native rates (Fleer etai, 1981 Van den Berghe/a/., 1989). The close spatial coupling of Asp-49/35 with its respective catalytic histidine (His-48/34) ensures a fixed active site geometry. Stability of this bihelical substructure is critical because His-48/34 is supported by a side chain from an adjacent segment of the same helix (Tyr-52) and must hydrogen bond with a residue from the opposed helix (Asp-99/64) for function. 

---