Histidine imidazole group

FIGURE 21.18 (a) The Cn site of cytochrome oxidase. Copper ligands inclnde two histidine imidazole groups and two cysteine side chains from the protein, (b) The coordination of histidine imidazole ligands to the iron atom in the heme a center of cytochrome oxidase. 

X-ray diffraction studies on several forms of the enzyme have demonstrated that the active site is composed of a pseudo-tetrahedral zinc center coordinated to three histidine imidazole groups and either a water molecule [(His)3Zn-OH2]2+ (His = histidine), or a hydroxide anion [(His)3Zn-OH] +, depending upon pH (156,157). On the basis of mechanistic studies, a number of details of the catalytic cycle for carbonic anhydrase have been elucidated, as summarized in Scheme 22... 

BSA possesses a total of 59 lysine e-amine groups (with only 30-35 of these typically available for derivatization), 1 free cysteine sulfhydryl (with an additional 17 disulfides buried within its three-dimensional structure), 19 tyrosine phenolate residues, and 17 histidine imidazole groups. The presence of numerous carboxylate groups gives BSA its net negative charge (pi 5.1). 

In its active form CheA undergoes autophosphorylation, that is, the phosphorylation of a histidine imidazole group in one of its subunits by the protein kinase active site of an adjacent subunit. The phospho group is then transferred from phospho-CheA to another protein, CheY. Phospho-CheY interacts with the flagellar motor proteins (Chapter 19) periodically causing a reversal of direction of the bacterial flagella. As a result the bacteria tumble and then usually move... 

Despite the fact that carbonic anhydrase was the first zinc metalloenzyme identified1233 and a good deal is known of its structure, there is still controversy about the nature of the various active-site species and the detailed mechanisms of their action. In particular, the identity of the group with a pXa of 7 that is involved in the mechanism, and the stereochemistry around the zinc ion during catalysis, are still in dispute. The various mechanisms proposed assume either ionization of a histidine imidazole group (bound or not to the zinc) and nucleophilic attack on C02 by the coordinated imidazolate anion,1273,1274 or ionization of the Znn-coordinated water and nucleophilic attack on C02 by OH. 1271 Many papers on this problem have appeared recently and the extensive literature is the subject of the several review articles referred to above. 

Figure I. Composite diagram summarizing the roles of histidine imidazole groups at the active sites of the phosphotransferase enzymes creatine kinase (CrK), pyruvate kinase (PK), and adenylate...

The second copper center near the heme as iron, which is readily assignable as Cub, has three histidine imidazole groups as the ligands (Fig. 3). The copper atom in this site is located approximately at the center of the triangle formed by three imidazole nitrogen atoms coordinated to the copper atom. The position of the copper atom is about 1.0 A from the heme normal for heme as. The Cub—Feaj distance is 4.7 A. In the fully oxidized state, a peroxide is bridged between the two metal atoms (Fig. 3). The To—Td difference Fourier map of the oxidized... 

The active site has a zinc(II) ion bonded to three histidine imidazole groups and a fourth site occupied by water or hydroxide ion, L3Zn OH, where L = imidazole N from histidine. Experimentally, the enzyme loses activity below pH 7, indicating that an ionizable group of pK 1 is part of the active site. In addition, it is known that the product of the hydration of CO2 is HCO3 , would be expected in neutral or basic solution. 

As an example, consider papain, a protease from papaya juice that is used to treat meat to make it more tender and easier to chew. The pH optimum of the enzyme is 6.2, and analysis of the pH dependence indicates that two groups, with pKg s of 4.2 and 8.2, are required for catalysis. One group must be protonated cmd the other deprotonated for catalysis to occur. The active site of papain contains a cysteine thiol and a histidine imidazole group. There are two possible ways to write the protonation state of the active site ... 

Mimics of carbonic anhydrase exemplify the genre further. Here, a simple saturated triaza-macrocycle replaces the histidine imidazole groups of the protein, yet the shape... 

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