Histidine imidazole

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). 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

A typical role for the histidine imidazole ring is shown below, in the enzyme mechanism for a general base-catalysed hydrolysis of an ester. The imidazole nitrogen acts as a base to remove a proton from water, generating hydroxide that attacks the carbonyl. Subsequently, the alkoxide leaving group is reprotonated by the imidazolium... 

The active site of the enzyme contains two distinct regions an anionic region that contains a glutamic acid residue, and a region in which a histidine imidazole ring and a serine hydroxyl group are particularly important. 

L-histidine Imidazole alkaloids Imidazole Histamine Pilocarpine Pilosine... 

The active site of manganese SOD contains a single five-coordinate Mn bound to three histidine imidazoles, a monoden-tate aspartate carboxylate, and a solvent molecule (H2O or OH , depending on the oxidation state) [103]. The catalytic cycle of the enzyme, which is illustrated in Fig. 16, incorporates the following oxidative and reductive steps ... 

Arrangement of subunits in nitrite reductase from Achromohacter cycloclastes. Domains are denoted D1 and D2, copper sites are shaded spheres, and Cu ligands are denoted by one-letter abbreviations C for cysteine sulfur and H for histidine imidazole nitrogen. From Fenderson et al. (1991). 

The crystal structure of this protein has been shown to depend on the salt solution from which it is crystallized.45 When crystallized from (NH4)2S04, the insulin hexamer is held together in part by two Zn2+ ions. These can be visualized as being at either end of a cylinder, and each Zn2 has as a ligand one histidine imidazole nitrogen atom from one of three chains. Thus a histidine (his B.10) of each of the six chains is coordinated to Zn2+, three at each Zn2+. The Zn2+ ions occupy octahedral (trigonally distorted) sites overall. 

In contrast to ferrocenes, osmium and ruthenium complexes are capable of forming coordinative bonds with donor centers of GO including histidine imidazoles. There are therefore two ways of bringing coordinated transition metals onto enzyme surfaces, i.e., via natural and artificial donor sites. Artificial centers are commonly made of functionalized pyridines or imidazoles, which must be covalently attached to GO followed by the complexation of an osmium or... 

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... 

Mechanisms. Studies of model reactions473-476 and of electronic, Raman,456 477 478 ESR,479/480 and NMR spectra and kinetics481 have contributed to an understanding of these enzymes.459 461 464 482 483 For these copper amine oxidases the experimental evidence suggests an aminotransferase mechanism.450 453 474 4743 d Tire structure of the E.coli oxidase shows that a single copper ion is bound by three histidine imidazoles and is located adjacent to the TPQ (Eq. 15-53). Asp 383 is a conserved residue that may be the catalytic base in Eq. 15-53.474b A similar mechanism can be invoked for LTQ and TTQ. 

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. 

Wallenfels (64) has proposed that proton abstraction and addition by the concerted action of a histidine imidazole nitrogen and the SH group could form the basis for the enzyme-catalyzed mutarotation. Amino acid analysis indicates the presence of ten histidine residues per molecule of enzyme (88). 

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