Metalloenzyme

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

Metallocene (Section 14 14) A transition metal complex that bears a cyclopentadienyl ligand Metalloenzyme (Section 27 20) An enzyme in which a metal ion at the active site contributes in a chemically significant way to the catalytic activity... 

Metallocomplexes Metalloenzymes Metalloid peroxides Metalloimmunoassays Metallomesogens... 

Karlin, K.D. Metalloenzymes, structural motifs, and inorganic models. Science 261 701-708, 1993. 

METALLOPROTEINS. Metalloproteins are either metal storage forms, as in the case of ferritin, or enzymes in which the metal atom participates in a catalyti-cally important manner. We encounter many examples throughout this book of the vital metabolic functions served by metalloenzymes. 

Many enzymes require metal ions for maximal activity. If the enzyme binds the metal very tightly or requires the metal ion to maintain its stable, native state, it is referred to as a metalloenzyme. Enzymes that bind metal ions more weakly, perhaps only during the catalytic cycle, are referred to as metal activated. One role for metals in metal-activated enzymes and metalloenzymes is to act as electrophilic catalysts, stabilizing the increased electron density or negative charge that can develop during reactions. Among the enzymes that function in this... 

In this review, we will describe the model studies of hydrolytic metalloenzymes of the second category, emphasizing those of micellar systems. Here it should be mentioned that such micellar models have been studied only little in the past so that the major,part of this review bases on our own work. 

An artificial metalloenzyme (26) was designed by Breslow et al. 24). It was the first example of a complete artificial enzyme, having a substrate binding cyclodextrin cavity and a Ni2+ ion-chelated nucleophilic group for catalysis. Metalloenzyme (26) behaves a real catalyst, exhibiting turnover, and enhances the rate of hydrolysis of p-nitrophenyl acetate more than 103 fold. The catalytic group of 26 is a -Ni2+ complex which itself is active toward the substrate 1, but not toward such a substrate having no metal ion affinity at a low catalyst concentration. It is appearent that the metal ion in 26 activates the oximate anion by chelation, but not the substrate directly as believed in carboxypeptidase. 

Micelles in water are described as spherical aggregates of a surfactant monomer27 30). They somewhat resemble to enzyme proteins in structures and functions, although the details are yet the subjects of recent controversies 29,30). There are numerous studies of micellar models of enzymes 28), but the examples of those of metalloenzymes are very few 31 37). In particular, there are no examples of micellar models of carboxypeptidase or carbonic anhydrase except ours 36,37). 

Several model systems related to metalloenzymes such as carboxypeptidase and carbonic anhydrase have been reviewed. Breslow contributed a great deal to this field. He showed how to design precise geometries of bis- or trisimidazole derivatives as in natural enzymes. He was able to synthesize a modified cyclodextrin having both a catalytic metal ion moiety and a substrate binding cavity (26). Murakami prepared a novel macrocyclic bisimidazole compound which has also a substrate binding cavity and imidazole ligands for metal ion complexation. Yet the catalytic activities of these model systems are by no means enzymic. 

Scheme 5. Summarized mechanism of the catalysis of the micellar model of a metalloenzyme...

Cobalt(II) in metalloenzymes a report of structure-function relationships. S. Lindskog. Struct. Bonding (Berlin), 1970,8,154-196 (142). 

The ribonucleotide reductases — a unique group of metalloenzymes essential for cell proliferation. M. Lammers and H. Follmann, Struct. Bonding (Berlin), 1983. 54, 27-91 (354). 

Classification of Metalloenzymes in Terms of the Interplay Between the General Base and the Metal... 

Electron-electron repulsion integrals, 28 Electrons bonding, 14, 18-19 electron-electron repulsion, 8 inner-shell core, 4 ionization energy of, 10 localization of, 16 polarization of, 75 Schroedinger equation for, 2 triplet spin states, 15-16 valence, core-valence separation, 4 wave functions of, 4,15-16 Electrostatic fields, of proteins, 122 Electrostatic interactions, 13, 87 in enzymatic reactions, 209-211,225-228 in lysozyme, 158-161,167-169 in metalloenzymes, 200-207 in proteins ... 

Metal ions, effect of size, 200-205 Metalloenzymes, see also Enzyme cofactors classification of, by cofactor and coupled general base, 205-207, 206 electrostatic interactions in, 205-207 SNase, 189-197... 

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