Zinc enzymes general mechanisms

Fig. 23. A general mechanism of CO2 hydration as catalyzed by carbonic anhydrase II. Certain structural details (e.g., the function of pentacoordinate zinc or the degree of CO2—Zn interaction in enzyme-substrate association) remain to be elucidated.

Subsequent to substrate binding, a promoted-water mechanism is favored for the hydrolysis of the scissile peptide linkage based on the results of chemical, kinetic, and structural investigations of carboxypeptidase A. A general mechanism is shown in Fig. 31, in which the zinc-bound water of the native enzyme is a nucleophile promoted both by zinc and by the general base Glu-270 (Christianson and Lipscomb, 1989). 

FIGURE 9. General reaction mechanism of zinc enzymes. L1-L3 represent any amino acid residues coordinated to the zinc... 

The reader is referred to the following sources of further information on the specific cocatalytic enzymes superoxide dismutase, and the reduced form, alkahne phosphatases, nuclease Pl, purple acid phosphatase, amidohydrolase, leucine amtnopeptidase, general comments on the mechanisms of the phosphatases and aminopeptidases, and other cocatalytic zinc enzymes. ... 

The mechanism of action of mononuclear zinc enzymes depends on the Zn +-OH2 centre, which can participate in the catalytic cycle in three distinct ways (Figure 12.2) — either by ionisation, to give zinc-bound hydroxyl ion (in carbonic anhydrase), polarisation by a general base (in carboxypeptidase), or displacement of... 

Enzyme Models .—Two general mechanisms have been proposed for hydrolytic reactions catalysed by bovine pancreatic carboxypeptidase A the first involves formation of an anhydride intermediate and the second involves the residue Glu-270 as a general base. Work on model systems and the enzyme indicates that the general base mechanism is the correct one and a consistent mechanism (Scheme 4) has been proposed in which both zinc and Arg-145 interact with the substrate. 

The general physical and chemical properties of several of the zinc-metalloenzymes discussed here have been the subjects of excellent comprehensive reviews by Coleman (30), Lindskog et al. (31), Lindskog (32), Hartsuck and Lipscomb (33), Quiocho and Lipscomb (34), and Sund and Theorell (35). Therefore, it would serve no purpose to attempt a comprehensive review of zinc-metalloenzymes. For this reason, the present work is confined primaril to a review of the literature which pertains to the role(s) played by zinc ion in enzyme catal5dic mechanism. This work is further restricted by limiting discussion to consideration of only those enzyme systems for which high resolution 3-dimensional X-ray structures are available. 

GENERAL MECHANISM OF HYDROLYSIS CATALYZED BY ZINC ENZYMES... 

Other metals such as magnesium, silver, chromium, nickel, manganese, zinc, and copper are aU regulated by different enzymes, but the general mechanism exhibits characteristics like those described above. 

This zinc metalloenzyme [EC 1.1.1.1 and EC 1.1.1.2] catalyzes the reversible oxidation of a broad spectrum of alcohol substrates and reduction of aldehyde substrates, usually with NAD+ as a coenzyme. The yeast and horse liver enzymes are probably the most extensively characterized oxidoreductases with respect to the reaction mechanism. Only one of two zinc ions is catalytically important, and the general mechanistic properties of the yeast and liver enzymes are similar, but not identical. Alcohol dehydrogenase can be regarded as a model enzyme system for the exploration of hydrogen kinetic isotope effects. 

Summarising, three possible mechanisms of action were proposed for the observed metal inhibition of RubisCo (1) SH-interaction (Cd, Cu and Pb Stiborova et al., 1986 Stiborova, 1988), (2) substitution of zinc for magnesium (Van Assche and Clusters, 1986b) and (3) a general cadmium-induced enzyme deficiency (Kremer and Markham, 1982). 

These experimental studies leave major aspects of the mechanism unresolved. The most important one is whether the catalysis proceeds via a general base mechanism or via a nucleophilic attack by an enzyme residue on the scissile bond. Although the experimental studies of carboxypeptidase A are providing essential information, they cannot show what species are actually involved in the reaction. For example, the suggestion that the zinc bound water acts as the nucleophile was based on the results of x-ray structures of unproductive and static complexes of carboxypeptidase A formed with pseudosubstrates and inhibitors (Christianson and Lipscomb 1985). 

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