Zinc ions systems

Uncovering of the three dimentional structure of catalytic groups at the active site of an enzyme allows to theorize the catalytic mechanism, and the theory accelerates the designing of model systems. Examples of such enzymes are zinc ion containing carboxypeptidase A 1-5) and carbonic anhydrase6-11. There are many other zinc enzymes with a variety of catalytic functions. For example, alcohol dehydrogenase is also a zinc enzyme and the subject of intensive model studies. However, the topics of this review will be confined to the model studies of the former hydrolytic metallo-enzymes. 

When a zinc strip is dipped into the solution, the initial rates of these two processes are different. The different rates of reaction lead to a charge imbalance across the metal-solution interface. If the concentration of zinc ions in solution is low enough, the initial rate of oxidation is more rapid than the initial rate of reduction. Under these conditions, excess electrons accumulate in the metal, and excess cationic charges accumulate in the solution. As excess charge builds, however, the rates of reaction change until the rate of reduction is balanced by the rate of oxidation. When this balance is reached, the system is at dynamic equilibrium. Oxidation and reduction continue, but the net rate of exchange is zero Zn (.S ) Zn (aq) + 2 e (me t a i)... 

The creation of boundaries in a potential-pH diagram results in the formation of areas, in each of which a particular species is thermodynamically stable. For example, in area X of the zinc-water systems (Figure 5.2 A), Zn2+ ions are thermodynamically stable species. This implies that if any other zinc species in the system is exposed to conditions of potential and pH to correspond to this area, it will tend to be converted to the Zn2+ ionic species. This clearly means that zinc present in the system is in either a metallic or oxide state, and so would tend to pass into solution. 

For example, consider a system in which metallic zinc is immersed in a solution of copper(II) ions. Copper in the solution is replaced by zinc which is dissolved and metallic copper is deposited on the zinc. The entire change of enthalpy in this process is converted to heat. If, however, this reaction is carried out by immersing a zinc rod into a solution of zinc ions and a copper rod into a solution of copper ions and the solutions are brought into contact (e.g. across a porous diaphragm, to prevent mixing), then zinc will pass into the solution of zinc ions and copper will be deposited from the solution of copper ions only when both metals are connected externally by a conductor so that there is a closed circuit. The cell can then carry out work in the external part of the circuit. In the first arrangement, reversible reaction is impossible but it becomes possible in the second, provided that the other conditions for reversibility are fulfilled. 

Zinc hydroxide and alkoxide species are particularly relevant to catalytic processes, often forming the active species. The cooperative effects of more than one zinc ion and bridged hydroxides are exploited in some enzymatic systems. Zinc alkyl phosphate and carboxylate materials have been important in the formation of framework compounds, often containing large amounts of free space for the inclusion of guest molecules. Aldehyde and ketone compounds are of low stability due to the poor donor capabilities of the ligands however, a number of examples have recently been characterized. 

The zinc-pyridoxal 5 -phosphate-2-amino-3-phosphonopropionic acid system exhibits deprotonation of the coordinated phosphonate group to bind to the zinc center at increased pH but cannot be coordinated to the zinc ion below pH 8.5 when it is protonated and hydrogen bound to the phosphate group.422... 

Theoretical calculations have been carried out on a number of zinc-containing enzymatic systems. For example, calculations on the mechanism of the Cu/Zn enzyme show the importance of the full protein environment to get an accurate description of the copper redox process, i.e., including the electronic effects of the zinc ion.989 Transition structures at the active site of carbonic anhydrase have been the subject of ab initio calculations, in particular [ZnOHC02]+, [ZnHC03H20]+, and [Zn(NH3)3HC03]+.990... 

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. 

Zinc may function to promote the nucleophilicity of a bound solvent molecule in both small-molecule and protein systems. The p/Ca of metal-free H2O is 15.7, and the p/Ca of hexaaquo-zinc, Zn (OH2)6. is about 10 (Woolley, 1975) (Table III). In a novel small-molecule complex the coordination of H2O to a four-coordinate zinc ion reduces the to about 7 (Groves and Olson, 1985) (Fig. 2). This example is particularly noteworthy since it has a zinc-bound solvent molecule sterically constrained to attack a nearby amide carbonyl group as such, it provides a model for the carboxypeptidase A mechanism (see Section IV,B). To be sure, the zinc ligands play an important role in modulating the chemical function of the metal ion in biological systems and their mimics. 

Currently, only a handful of examples of unique protein carboxylate-zinc interactions are available in the Brookhaven Protein Data Bank. Each of these entries, however, displays syn coordination stereochemistry, and two are bidentate (Christianson and Alexander, 1989) (Fig. 5). Other protein structures have been reported with iyw-oriented car-boxylate-zinc interactions, but full coordinate sets are not yet available [e.g., DNA polymerase (Ollis etal., 1985) and alkaline phosphatase (Kim and Wyckoff, 1989)]. A survey of all protein-metal ion interactions reveals that jyw-carboxylate—metal ion stereochemistry is preferred (Chakrabarti, 1990a). It is been suggested that potent zinc enzyme inhibition arises from syn-oriented interactions between inhibitor carboxylates and active-site zinc ions (Christianson and Lipscomb, 1988a see also Monzingo and Matthews, 1984), and the structures of such interactions may sample the reaction coordinate for enzymatic catalysis in certain systems (Christianson and Lipscomb, 1987). 

Fig. 34. Glu-72- Zn interactions in native carboxypeptidase A and in carboxypep-tidase A-inhibitor complexes (inhibitors have been reviewed by Christianson and Lipscomb, 1989). When substrates or inhibitors bind to the enzyme active site and interact with the zinc ion, the interaction of the metal with Glu-72 tends from bidentate toward uniden-tate coordination. The flexibility of protein-zinc coordination may be an important aspect of catalysis in this system, and the Glu-72->Zn - coordination stereochemistry observed here is consistent with the stereochemical analysis of carboxylate-zinc interactions from the Cambridge Structural Database (Carrell et al., 1988 see Fig. 4).

Maass and Eigen have shown for the zinc acetate system fos " 32 — 3.2 X 107 sec."1 The rate of loss of water from zinc equals the rate of loss of a carboxylate group from zinc. This means that the ion pair and inner sphere species of zinc acetate are present in solution in equal amounts. If we now examine the zinc oxalate system we see that we have similar rate steps. 

The intramolecular arrangement of these two zinc(II) ions in AP is more advantageous than the single-zinc(II) system to exercise the dual zinc(II) role. In the single zinc(II) system, the pAa value of 9.1 for the 25a = 25b equilibrium is higher than the reported p a value of 7.5 (2 <= 3) for the phosphoryl-serine intermediate. The dinuclear zinc(II) system is also essential in the initial interaction with phosphomonoes-ter see Section IV. 

J. A Fee Copper Proteins - Systems Containing the Blue" Copper Center. - M.F.Dunn Mechanisms of Zinc Ion Catalysis in Small Molecules and Enzymes. - W. Schneider Kinetics and Mechanism of Metalloporphyrin Formation. - M. Orchin, D. M. Bollinger Hydro gen-Deuterium Exchange in Aromatic Compounds. 

Reduction of l,10-phenanthroline-2-aldehyde to 1,10-phenanthroline-2-carbinol is efficiently accomplished by a dihydronicotinamide derivative in acetonitrile solution catalyzed by zinc ions. This was the first example of the reduction of an aldehyde by a NADH analog in a nonenzymic system. It also supports the catalytic function of the metal ion in the enzymic system.359 l,10-Phenanthroline-2-carbinol, obtained by sodium borohydride reduction of 2-carbomethoxy-1,10-phenanthroline, is phosphorylated by adenosine triphosphate in the presence of zinc ions.360... 

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