Or other bonds by elimination leaving double bonds or

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or other bonds by elimination, leaving double bonds, or con- versely add groups to double bonds. Carbonic anhydrase has a molecular weight around 30 kDa, and is among the most-studied metalloenzymes. It catalyzes the deceivingly simple CO 2 hydration reaction. The subtleties of its biological func- tion, unraveled by a combination of techniques, make it an ideal example for bioinorganic chemistry. Section IV is fully dedicated to this enzyme. Alcohol dehydrogenase is a 90-kDa enzyme that catalyzes the reversible transfer of a hydride ion from alcohols to NAD +. Although it is a redox enzyme (in fact, classified as an oxidoreductase) and not a hydrolytic one, it will illustrate a different use that Nature makes of zinc to catalyze nucleophilic attack at carbon (Section V.C). Finally, the enzymatic transfer of organic radicals by enzymes involving coenzyme B 12 will be briefly considered.
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40 2/ THE REACTION PATHWAYS OF ZINC ENZYMES AND RELATED BIOLOGICAL CATALYSTS III. STRATEGIES FOR THE INVESTIGATION OF ZINC ENZYMES A. Why Zinc? Zinc has a specific role in bioinorganic processes because of the peculiar prop- erties of the coordination compounds of the zinc(II) ion. (1) Zinc(II) can easily be four-, five-, or six-coordinate, without a marked preference for six coordination. The electronic configuration of zinc(II) is 3d 10 with two electrons per orbital. In coordination compounds, there is no ligand- field stabilization energy, and the coordination number is determined by a bal- ance between bonding energies and repulsions among the ligands. Tetrahedral four-coordinate complexes have shorter metal-donor distances than five-coordi- nate complexes, and the latter have shorter ones than six-coordinate complexes (Table 2.2), whereas the ligand repulsion increases in the same order. The re- Table 2.2 Average zinc(II)-donor atom distances (A) for some common zinc(II) ligands in four-, five-, and six-coordinate complexes. 5 Coordination number Ligand 4 5 6 H 2 O 2.00 2.08 2.10 R-COO - 1.95 2.02 2.07 Imidazole 2.02 2.08 Pyridine 2.06 2.12 2.11 R-NH z 2.06 2.15 R,R'NH 2.19 2.27 pulsion can be both steric and electronic. In enzymes, zinc(II) usually has co- ordination numbers smaller than six, so that they have available binding sites in their coordination spheres. Substrate can in principle bind to zinc by substituting for a coordinated water or by increasing the coordination number. This behavior would be typical of Lewis acids, and, indeed, zinc is the most common Lewis acid in bioinorganic chemistry. Zinc could thus substitute for protons in the task of polarizing a substrate bond, e.g., the carbonyl C-O bond of peptides and esters, by accepting a substrate atom (oxygen) as a ligand. This has been shown to be possible in model systems. Relative to the proton, a metal ion with an available coordination position has the advantage of being a "superacid," 6 in the sense that it can exist at pH values where the H 3 0 + concentration is ex- tremely low. Also, relative to the proton, the double positive charge partly com- pensates for the smaller electrophilicity due to the smaller charge density.
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