Apocarbonic anhydrase

Work from Sturtevant s laboratory detailed the kinetics and thermodynamics of zinc binding to apocarbonic anhydrase (carbonate dehydratase) selected data are recorded in Table II (Henkens and Sturtevant, 1968 Henkens etal., 1969). The thermodynamic entropy term A5 at pH 7.0 is 88 e.u. (1 e.u. = 1 cal/mol-K), and this is essentially matched by the binding of zinc to the hexadentate ligand cyclohexylenediamine tetraacetate where AS = 82 e.u. At pH 7.0 the enthalpy of zinc-protein association is 9.8 kcal/mol, but this unfavorable term is overwhelmed by the favorable entropic contribution to the free energy (AG = AH - T AS), where —TAS = -26.2 kcal/mol at 298 K (25°C). Hence, the kinetics and thermodynamics of protein-zinc interaction in this example are dominated by very favorable entropy effects. 

Data are from Henkens and Sturtevant (1968) and Henkens et at. (1969). apo-CA, Bovine apocarbonic anhydrase II CHDTA, cyclohexylenediamine tetraacetate. 

The observation that Zn(II) incorporation into apocarbonic anhydrase in the presence of dansylamide raises the fluorescence of the dansyl-amide (69) has led to the adoption of the dansyl group as a pendant accessory to cyclen to generate a Zn(II)-sensitive fluorophore 54 (70-72). 

The results are of some significance as far as metalloenzymes are concerned as the Lewis acidity of the metal ion can presumably be tailored to a degree by the appropriate degree of determining metal ion specificities of metalloenzymes. Metal ions with high binding constants for apoenzymes may be poor Lewis acid catalysts. 

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