Marcus rate theory applied to enzymatic proton transfer

The hydration of CO2 and the dehydration of HCO3- catalyzed by the carbonic anhydrases is accompanied by the transfer of protons between solution and the zinc-bound water molecule in the active site. This transfer is facilita ted by amino acid residues of the enzyme which act as intramolecular proton shuttles; variants of carbonic anhydrase lacking such shuttle residues are enhanced or rescued in catalysis by intermolecular proton transfer from do nors such as imidazole in solution. The resulting rate constants for proton transfer when compared with the values of the pK(a) of the donor and accep tor give Bronsted plots of high curvature. These data are described by Marc us theory which shows an intrinsic barrier for proton transfer from 1 to 2 kcal/mol and work terms or thermodynamic contributions to the free energy o f reaction from 4 to 10 kcal/ mol. The interpretation of these Marcus param eters is discussed in terms of the well-studied pathway of the catalysis an d structure of the enzymes. (C) 2000 Elsevier Science B.V. All rights reser ved.