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 facilitated 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 donors 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 acceptor give Bronsted plots of high curvature. These data are described by Marcus 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 of reaction from 4 to 10 kcal/ mol. The interpretation of these Marcus parameters is discussed in terms of the well-studied pathway of the catalysis and structure of the enzymes. (C) 2000 Elsevier Science B.V. All rights reserved.