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.