Properties of intramolecular proton transfer in carbonic anhydrase III

We investigated the efficiency of glutamic acid 64 and aspartic acid 64 as proton donors to the zinc-bound hydroxide in a series of site-specific mutants of human carbonic anhydrase III (HCA III). Rate constants for this intramolecular proton transfer, a step in the catalyzed dehydration of bicarbonate, were determined from the proton-transfer-dependent rates of release of (H2O)-O-18 from the enzyme measured by mass spectrometry. The free energy plots representing these rate constants could be fit by the Marcus rate theory, resulting in an intrinsic barrier for the proton transfer of Delta G(o)(double dagger) = 2.2 +/- 0.5 kcal/mol, and a work function or thermodynamic contribution to the free energy of reaction w(r) = 10.8 +/- 0.1 kcal/mol. These values are very similar in magnitude to the Marcus parameters describing intramolecular proton transfer from His(64) and His(67) to the zinc-bound hydroxide in mutants of HCA III. That result and the equivalent efficiency of Glu(64) and Asp(64) as proton donors in the catalysis by CA III demonstrate a lack of specificity in proton transfer from these sites, which is indirect evidence of a number of proton conduction pathways through different structures of intervening water chains. The dominance of the thermodynamic contribution or work function for all of these proton transfers is consistent with the view that formation and breaking of hydrogen bonds in such water chains is a limiting factor for proton translocation.