Solvent deuterium isotope effects in the catalysis of oxygen-18 exchange by human carbonic anhydrase II

By measuring the rate of exchange at chemical equilibrium of 18O between HCO3- and H2O catalyzed by human carbonic anhydrase II in the absence of buffers, we have determined the rate of release from the enzyme of water bearing substrate oxygen. The ratio of this rate measured in H2O to the rate measured in D2O, the solvent deuterium isotope effect, is between 4 and 9 in the range of pH(D) from 5.8 to 8.0, with a value of 8.0 +/- 0.7 at pH(D) 6.6 (uncorrected pH meter reading). The magnitude of this isotope effect at pH(D) 6.6 has an exponential dependence on the atom fraction of deuterium in solvent water. We conclude that an intramolecular proton transfer between a proton shuttle group on the enzyme and the active site is rate limiting for the release from the enzyme of water bearing substrate oxygen and involves a change in bonding of more than one proton. In contrast, the solvent deuterium isotope effect on the intermolecular proton transfer between the external buffer imidazole and the active site (or proton shuttle group) of the enzyme is small, 2.3 at pH(D) 7.0, as determined from initial velocity experiments. With a rate constant near 9 X 10(8) M-1 s-1, this intermolecular transfer is limited to a significant extent by diffusion processes.