Carbon dioxide hydration activity of carbonic anhydrase: paradoxical consequences of the unusually rapid catalysis.

The kinetic parameters for carbon dioxide hydration catalysis by carbonic anhydrase (EC 4.2.1.1) present an apparent paradox. The assumption of H(2)CO(3) as the hydration product requires the rate of recombination of H(2)CO(3) with enzyme to be faster than the diffusion limit. The alternative assumption of HCO(3) (-) as the product of hydration likewise requires active-site ionization rates to exceed the diffusion limit. We previously postulated the presence of special means for rapid active-site ionization. It is shown here that when proton transfer between enzyme and buffer species is taken into account, there is no need to invoke rates exceeding the diffusion limit. Bicarbonate ion thus appears as the most probable hydration product and dehydration substrate.