Energetics of triosephosphate isomerase: the nature of the proton transfer between the catalytic base and solvent water

The isomerization of specifically deuterium-labeled [1(R)-2H5dihydroxyacetone phosphate to D-glyceraldehyde 3-phosphate, catalyzed by the enzyme triosephosphate isomerase, has been studied. It is shown that the extent of transfer of the 2H label from the substrate to the product D-glyceraldehyde 3-phosphate is (after complete reaction) the same as that of the corresponding transfer of 3H. The absence of an isotope effect shows that the exchange process of the tstopically labeled enzyme carboxyl group, -COOL H2O leads to -COOH + LOH, does not tnvolve a rate-limiting transition state in which L is the flight. Possible modes for the nature of the ionization of -COOL in 1H2O are discussed.