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

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Abstract
The isomerization of specifically 2H-labeled [1(R)-2H]dihydroxyacetone phosphate to D-glyceraldehyde 3-phosphate, catalyzed by the enzyme triosephosphate isomerase, was studied. 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 isotopically labeled enzyme carboxyl group, -COOL + H2O .fwdarw. -COOH + LOH, does not involve a rate-limiting transition state in which L is in flight. Possible modes for the nature of the ionization of -COOL in 1H2O are discussed.