The nature of the proton transfer from an acid group at the active site of an enzyme, to solvent water. The extent of 2H and 3H transfer in the reaction catalysed by triose phosphate isomerase
Abstract
The extent of transfer of a 2H and of a 3H label in 1-R-[2H or 3H]-dihydroxyacetone phosphate to the 2-position of D-glyceraldehyde 3-phosphate in the reaction catalysed by triose phosphate isomerase has been determined. Since the enzymic base that abstracts the substrate's isotopic label is a carboxylate group, this enzyme-substrate system effectively provides —COO2H and —COO3H in a solution of 1H2O, and allows an investigation of the transfer of 2H and 3H from the carboxyl group to unlabelled water. From the value of the fractionation factor for this proton transfer, it is evident that the mechanism of exchange of isotope on —COOL with the protons of the solvent does not involve a transition state in which L is in flight.