Oxygen isotope effects on the ribulosebisphosphate oxygenase reaction

The oxygen isotope effect at the substrate O2 on the oxygenase reaction of ribulose bisphosphate carboxylase/oxygenase from spinach is pH and metal dependent. The pH dependence between pH 7.4 and 8.9 is different with Mg2+ (steady decrease in this isotope effect from 1.036 to 1.030) and Mn2+ (minimum isotope effect of 1.028 at pH 8.0). Deuteration of the substrate ([3-2H]ribulose bisphosphate) has no influence on the isotope effect. The results are interpreted as a direct participation of the metal ion in the oxygen-sensitive step, i.e. carbon-oxygen bond formation and the stabilization of the intermediates. In the overall reaction oxygen addition is a major rate-limiting step, and the observed isotope effect is probably close to the intrinsic oxygen isotope effect of the reaction. The basic mechanisms for carboxylation and oxygenation of ribulose bisphosphate appear to be the same.