Binding causes the remote [5'-3H]thymidine kinetic isotope effect in human thymidine phosphorylase

The remote 5'-3H V/K kinetic isotope effect (KIE) observed in human thymidine phosphorylase (6.1%) is significantly larger than can be explained by the reaction chemistry. One hypothesis connects the 5'-3H KIE in purine nucleoside phosphorylase to that enzyme's SN1 transition state. The transition state of thymidine phosphorylase, however, is an SN2 nucleophilic displacement. Here we report equilibrium binding isotope effects sufficiently large to explain the presence of this substantial KIE in thymidine phosphorylase.