Activation of Triplet Dioxygen by Glucose Oxidase: Spin−Orbit Coupling in the Superoxide Ion

Abstract

Hybrid density functional calculations have been performed for the reductive activation of dioxygen by glucose oxidase, for which recent experiments have shown substantial kinetic 18O isotope effects but no deuterium isotope effect. The present analysis of the mechanism suggests that this surprising isotope effect is best explained if the rate-determining step is the triplet → singlet interconversion that follows after the electron transfer and the superoxide ion production. The oxygen isotope effect is rationalized by an analysis of the spin−orbit coupling in the radical pair M • +···O2•-, where M is the FADH2 cofactor. For the electron transfer between the M and O2, the presence of the protonated His516 plays a crucial role by strongly increasing the electron affinity of O2, which makes the electron transfer exothermic and allows it to occur without any barrier. The chemical step where hydrogen peroxide is formed has a computed free-energy barrier of only 6.6 kcal/mol.