Activation of triplet dioxygen by glucose oxidase:: Spin-orbit coupling in the superoxide ion

Hybrid density functional calculations have been performed for the reductive activation of dioxygen by glucose oxidase, for which recent experiments have shown substantial kinetic O-18 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.+..O-2(.-), where M is the FADH(2) cofactor. For the electron transfer between the M and O-2, the presence of the protonated His516 plays a crucial role by strongly increasing the electron affinity Of O-2, 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.