Electron transfer reactions in the soluble methane monooxygenase of Methylococcus capsulatus (Bath)

Abstract

Aerobic stopped-flow experiments have combined that component C is the methane monooxygenase component responsible for interaction with NADH. Reduction of component C by NADH is not the rate-limiting step for component C in the methane monooxygenase reaction. Removal and reconstitution of the redox center of component C suggests a correlation between the presence of the FAD and Fe2S2 redox centers and NADH:acceptor reductase activity and methane monooxygenase activity, respectively, consistent with the order of electron flow: NADH .fwdarw. FAD Fe2S2 .fwdarw. component A. This order suggests that component C functions as a 2e-1/1e-1 transformase, splitting electron pairs from NADH for transfer to component A via the 1-electron-carrying Fe2S2 centers. Electron transfer was demonstrated between the reductase component, component C and the oxygenase component, component A, of the methane monooxygenase complex from M. capsulatus by 3 separate methods. This intermolecular electron transfer step is not rate-determining for the methane monooxygenase reaction. Intermolecular electron transfer was independent of component B, the 3rd component of the methane monooxygenase. Component B is required to switch the oxidase activity of component A to methane monooxygenase activity, suggesting that the role of component B is to couple substrate oxidation to electron transfer, via the methane monooxygenase components.