Kinetic and spectroscopic characterization of intermediates and component interactions in reactions of methane monooxygenase from Methylococcus capsulatus (Bath)

Abstract

We describe mechanistic studies of the soluble methane monooxygenase (sMMO) enzyme system from Methylococcus capsulatus (Bath). Interactions among the three sMMO components, the hydroxylase (H), reductase (R), and protein B (B), were investigated by monitoring conversion of nitrobenzene to nitrophenol under both single turnover and catalytic conditions. During catalytic turnover, hydroxylation occurs to afford 3-nitrophenol (43%) and 4-nitrophenol (57%), whereas hydroxylation takes place exclusively (> 95%) to give 4-nitrophenol under single turnover conditions in the absence of reductase. Protein B exerts a strong influence on single turnover reactions of nitrobenzene, with optimal rate constants and yields obtained by using 1.5-2 equiv of protein R per equivalent of hydroxylase. The temperature dependence of these kinetic values was determined. Changes in dioxygen concentration and pH, as well as exchange of solvent accessible protons with D{sub 2}O, did not significantly affect the rate constants for either of these processes, the implications of which for the kinetic mechanism are discussed. From the present and related evidence, structures for H{sub peroxo} and Q are proposed. 54 refs., 11 figs., 4 tabs.