Oxygen Kinetic Isotope Effects in Soluble Methane Monooxygenase

Abstract

Soluble methane monooxygenase (sMMO) contains a nonheme, carboxylate-bridged diiron site that activates dioxygen in the catalytic oxidation of hydrocarbon substrates. Oxygen kinetic isotope effects (KIEs) have been determined under steady-state conditions for the sMMO-catalyzed oxidation of CH3CN, a liquid substrate analog. Kinetic studies of the steady-state sMMO reaction revealed a competition between fully coupled oxygenase activity, which produced glycolonitrile (HOCH2CN) and uncoupled oxidase activity that led to water formation. The oxygen KIE was measured independently for both the oxygenase and oxidase reactions, and values of 1.0152 ± 0.0007 and 1.0167 ± 0.0010 were obtained, respectively. The isotope effects and separate dioxygen binding studies do not support irreversible formation of an enzyme-dioxygen Michaelis complex. Additional mechanistic implications are discussed in the context of previous data obtained from single turn-over and steady-state kinetic studies.