Structure, reactivity, and regulation of methane monooxygenase catalytic cycle intermediates

Methane monooxygenase from M. trichosporium consists of a hydroxylase, an NADH coupled oxidoreductase, and a regulatory "B" component. The hydroxylase utilizes a binuclear iron cofactor for oxygen activation and substrate oxidation. Reduction of the binuclear cluster to the Fe(II)Fe(II) state, followed by exposure to oxygen and substrate results in turnover. The B component and reductase greatly enhance catalytic rate and efficiency. Monitoring the reaction by transient kinetic techniques has allowed some of the details of the chemistry and regulatory effects to be elucidated. Reductive activation of dioxygen culminates in Compound Q, the most reactive transient intermediate yet observed in any oxygenase. Kinetic isotope effects and the stereochemieal course of the reaction of Q with substrate support a hydrogen atom abstraction process, followed by radical recombination to form product. MSssbauer spectroscopy of Q, trapped by rapid freezing, shows.parameters = 0.17 mm/s and AEQ = 0.53 mm/s, suggesting an Fe(IV)Fe(IV) unit, with both irons in very similar environments. EXAFS shows a very short Fe-Fe distance of 2.47/, and one short (1.77/.) oxygen bond to each iron. Through comparison with inorganic, biomimetric models, these data indicate that Q contains a diferryl dioxo-bridged "diamond" core unit. This is the first structural characterization of a reactive intermediate of any metallooxygenase. Sponsored by NIH grant GM40466.