2-STEP CONCERTED MECHANISM FOR ALKANE HYDROXYLATION ON THE FERRYL ACTIVE-SITE OF METHANE MONOOXYGENASE

A two-step concerted mechanism for the conversion of methane to methan ol catalyzed by soluble methane monooxygenase (sMMO) is discussed. We propose that the enzymatic reaction mechanism is essentially the same as that of the gas-phase methane-methanol conversion by the bare FeOcomplex. In the initial stage of our mechanism, the ferryl (Fe-O) ''ir on'' active site of intermediate Q and substrate methane come into con tact to form the initial Q(CH,) complex with an OFe-CH4 bond. The C-H bonds of methane are significantly weakened by the formation of a five -coordinate carbon species, through orbital interactions between a C-3 y- or D-2d-distorted methane and the Fe-O active site. The important t ransition state for an H alum abstraction exhibits a four-centered str ucture. The generated intermediate involves an HO-Fe-CH3 moiety, and i t is then converted into the final product complex including methanol as a ligand through a methyl migration that occurs via a three-centere d transition state. The two-step concerted mechanism is consistent wit h recent experiments on regioselectivity of enzyme-catalyzed alkane hy droxylations.