A two-step concerted mechanism for the conversion of methane to methanol 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 FeO+ complex. In the initial stage of our mechanism, the ferryl (Fe-O) 'iron' active site of intermediate Q and substrate methane come into contact to form the initial Q(CH4) 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(3v)- or D(2d)-distorted methane and the Fe-O active site. The important transition state for an H atom abstraction exhibits a four-centered structure. The generated intermediate involves an HO-Fe-CH3 moiety, and it is then converted into the final product complex including methanol as a ligand through a methyl migration that occurs via a three-centered transition state. The two-step concerted mechanism is consistent with recent experiments on regioselectivity of enzyme-catalyzed alkane hydroxylations.
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