Enzymatic Methane Hydroxylation: SMMO and pMMO

Catalytic function ofmethane monooxygenase (MMO)for the conversion of methane to methanol at ambient pressure and temperature has fascinated many researchers. This chapter will review quantum chemical approaches, especially by means of density functional theory (DFT) calculations, to elucidate mechanisms for  the methane hydroxylation by active sites of MMO (soluble and particulate MMOs) with a focus on how a methane C–H bond is activated. Because of the uncertainty in the coordination environment of transition metal cations in the active site that has a direct reactivity toward substrate methane, different computational models in various spin states have been used. Reflecting from these differences, a variety of mechanisms for the methane hydroxylation on MMO has been proposed: Radicalrelated mechanisms (radical-rebound and nonsynchronous concerted mechanisms) and nonradical mechanism. These proposed mechanisms will be compared. In addition, key factors in determining the reactivity ofMMOwill be discussed on the basis of molecular orbital theory applied to transition-metal containing active sites.