The aerobic oxidation of methane to methanol in methanotrophs is marshaled by the diiron sMMO and dicopper pMMO enzymes. The soluble nature of sMMO has enabled an extensive investigation of its catalytic cycle, leading to the discovery of 8 intermediates in its single turnover reaction. These intermediates can be classified into three groups that illustrate the critical events during catalysis; (i) binding of O2 to the diiron cluster (Hred  , O  , P*  ), (ii) activation of O2 leading to the formation of the strongest biological oxidant (P  , Q  ), and (iii) single oxygen atom insertion into the C-H bond of methane (Q  , QS  , T  ). Recent results from our laboratory have shed light upon the manner of O2 binding to diiron clusters in biology by elucidating the electronic nature of P*  . P*  is a diferrous species that has lost the characteristic g = 16 integer spin EPR signal of the ferromagnetically coupled diferrous cluster of O  . We propose that O2 replaces a labile bridging water molecule in a coordination mode with less than complete electron transfer from the iron atoms. We find no kinetic or spectroscopic evidence for a superoxo intermediate during subsequent formation of the diferric cis-�-1,2-peroxide species P  . The detailed electronic structure and basis for reactivity of Q are being investigated using a range of new spectroscopic and kinetic techniques.

Steps in the generation of reactive intermediates in soluble methane monooxygenase (sMMO)

248th National Meeting (2014)

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