Methyl-coenzyme M reductase (MCR), the rate-limiting enzyme in methanogenesis and anaerobic methane oxidation, is responsible for the production of over one billion tons of methane per year. We have demonstrated that mechanism of methane synthesis involves a methyl radical/Ni(II)-thiolate  intermediate(s). When the reaction between the EPR-active Ni(I) enzyme with substrates was studied by transient kinetic, spectroscopic and computational methods, formation of an EPR-silent Ni(II)-thiolate intermediate was positively identified by magnetic circular dichroism spectroscopy. There was no evidence for an EPR-active methyl-Ni(III) species. Temperature-dependent transient kinetic studies revealed that the activation energy for the initial catalytic step closely matched the value computed by density functional theory for Mechanism II.  Thus, our results demonstrate that biological methane synthesis occurs by generation of a methyl radical. We also describe elucidation of the steps in the reverse reaction, involving activation of methane.

Radical mechanism of the nickel enzyme methyl-CoM reductase, which catalyzes the synthesis and activation of methane

254th National Meeting (2017)

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