Geometric and Electronic Structures of the Ni(I) and Methyl-Ni(III) Intermediates of Methyl-Coenzyme M Reductase

Methyl-coenzyme M reductase (MCR) from methanogenic archaea catalyzes the terminal step in biological methane synthesis. Using coenzyme B (CoBSH) as the two-electron donor, MCR reduces methyl-coenzyme M (methyl-SCoM) to form methane and the heterodisulfide product, CoBS-SCoM. MCR contains an essential redox active nickel tetrapyrrolic cofactor called coenzyme F430 at its active site, which is active in the reduced Ni(I) state (MCRred1). All of the biologically generated methane, amounting to 1 billion tons per annum globally, is formed by MCR. Furthermore, recent evidence indicates that anaerobic methane oxidation is also catalyzed by MCR and occurs by a reversal of the methane synthesis reaction. Methane is a potent greenhouse gas, trapping 20 times more heat than CO2. In addition, methane is also an important and clean fuel as it produced the least amount of CO2 per unit of heat released. Thus, it is critically important to understand the mechanism of formation of the smallest hydrocarbon in nature.