Intermediates in the catalytic cycle of methyl coenzyme M reductase: Isotope exchange is consistent with formation of a σ-Alkane-Nickel Complex

The key nickel enzyme for methanogenesis (MCR) catalyzes the formation of CH₃D and CH₂D₂ in a deuterated medium. CH ₂D₂ is formed by an exchange of deuterium into the S-methyl group of the substrate. Deuterium is incorporated at both carbon atoms of the S-ethyl group of ethyl coenzyme M, and a ¹³C label is rapidly scrambled within the ethyl group (see scheme). Thus, at least one intermediate is formed and the isotope exchange pattern is consistent with formation of a σ-alkane-nickel complex.