Elucidating the process of activation of methyl-coenzyme M reductase

Methyl-coenzymeMreductase (MCR) catalyzes the reversible reduction of methyl-coenzyme M(CH₃-S-CoM) and coenzyme B (HS-CoB) to methane and heterodisulfide CoM-S-S-CoB (HDS). MCR contains the hydroporphinoid nickel complex coenzyme F₄₃₀ in its active site, and the Ni center has to be in its Ni(I) valence state for the enzyme to be active. Until now, no in vitro method that fully converted the inactive MCR_silent-Ni(II) form to the active MCR_red1-Ni(I) form has been described. With the potential use of recombinant MCR in the production of biofuels and the need to better understand this enzyme and its activation process, we studied its activation under nonturnover conditions and achieved full MCR activation in the presence of dithiothreitol and protein components A2, an ATP carrier, and A3a. It was found that the presence of HDS promotes the inactivation of MCR_red1, which makes it essential that the activation process is isolated from the methane formation assay, which tends to result in minimal activation rates. Component A3a is a multienzyme complex that includes the mcrC gene product, an Fe-protein homolog, an iron-sulfur flavoprotein, and protein components involved in electron bifurcation. A hypothetical model for the cellular activation process of MCR is presented.