Methyl-coenzyme M reductase (MCR) is the enzyme responsible for microbial formation of methane. It is a hexamer composed of 2 alpha (Pfam:PF02249), 2 beta (this family), and 2 gamma (Pfam:PF02240) subunits with two identical nickel porphinoid active sites [1]. The C-terminal domain of MCR beta has an all-alpha fold with buried central helix.

1: Crystal structure of methyl-coenzyme M reductase: the key enzyme of biological methane formation Ermler U, Grabarse W, Shima S, Goubeaud M, Thauer RK; Science 1997;278:1457-1462. PMID:9367957