Component of the methyl-coenzyme M reductase (MCR) I that catalyzes the reductive cleavage of methyl-coenzyme M (CoM-S-CH3 or 2-(methylthio)ethanesulfonate) using coenzyme B (CoB or 7-mercaptoheptanoylthreonine phosphate) as reductant which results in the production of methane and the mixed heterodisulfide of CoB and CoM (CoM-S-S-CoB). This is the final step in methanogenesis. coenzyme B + methyl-coenzyme M = coenzyme M-coenzyme B heterodisulfide + methane Binds 2 coenzyme F430 non-covalently per MCR complex. Coenzyme F430 is a yellow nickel porphinoid. Methyl-coenzyme-M reductase is activated when the enzyme-bound coenzyme F430 is reduced to the Ni(I) oxidation state. One-carbon metabolism; methyl-coenzyme M reduction; methane from methyl-coenzyme M: step 1/1. MCR is a hexamer of two alpha, two beta, and two gamma chains, forming a dimer of heterotrimers. There are two MCR complexes in this bacteria. MCR II is expressed in the early growth phase. Late growth cells contain mostly MCR I. The alpha subunit contains six modified amino acids near the active site region. Is methylated on His-257, Arg-271, Gln-400 and Cys-452, probably by the action of specific S-adenosylmethionine-dependent methyltransferases. Also contains a thioglycine at position 445, forming a thiopeptide bond. Contains a didehydroaspartate residue at position 450 (By similarity). The methylation on C5 of Arg-271 is a post-translational methylation not essential in vivo, but which plays a role for the stability and structural integrity of MCR (By similarity). Belongs to the methyl-coenzyme M reductase alpha subunit family.