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 (PubMed:11023796). Methyl-coenzyme-M reductase is activated when the enzyme-bound coenzyme F430 is reduced to the Ni(I) oxidation state (By similarity). 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. The alpha subunit contains five modified amino acids near the active site region (PubMed:27467699, PubMed:11023796). Is methylated on His-271, Arg-285 and Cys-472, probably by the action of specific S-adenosylmethionine-dependent methyltransferases. Also contains a thioglycine at position 465, forming a thiopeptide bond (PubMed:27467699, PubMed:11023796). Contains a didehydroaspartate residue at position 470 (PubMed:27467699). The methylation on C5 of Arg-285 is a post-translational methylation not essential in vivo, but which plays a role for the stability and structural integrity of MCR (By similarity). Does not show a methylation at Gln-420, as shown for M.marburgensis (PubMed:11023796). Belongs to the methyl-coenzyme M reductase alpha subunit family.