Selenocysteine, Pyrrolysine, and the Unique Energy Metabolism of Methanogenic Archaea
Figure 2
Scheme of methylotrophic methanogenesis. Methyl groups from methanol, TMA, DMA, and MMA are mobilized into metabolism by the action of a substrate-specific methyltransferase interacting with its cognate corrinoid protein in which the cofactor’s cobalt ion cycles between methyl-Co(III) and Co(I) states. Methyltransferases that are pyrrolysyl-proteins are marked with an asterisk. The corrinoid cofactor is then demethylated by the action of a methylcobamide:CoM methyltransferase such as MtbA (for methylamines) or MtaA (for methanol). Adventitious oxidation can inactive the corrinoid proteins to the Co(II) state, which can be reductively reactivated by RamA (for methylamines) and possibly by RamA homologs for other pathways. Reducing equivalents in the form of hydrogen, F420H2, or F from the oxidation of methyl-CoM are used to reduce methanophenazine (MP) and subsequently CoM-S-S-CoB, thereby generating ATP via electron transport phosphorylation and the free HS-CoM and HS-CoB cofactors; CoM-S-S-CoB is then recycled by the reduction of methyl-CoM to methane. See Figure 1 for cofactor abbreviations.
