Three pathways for methanogenesis. Methanogenesis is a form of anaerobic respiration using a variety of one-carbon (C-1) compounds or acetic acid as a terminal electron acceptor. All three pathways converge on the reduction of methyl-CoM to methane (CH4). Many methanogens can reduce CO2 to methane using electrons derived by oxidizing H2 (the hydrogenotrophic pathway, red arrows). Others can utilize C-1 compounds such as methanol or methylamines with one molecule of C-1 compound being oxidized to provide electrons for reducing three additional molecules to methane (the methylotrophic pathway, green arrows). Still other methanogens split acetate into a methyl group and an enzyme-bound CO, with the CO subsequently oxidized to provide electrons for the reduction of the methyl group to methane (the acetoclastic pathway, blue arrows). In all cases, an electrochemical gradient is generated for use in ATP synthesis. Most methanogens possess only one of the three methanogenic pathways. Methanosarcina species possess all three. CoM, coenzyme M; H4SPT, tetrahydrosarcinapterin; MF, methanofuran.
