Biochemistry and bioenergetics of biomethanation

Biomethanation comprises of four consecutive stages, viz., hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Methanogenesis, the final stage of biomethanation, essentially occurs in an anaerobic condition involving the reduction of substrates like CH3COOH, CO2, CH3OH and CH3NH2 as terminal electron acceptors, wherein H2 acts as the electron donor by methanogenic archaea. However, in absence of H2, the CH3OH and CH3NH2 can act as both electron donors and acceptors to generate CH4. These anaerobic respiration processes are accompanied with the proton motive and/or sodium motive forces to drive ATP synthesis by A0A1 ATP synthase. Irrespective of the pathway, the final step of methanogenesis is catalysed by Methyl CoM reductase (Mcr). Mcr is a hexamer composed of two each of α-, β- and γ-subunits along with two Ni-containing F430 cofactors. The mechanisms of action of Mcr proceed through three pathways, each of which differs by the type of intermediates. The mechanism I involves the formation of methyl-N(III), mechanism II Ni(II)-thiolate, and mechanism III Ni(III)-S-CoM intermediates.