Fig. 5.
Reactions, coenzymes, and enzymes involved in CO2 reduction with 4 H2 to CH4 in M. marburgensis. Reduced ferredoxin (Fd), which is required for CO2 reduction to formylmethanofuran (CHO-MFR), is regenerated in the MvhADG/HdrABC catalyzed reaction. If the latter is not completely coupled, then less reduced Fd is regenerated than required. In this case the energy-converting hydrogenases (EhaA-T and/or EhbA-Q) that catalyze the sodium motive force-driven Fd reduction with H2 are proposed to step in. Eha and Ehb mainly have an anabolic function in providing the reduced Fd required for the reduction of CO2 to CO and of acetyl-CoA + CO2 to pyruvate. In M. marburgensis there are several ferredoxins that are considered to form a pool into which electrons are fed and from which electrons can be taken out (Discussion) (9). MFR, methanofuran; H4MPT, tetrahydromethanopterin; CH≡H4MPT+, methenyl-H4MPT+; CH2 = H4MPT, methylene-H4MPT; CH3-H4MPT, methyl-H4MPT; MtrA-H, methyl transferase; FwdA-DFGH/FwdAFmdBCE, formyl-methanofuran dehydrogenase; AhaA-IK, A1A0 ATP synthase.