5.B.16.  The Acetogenic Electron Transfer Chain (A-ETC) Family 

Acetogenic bacteria are a group of strictly anaerobic bacteria that make a living by acetate formation from two molecules of CO₂ via the Wood-Ljungdahl pathway (WLP). The free energy change of this reaction is small and allows the synthesis of only a fraction of an ATP.  Kremp et al. 2022 described an electron transport chain in the cytochrome- and quinone-containing acetogen Sporomusa ovata that leads from molecular hydrogen as an electron donor to an intermediate of the WLP, methylenetetrahydrofolate (methylene-tetrahydrofolate [THF]), as an electron acceptor. The catalytic site of the hydrogenase is periplasmic and likely linked cytochrome b to the membrane. The MetVF-type methylenetetrahydrofolate reductase links proteins MvhD and HdrCBA to the cytoplasmic membrane. Membrane preparations catalyzed the H₂-dependent reduction of methylene-THF to methyl-THF. In this model, a transmembrane electrochemical H⁺ gradient is established by both scalar and vectorial protons that leads to the synthesis of 0.5 mol ATP/mol methylene-THF by a H⁺-F₁F_o ATP synthase. This H₂- and methylene-THF-dependent electron transport chain may be present in other cytochrome-containing acetogens as well and represents a third way of chemiosmotic energy conservation in acetogens, but only in addition to the well-established respiratory enzymes Rnf and Ech. Figure 6 in this paper shows the presumed electron transfer chain, which in order of electron receipt includes:  1, the hydrogenase (in the periplasm), 2, cytochrome b (crosses the membrane), 3, the quinone, 4, MvhD, 5, HdrA, 6, HdrC, 7. HdrB, 7, MetV, and 8, MetF.  Not all of these proteins were found in the NCBI protein database.

Acetogenic bacteria grow by making acetate from CO₂ and are considered the first life forms on Earth since they couple CO₂ reduction to the conservation of energy. How this is achieved has been an enigma, but two respiratory enzymes, a ferredoxin:NAD⁺ oxidoreductase (Rnf) and a ferredoxin:H⁺ oxidoreductase (Ech), have been found in cytochrome-free acetogenic model bacteria. However, some acetogens contain cytochromes, and there has been a long-standing assumption of a cytochrome-containing electron transport chain in those acetogens. Kremp et al. 2022 provided evidence for a respiratory chain in Sporomusa ovata that has a cytochrome-containing hydrogenase as the electron donor and a methylenetetrahydrofolate reductase as the terminal electron acceptor. This is the third way of chemiosmotic energy conservation found in acetogens.