Fig. 7

Theoretical model of formate-induced CO tolerance in n-butyrate and n-caproate producers performing reverse β-oxidation. The CO-inhibited enzymes in the lower box are typically involved in energy conservation in anaerobic acidogenic bacteria that lack cytochromes. Chain-elongating bacteria can compensate for this inhibition by oxidizing formate, thereby providing reduction equivalents needed for reverse β-oxidation. Dashed arrows represent sequential metabolic steps and gray arrows represent CO-inhibited processes. 3-HACAD 3-hydroxy-acyl-CoA dehydrogenase, ACAD acyl-CoA dehydrogenase, Ebh electron bifurcation hydrogenase, Ech energy-conserving hydrogenase, ETF electron-transferring flavoproteins A and B, Fdh formate dehydrogenase, Fd-Hyd Fd-dependent hydrogenase, NAD-Hyd NAD-dependent hydrogenase, Nfn NAD(P)⁺ transhydrogenase, Pfl pyruvate formate lyase