Fig. 1.

Schematic of the metabolism of acetogenic and solventogenic clostridia. Oxidative metabolic modules for the generation of reduction equivalents and intermediates are depicted in yellow. Reductive metabolic modules consuming reduction equivalents and synthesizing products are displayed in light blue. Redox balancing modules for the balancing of formed and consumed reduction equivalents are marked in green. Products of reductive metabolic modules are framed by black boxes. a Reduction of CO₂ to formate can use H₂, Fd²⁻, NADPH or even 0.5 Fd + 0.5 NADPH; b NADH is used for the reduction of H2C-THF to H3C-THF in the non-clostridial acetogen Acetobacterium woodii. In C. autoethanogenum, 2 NADH are most likely used to reduce Fd and H2C-THF in an electron bifurcating reaction [300]. c The translocation of Na⁺ by Ech in some species is likely but experimental evidence is missing [258]. d Subsequent steps for the reduction of acetoacetyl-CoA to butyryl-CoA are catalyzed by 3-hydroxyacyl-CoA dehydrogenase, crotonase and acyl-CoA dehydrogenase. e Subsequent steps for the reduction of butyryl-CoA to hexanoyl-CoA are catalyzed by thiolase, 3-hydroxyacyl-CoA dehydrogenase, crotonase and acyl-CoA dehydrogenase. 23BDH 2,3-butanediol dehydrogenase; 3-HPA 3-hydroxypropionaldehyde; 3PG glycerate 3-phosphate; AAD alcohol/aldehyde dehydrogenase; AADC acetoacetate decarboxylase; ACS acetyl-CoA synthase; ADH alcohol dehydrogenase; AK acetate kinase; ALDC acetolactate decarboxylase; ALDH aldehyde dehydrogenase; ALDO fructose biphosphate aldolase; ALS acetolactate synthase; BK butyrate kinase; BPG 1,3-bisphosphoglycerate; CoAT CoA transferase; CFeSP corrinoid iron–sulfur protein; DHA dihydroxyacetone; DhaB glycerol dehydratase; DhaD glycerol dehydratase; DhaK DHA kinase; DHAP dihydroxyacetone phosphate; DhaT 1,3-propanediol oxidoreductase; ECH energy-converting hydrogenase complex; ENO enolase; F6P fructose 6-phosphate; FBP fructose 1,6-bisphosphate; Fd ferredoxin; FDH formate dehydrogenase; FL formate-H₂ lyase; FTS formyl-THF synthase; G3P glyceraldehyde 3-phosphate; G6P glucose 6-phosphate, GAPDH glyceraldehyde phosphate dehydrogenase; GK hexokinase; GPI phosphoglucose isomerase; HYD hydrogenase; HYDABC(D) electron-bifurcating hydrogenase; LDH lactate dehydrogenase; MTC methenyl-THF cyclohydrolase; MTD methylene-THF dehydrogenase; MTR methyl transferase; MTRS methylene-THF reductase; NAD(P)FOR NAD(P)H:Ferredoxin oxidoreductase; NFN electron-bifurcating transhydrogenase; PFK-1 phosphofructokinase; PFOR pyruvate:ferredoxin oxidoreductase; PGK phosphoglycerate kinase; PGM phosphoglycerate mutase; PEP phosphoenolpyruvate; PK pyruvate kinase; PTA phosphotransacetylase; PTB phosphotransbutyrylase; RNF Rnf complex; TPI triosephosphate isomerase