Fig. 1

Stoichiometry and thermodynamics of fermentation in human gut butyrogens. A Generic fermentation pathways in human gut butyrogens that can yield combinations of H₂, CO₂, formate, lactate, acetate, and butyrate (adapted from Louis and Flint, 2017 [18]). Fd ferredoxin, LDH lactate dehydrogenase, PFOR pyruvate:ferredoxin oxidoreductase, PFL pyruvate formate-lyase, Ack acetate kinase, Hyd ferredoxin hydrogenase, Bhbd hydroxybutyryl-CoA dehydrogenase, Etf-Bcd butyryl-CoA dehydrogenase electron-transferring flavoprotein complex, But butyryl-CoA:acetyl CoA transferase, Rnf complex. The division of acetyl-CoA between acetate production (green box) and butyrate production (blue box) is variable; an equal division is shown as a representative case with simple stoichiometry. The division of pyruvate between the PFOR and PFL routes of acetyl-CoA formation is also variable. Stoichiometry is balanced from glucose to either lactate (orange box) or to butyrate and acetate, via either PFOR or PFL. B Illustration of the effect of [H₂] on ∆G of glucose fermentation to butyrate and acetate. A range of possible fermentation balances are shown, with the net molar production or consumption of butyrate (but), acetate (ace), and H₂ per glucose indicated. Dotted horizontal lines indicate theoretical ∆G thresholds for formation of a given number of ATP, assuming ∆G of ADP phosphorylation as + 70 kJ/mol [19]. Dashed line indicates the least butyrate formation likely to be feasible for human gut butyrogens. Representative physiological conditions were used as indicated. See Supplementary Table 4 for complete reactions and thermochemical parameters