Clostridial acidogenesis product profiles are discontinuous: a thermodynamic hypothesis

At neutral pH, Clostridial fermentative catabolism is typically acidogenic, with a product profile dominated by acetate and butyrate. H2 acts as a terminal electron acceptor via hydrogenases, which increases ATP-producing potential from glucose. Acetate production is characterised by both higher ATP and H2 yields, rendering it desirable but more thermodynamically limited. For this reason, is commonly understood that Clostridia can adjust the ratio of acetate to butyrate (Ace:But) produced to maximise ATP while maintaining sufficient pathway driving forces to sustain a high flux. We identify three redox-balanced product profiles that underlie the spectrum of Clostridial catabolic Ace:But ratios: Homoacetic (Ace:But = 2:0), Equimolar (0.67:0.67), and Homobutyric (0:1). To reach Ace:But ratios intermediate to these, the elementary flux modes (EFMs) underlying the aforementioned product profiles must be blended. We performed a maximum-minimum driving force (MDF) analysis to test the thermodynamic favourability of the pathways underlying different Ace:But ratios at varying H2 partial pressures (pH2). We find that blended EFMs are less efficient than their constituent EFMs at all pH2, allocating excessive driving force (DF) to certain reactions, thereby lowering the DF of others. This is, in part, due to the co-occurrence of hydrogenases with different optimal redox carrier ratios. One hydrogenase inevitably has very high DF, which decreases the DF available for other reactions. This leads to a lower minimum DF and a higher enzyme cost for operating blended EFMs. This implies that certain discrete Ace:But ratios are most favourable for large ranges of pH2, contradicting the continuity assumption in literature.