(A) Maximum-minimum thermodynamic driving force (MDF) for each carbon source for growth-only and production-only extremes without complete carbon conversion (i.e. carbon dioxide excretion allowed).

For all carbon sources except carbon dioxide, hydrogen exchange was blocked so that energy and carbon could be derived only from the given carbon source. Positive MDFs - required for thermodynamic feasibility - are possible for all carbon sources. (B) MDF comparison for PHB production with forced complete carbon conversion (‘Complete’) and incomplete carbon conversion (‘Incomplete’). In the complete conversion case, carbon dioxide excretion was blocked and hydrogen exchange was unconstrained, as in Fig 1C . Complete conversion is thermodynamically feasible for all carbon sources, but incomplete conversion is more thermodynamically favourable for all but acetate and butyrate. (C) Flux distributions for acetate transformation to PHB with incomplete carbon conversion (top panel) and complete carbon conversion (bottom panel) with relative flux indicated. In the unforced scenario, 40% of assimilated acetate must be converted to reducing equivalents via the carbon dioxide-producing TCA cycle. The requirement of fumarase (‘FUMC’) and malate dehydrogenase (‘MDH’) activities for energy generation yields an MDF of of 4.4 kJ/mol. All reactions shown in red operate at the MDF. In the complete conversion scenario, hydrogen oxidation provides sufficient reducing power for all of the assimilated acetate to be converted to PHB with no carbon dioxide generation, and therefore no reassimilation required. This reduces the thermodynamic constraint, allowing for operation at an MDF of 6.5 kJ/mol. ACS: acetyl-CoA synthase; PHAA: acetoacetyl-CoA thiolase; PHAB: (R)-3-Hydroxybutanoyl-CoA:NADP+ oxidoreductase; PHACE: PHB synthase; ADK: adenylate kinase; AcCoA: acetyl-CoA; AcAcCoA: acetoacetyl-CoA; 3HBCoA: (R)-3-hydroxybutyryl-CoA.