Table 1.
Possible glucose fermentation pathways to VFA (Ac-, Pr- and Bu- for acetate, propionate and butyrate, respectively), volatile fatty acid (VFA) interconversions, hydrogenase-catalyzed NADH oxidation and methanogenesis in the rumen and their yield of ATP (YATP), number of NADH to be oxidized with H2 formation (YNADH, mol per mol of glucose), the standard reaction Gibbs energy (ΔGo in kJ⋅mol−1, standardized to concentrations of 1 M, pH of 0, gas pressure of 1 bar) adjusted to 312K, and the average stoichiometric number χ.
Fig 1.
Thermodynamic potential factor (FT) as a function of PH2 for glucose fermentation pathways and methanogenesis.
The black line for glucose to VFA is valid for the reactions a to d (yielding acetate, propionate or butyrate), the solid and dotted gray lines represent methanogenesis with 0.5 and 1.5 mol of ATP per mol of CH4, respectively; a more detailed description of the glucose fermentation pathways to VFA and methanogenesis is given in Table 1. Confidence intervals represent uncertainty of FT to variation in inputs other than PH2. Vertical lines demarcate the rumen physiological range of PH2. A log scale is used to plot the x-axis.
Fig 2.
Thermodynamic potential factor (FT) as a function of PH2 for a) NADH oxidation without ferredoxin oxidation and b) NADH oxidation with ferredoxin oxidation and the FdRED2- to FdOX ratio constant at 9.
Line type represents NAD+ to NADH ratio and line color represents intracellular pH equal to 6.2 (gray) and 7.0 (black). Vertical lines demarcate the rumen physiological range of PH2. A log scale is used to plot the x-axis.
Fig 3.
Thermodynamic potential factor (FT) as a function of PH2 for VFA interconversions.
Conversions comprise a) acetate to propionate, b) propionate to acetate, c) butyrate to acetate. The 95% confidence intervals represent uncertainty of FT to variation in inputs other than PH2. Vertical lines demarcate the rumen physiological lower bound of PH2. A log scale is used to plot the x-axis.