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Fig. 5 | Biotechnology for Biofuels

Fig. 5

From: Continuous n-valerate formation from propionate and methanol in an anaerobic chain elongation open-culture bioreactor

Fig. 5

The hypothetically proposed mechanism for methanol-based propionate elongation to n-valerate [49]. Within the Wood–Ljungdahl pathway, one methanol is oxidized via the THF route to formate/CO, whilst another methanol is supplied to the ACS complex via a CH3–[Co]-enzyme intermediate [45, 46]. The ACS complex then catalyzes the formation of acetyl-CoA. Depending on the intracellular potential, formate could either be directly utilized for the formation of CO (dotted line) [46], or alternatively CO formation would require the bifurcating hydrogenase as well as an Rnf complex to balance the redox compounds (dashed line) [45]. The formed acetyl-CoA is then likely used in a thiolase-driven condensation step with propionyl-CoA to form 3-ketopentanoyl-CoA, similar to the reverse beta-oxidation mechanism in C. kluyveri [6]. The two NADHs generated during the oxidation of methanol are subsequently used to reduce 3-ketopentanoyl-CoA to 3-hydroxypentanoyl-CoA and to reduce pent-2-enoyl-CoA to pentanoyl-CoA. Because the methanol-based chain elongation of propionate to n-valerate (Table 1R3) has a ΔG of − 106.1 kJ/reaction, an ATP yield of 1.5 ATP would be expected (106.1 kJ/~ 70 kJ/ATP [50] = 1.5 ATP). This suggests that additional energy would be gained via a proton/Na+ motive force (pmf) that is likely generated at the oxidation of CH3-THF [50]. Potentially, additional bifurcation steps within the reverse beta-oxidation part might be necessary, depending on the intracellular redox potentials of the redox cofactors [50, 51]

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