Metabolism of wild-type butanol-producing clostridia A typical characteristic of butanol production by clostridia is biphasic fermentation, as shown in Fig. 1, i.e., comprising an acid ogenic phase and a solventogenic phase (27).
Hexose and pentose sugars are metabolized to pyruvate via different pathways (26). Hexose sugars are degraded to pyruvate by the EmbdeneMeyerhofeParnas (EMP) pathway.
Through the EMP pathway, 1 mol of hexose yields 2 mol of pyruvate with a net production of 2 mol each of ATP and NADH (28).
In contrast, pentose sugars are metabolized via the pentose phosphate(PP)/glycolic pathway.
Fructose 6-phosphate and xylulose 5-phosphate are converted to glyceraldehyde 3-phosphate by transaldolase and transketolase, and the glyceraldehyde 3-phosphate then enters the EMP pathway.
During this process, the conversion of 3 mol of pentose sugars yields 5 mol of pyruvate, 5 mol of ATP, and 5 mol of NADH (29).
After pyruvate formation, the strains enter an exponential growth phase with active hydrogen evolution and acetate and butyrate production, which is called the acidogenic phase.
The acetyl-coenzyme A (A-CoA) node serves as an important switch point in the acid- and solvent-producing pathways(30).
Acetate is converted from A-CoA by phosphotransacetylase (pta) and acetate kinase (ack).
Phosphotransbutyrylase (ptb) and butyrate kinase (buk) are responsible for butyrate formation.
The accumulation of acids leads to a decrease in the external pH in the
fermentation broth.
Consequently, organic acids are reassimilated into solvents of mainly acetone, butanol, and ethanol at a typical ratio of 3:6:1 (31), which is named as solventogenic phase.
Simultaneously, the external pH increases because of acid uptake.
Metabolism of wild-type butanol-producing clostridia A typical characteristic of butanol production by clostridia is biphasic fermentation, as shown in Fig. 1, i.e., comprising an acid ogenic phase and a solventogenic phase (27).
Hexose and pentose sugars are metabolized to pyruvate via different pathways (26). Hexose sugars are degraded to pyruvate by the EmbdeneMeyerhofeParnas (EMP) pathway.
Through the EMP pathway, 1 mol of hexose yields 2 mol of pyruvate with a net production of 2 mol each of ATP and NADH (28).
In contrast, pentose sugars are metabolized via the pentose phosphate(PP)/glycolic pathway.
Fructose 6-phosphate and xylulose 5-phosphate are converted to glyceraldehyde 3-phosphate by transaldolase and transketolase, and the glyceraldehyde 3-phosphate then enters the EMP pathway.
During this process, the conversion of 3 mol of pentose sugars yields 5 mol of pyruvate, 5 mol of ATP, and 5 mol of NADH (29).
After pyruvate formation, the strains enter an exponential growth phase with active hydrogen evolution and acetate and butyrate production, which is called the acidogenic phase.
The acetyl-coenzyme A (A-CoA) node serves as an important switch point in the acid- and solvent-producing pathways(30).
Acetate is converted from A-CoA by phosphotransacetylase (pta) and acetate kinase (ack).
Phosphotransbutyrylase (ptb) and butyrate kinase (buk) are responsible for butyrate formation.
The accumulation of acids leads to a decrease in the external pH in the
fermentation broth.
Consequently, organic acids are reassimilated into solvents of mainly acetone, butanol, and ethanol at a typical ratio of 3:6:1 (31), which is named as solventogenic phase.
Simultaneously, the external pH increases because of acid uptake.
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