Thermosynechococcus is also naturally transformable. The metabolic pathway of
ethanol synthesis is briefly summarized: After fixation of inorganic carbon by
Calvin cycle, it forms phosphoglycerate that is converted into pyruvate by two
enzymes (pyruvate decarboxylase and alcohol dehydrogenase), and finally into
ethanol. Therefore, the “photofermentation” process for obtaining ethanol includes
two stages: photosynthesis and fermentation. Each stage has its key factors that
determine the efficiency of the process and the metabolic needs of the cyanobacteria. In any case, this route requires the use of genetically modified
microorganisms.
Figures 3 and 4 present the schematic diagram of the assumed fermentative
pathways operating in dark-incubated wild type Chlamydomonas reinhardtii and
mutant PFL1-deficient strain 48F5 (Philipps et al. 2011). In fermenting C. reinhardtii wild type cells (CC-124), pyruvate from glycolytic glucose oxidation, serves
as substrate for several enzymes. Pyruvate formate lyase (PFL1) cleaves pyruvate
into formate and acetyl CoA. Acetyl CoA is converted to acetate by the successive
action of phosphotransacetylase (PTA) and acetate kinase (ACK), resulting in ATP
production, or to ethanol by a bifunctional aldehyde/alcohol dehydrogenase
(ADH1), resulting in oxidation of NAD(P)H. Pyruvate decarboxylase (PDC)
decarboxylates pyruvate yielding acetaldehyde, which is further reduced to ethanol
by alcohol dehydrogenase (ADH). Another pathway leads to D-lactate production
by the action of D-lactate dehydrogenase (D-LDH). Pyruvate ferredoxin oxidoreductase (PFR1) oxidatively decarboxylates pyruvate, resulting in reduced ferredoxin (FDX), CO 2 , and acetyl CoA. The latter can probably be metabolized by PTA
and ACK or ADH1 (indicated by a dotted line). Reduced FDX could then function
as an electron donor for the hydrogenase (HYD1), resulting in hydrogen evolution
in the dark.
Fig. 3 Fermentative pathways of wild type Chlamydomonas reinhardtii and mutant
PFL1-deficient strain 48F5
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241
ethanol synthesis is briefly summarized: After fixation of inorganic carbon by
Calvin cycle, it forms phosphoglycerate that is converted into pyruvate by two
enzymes (pyruvate decarboxylase and alcohol dehydrogenase), and finally into
ethanol. Therefore, the “photofermentation” process for obtaining ethanol includes
two stages: photosynthesis and fermentation. Each stage has its key factors that
determine the efficiency of the process and the metabolic needs of the cyanobacteria. In any case, this route requires the use of genetically modified
microorganisms.
Figures 3 and 4 present the schematic diagram of the assumed fermentative
pathways operating in dark-incubated wild type Chlamydomonas reinhardtii and
mutant PFL1-deficient strain 48F5 (Philipps et al. 2011). In fermenting C. reinhardtii wild type cells (CC-124), pyruvate from glycolytic glucose oxidation, serves
as substrate for several enzymes. Pyruvate formate lyase (PFL1) cleaves pyruvate
into formate and acetyl CoA. Acetyl CoA is converted to acetate by the successive
action of phosphotransacetylase (PTA) and acetate kinase (ACK), resulting in ATP
production, or to ethanol by a bifunctional aldehyde/alcohol dehydrogenase
(ADH1), resulting in oxidation of NAD(P)H. Pyruvate decarboxylase (PDC)
decarboxylates pyruvate yielding acetaldehyde, which is further reduced to ethanol
by alcohol dehydrogenase (ADH). Another pathway leads to D-lactate production
by the action of D-lactate dehydrogenase (D-LDH). Pyruvate ferredoxin oxidoreductase (PFR1) oxidatively decarboxylates pyruvate, resulting in reduced ferredoxin (FDX), CO 2 , and acetyl CoA. The latter can probably be metabolized by PTA
and ACK or ADH1 (indicated by a dotted line). Reduced FDX could then function
as an electron donor for the hydrogenase (HYD1), resulting in hydrogen evolution
in the dark.
Fig. 3 Fermentative pathways of wild type Chlamydomonas reinhardtii and mutant
PFL1-deficient strain 48F5
11 Biofuels from Microalgae: Bioethanol
241