Syngas-fermenting microorganisms such as C. Ijungdahlii (Phillips et al., 1994), C. carboxydivorans, C. autoethanogenum, and B. methylotrophicum (Bredwell et al., 1999) follow the acetyl-CoA pathway (sometimes referred to as Wood-Ljungdahl Pathway) to produce biofuels (Henstra et al., 2007). Microorganisms that produce the intermediate acetyl-CoA from carbonyl or carboxyl precursors are known as acetogens (Brown, 2006). Though many acetogenic microbes produce acetate from alcohols and fatty acids, some are capable of produc­ing organic acids and alcohols using CO2 and H2 (autotrophic acetogens) or CO (unicarbo — notrophic acetogens) as their substrates.

Figure 1 shows the simplified acetyl-CoA pathway leading to the production of bio-based products such as ethanol, butanol, and butyrate and acetic acids from syngas. The essential reducing equivalents (-CO, — CoA, — Co-CH3) are produced from H2 and CO by hydrogenase and CO dehydrogenase (CODH) enzymes, respectively. In addition, the bifunctional CODH enzyme produces a carbonyl group from the reaction of carbon dioxide and water (Henstra et al., 2007). The produced reducing equivalents are then converted to acetyl-CoA by acetyl — CoA synthase (ASC) complex.

During the metabolic pathway, intermediate acetyl-CoA performs two major roles—it acts as a precursor for the cell macromolecule, and it serves as an energy source. It is essential to maintain a strict anaerobic environment during the acetyl-CoA pathway to avoid the con­sumption of reducing equivalents by other metabolic pathways (e. g., aerobic respiration). After several successive reactions, CO2 is reduced to a methyl (-CH3) group with the expense of 6 electrons and adenosine triphosphate (ATP). The produced methyl groups then react with the coenzyme and produce — Co-CH3. During the later stage of the acetyl-CoA pathway,

the produced metabolites (-C0-CH3, — CoA) react with CO to produce acetate. The enzyme complex—acetyl-CoA synthase enhances the reaction rate. This reaction recovers the meta­bolic energy invested during the early stages of the pathway. Acetate is further reduced to produce ethanol.