Acetogenesis is a process through which acetyl-CoA or acetic acid is produced by prokaryote microorganisms either by the reduction of CO2 or by the reduction of organic acids, rather than by the oxidative breakdown of carbohydrates or ethanol, as with acetic acid bacteria. Acetyl-CoA can be incorporated into biomass or converted to acetic acid. The different bacterial species capable of acetogenesis are collectively termed acetogens. Reduction of CO2 to acetate by anaerobic bacteria occurs via the Wood–Ljungdahl pathway and requires an electron source (e.g., H2, CO, formate, etc.). Some acetogens can synthesize acetate autotrophically from carbon dioxide and hydrogen gas. Reduction of organic acids to acetate by anaerobic bacteria occurs via fermentation.

Discovery

In 1932, organisms were discovered that could convert hydrogen gas and carbon dioxide into acetic acid. The first acetogenic bacterium species, Clostridium aceticum, was discovered in 1936 by Klaas Tammo Wieringa. A second species, Moorella thermoacetica, attracted wide interest because of its ability, reported in 1942, to convert glucose into three moles of acetic acid.

Biochemistry

The precursor to acetic acid is the thioester acetyl CoA. The key aspects of the acetogenic pathway are several reactions that include the reduction of carbon dioxide to carbon monoxide (CO) and the attachment of CO to a methyl group (–CH3) and coenzyme A. The first process is catalyzed by enzymes called carbon monoxide dehydrogenase. The coupling of the methyl group (provided by methylcobalamin), the CO, and the coenzyme A is catalyzed by acetyl-CoA synthase. The global reduction reaction of into acetic acid by H2 is the following: The conversion of one mole of glucose into three moles of acetic acid is also a thermodynamically favorable reaction: However, what matters for the cell is how much ATP is generated. This depends on the substrate.

Applications

The unique metabolism of acetogens has significance in biotechnological uses. In carbohydrate fermentations, the decarboxylation reactions end in the conversion of organic carbon into carbon dioxide. In the production of biofuels, the need to reduce emissions as well as the need to be competitive mean that this inefficiency should perhaps be eliminated by using acetogens. Acetogenesis does not replace glycolysis with a different pathway, but rather captures the CO2 from glycolysis and uses it to produce acetic acid. Although three molecules of acetic acid can be produced in this way, production of three molecules of ethanol would require an additional reducing agent such as hydrogen gas.