Acetogenesis

Acetogenesis is a process through which acetate is produced 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.^[1]

The different bacterial species that are 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.^[2] 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.^[3]

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 and the attachment of the carbon monoxide to a methyl group. The first process is catalyzed by enzymes called carbon monoxide dehydrogenase. The coupling of the methyl group (provided by methylcobalamin) and the CO is catalyzed by acetyl CoA synthase.^[4]

2 CO₂ + 4 H₂ → CH₃COOH + 2H₂O

Applications

The unique metabolism of acetogens has significance in biotechnological uses. In carbohydrate fermentations, the decarboxylation reactions involved result in the loss of carbon into carbon dioxide. This loss is an issue with an increased requirement of minimization of CO₂ emissions, as well as successful competition for fossil fuels with biofuel production being limited by monetary value. Acetogens can ferment glucose without any CO₂ emissions and convert one molecule into three molecules of acetic acid, increasing product yield by 50%. Acetogenesis does not replace glycolysis with a different pathway, but rather captures the CO₂ from glycolysis and uses it for acetogenesis. Although three molecules of acetic acid can be produced in this way, to produce three molecules of ethanol would require a reducing agent such as hydrogen gas.^[5]

References

1. Angelidaki I, Karakashev D, Batstone DJ, Plugge CM, Stams AJ (2011). "16. Biomethanation and Its Potential". In Rosenzweig AC, Ragsdale SW (eds.). Methods in Enzymology. Methods in Methane Metabolism, Part A. Vol. 494. Academic Press. pp. 327–351. doi:10.1016/B978-0-12-385112-3.00016-0. ISBN 978-0-123-85112-3. PMID 21402222.
2. Singleton P (2006). "Acetogenesis". Dictionary of microbiology and molecular biology (3rd ed.). Chichester: John Wiley. ISBN 978-0-470-03545-0.
3. Ragsdale SW, Pierce E (December 2008). "Acetogenesis and the Wood-Ljungdahl pathway of CO(2) fixation". Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics. 1784 (12): 1873–98. doi:10.1016/j.bbapap.2008.08.012. PMC 2646786. PMID 18801467.
4. Ragsdale SW (August 2006). "Metals and their scaffolds to promote difficult enzymatic reactions". Chemical Reviews. 106 (8): 3317–37. doi:10.1021/cr0503153. PMID 16895330.
5. Schuchmann K, Müller V (July 2016). "Energetics and Application of Heterotrophy in Acetogenic Bacteria". Applied and Environmental Microbiology. 82 (14): 4056–69. Bibcode:2016ApEnM..82.4056S. doi:10.1128/AEM.00882-16. PMC 4959221. PMID 27208103.