acetogenesis
{{Short description|Biosynthesis of acetic acid by prokaryotes}}
Acetogenesis is a process through which acetyl-CoA or acetic acid is produced by anaerobic bacteria through the reduction of {{CO2}} via the Wood–Ljungdahl pathway. Other microbial processes that produce acetic acid (like certain types of fermentation or the oxidative breakdown of carbohydrates or ethanol by acetic acid bacteria) are not considered acetogenesis. The diverse bacterial species capable of acetogenesis are collectively called acetogens.
Reduction of {{CO2}} to acetic acid via the Wood–Ljungdahl pathway requires an electron source (e.g., H2, CO, formate, etc.).{{cite book | last1 = Singleton | first1 = Paul | name-list-style = vanc | title = Dictionary of microbiology and molecular biology | date = 2006 | publisher = John Wiley | location = Chichester | isbn = 978-0-470-03545-0 | edition = 3rd | chapter = Acetogenesis }} When acetogens are grown autotrophically, they synthesize acetic acid only through the Wood–Ljungdahl pathway; but when they are grown heterotrophically, they can produce additional acetic acid by oxidation of the carbon source (carbohydrates, organic acids, or alcohols). Once produced, acetyl-CoA can be incorporated into biomass or converted to acetic acid.
== 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,{{cite journal | vauthors = Ragsdale SW, Pierce E | title = Acetogenesis and the Wood-Ljungdahl pathway of CO2 fixation | journal = Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics | volume = 1784 | issue = 12 | pages = 1873–98 | date = December 2008 | pmid = 18801467 | pmc = 2646786 | doi = 10.1016/j.bbapap.2008.08.012 }} a process called homoacetate fermentation.
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 ({{CO2}}) to carbon monoxide (CO) and the attachment of CO to a methyl group (–CH{{sub|3}}) 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.{{cite journal | vauthors = Ragsdale SW | title = Metals and their scaffolds to promote difficult enzymatic reactions | journal = Chemical Reviews | volume = 106 | issue = 8 | pages = 3317–37 | date = August 2006 | pmid = 16895330 | doi = 10.1021/cr0503153 }}
The global reduction reaction of {{CO2}} into acetic acid by {{chem2|H2}} is the following:
: {{chem2|2 CO2 + 4 H2 → CH3COOH + 2 H2O}} {{Spaces|7}} ΔG° = −95 kJ/mol
The conversion of one mole of glucose into three moles of acetic acid is also a thermodynamically favorable reaction:
: {{chem2|C6H12O6 → 3 CH3COOH}} {{Spaces|25}} ΔG° = −310.9 kJ/mol
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 {{CO2}} emissions, as well as the need to be competitive, means 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.{{cite journal | vauthors = Schuchmann K, Müller V | title = Energetics and Application of Heterotrophy in Acetogenic Bacteria | journal = Applied and Environmental Microbiology | volume = 82 | issue = 14 | pages = 4056–69 | date = July 2016 | pmid = 27208103 | doi = 10.1128/AEM.00882-16 | pmc = 4959221 | bibcode = 2016ApEnM..82.4056S }}
== References ==
{{Reflist|2}}