Acetogenesis is a process through whichacetyl-CoA[1] oracetic acid is produced byanaerobic bacteria through thereduction ofCO2 via theWood–Ljungdahl pathway. Other microbial processes that produce acetic acid (like certain types offermentation or theoxidative breakdown ofcarbohydrates orethanol byacetic acid bacteria) are not considered acetogenesis.[citation needed] The diversebacterial species capable of acetogenesis are collectively calledacetogens.
Reduction of CO2 to acetic acid via theWood–Ljungdahl pathway requires anelectron source (e.g.,H2,CO,formate, etc.).[2] When acetogens are grownautotrophically, theysynthesize acetic acid only through the Wood–Ljungdahl pathway; but when they are grownheterotrophically, they can produce additional acetic acid by oxidation of the carbon source (carbohydrates, organic acids, or alcohols).[1] Once produced, acetyl-CoA can be incorporated intobiomass or converted to acetic acid.[1]
In 1932, organisms were discovered that could convert hydrogen gas and carbon dioxide intoacetic 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 convertglucose into threemoles of acetic acid,[3] a process called homoacetate fermentation.[1]
The precursor toacetic acid is thethioesteracetyl CoA. The key aspects of the acetogenicpathway are several reactions that include the reduction ofcarbon dioxide (CO2) tocarbon monoxide (CO) and the attachment of CO to amethyl group (–CH3) andcoenzyme A. The first process iscatalyzed byenzymes calledcarbon monoxide dehydrogenase. The coupling of the methyl group (provided bymethylcobalamin), the CO, and the coenzyme A is catalyzed byacetyl-CoA synthase.[4]
The globalreduction reaction of CO2 intoacetic acid byH2 is the following:
The conversion of onemole ofglucose into three moles of acetic acid is also a thermodynamically favorable reaction:
However, what matters for the cell is how muchATP is generated. This depends on thesubstrate.[1]
The uniquemetabolism of acetogens has significance in biotechnological uses. Incarbohydrate fermentations, thedecarboxylation reactions end in the conversion of organic carbon intocarbon dioxide. In the production ofbiofuels, 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 replaceglycolysis 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 ofethanol would require an additionalreducing agent such ashydrogen gas.[1]