# Homoacetate fermentation

Homoacetate fermentation is the bacterial conversion of sugars into acetic acid under anoxic conditions without the inclusion of oxygen (O 2 ) or other external electron acceptors . Because no external electron acceptor ( oxidant ) is used, it is a "real" fermentation (in contrast to acetic acid fermentation by acetic acid bacteria ).

Homoacetate fermentation is exergonic and serves the bacteria as an energy source . Homoacetate fermenters are certain species of the bacterial genus Clostridium , for example Clostridium aceticum , Clostridium thermoaceticum and Clostridium formicoaceticum , but also acetogenic bacteria such as Moorella thermoacetica .

## Sum equation and course

The reductive acetyl-CoA route, please see text for details. The metabolic pathway is not a cycle, but the coenzymes involved are regenerated in smaller cycles.

The sum equation for the complicated conversion of D- glucose to acetate , the deprotonated form of acetic acid, takes place over several intermediate stages :

${\ displaystyle \ mathrm {Glucose + 4 \ ADP + 4 \ P_ {i} \ longrightarrow 3 \ Acetate + 4 \ ATP}}$

Glucose is broken down into two molecules of pyruvate in what is known as " glycolysis " . These are then oxidatively decarboxylated so that - with consumption of coenzyme A - acetyl-CoA and formally hydrogen are formed. Acetate is finally produced via the intermediate stage acetyl phosphate with ATP gain, which is excreted and not further broken down via the citric acid cycle, as is the case with oxidative sugar breakdown . These processes produce two of the three acetate molecules.

The special thing about homoacetate fermentation is the formation of the third acetate molecule, which is known as the reductive acetyl-CoA route (see there). It is built up reductively from the two carbon dioxide molecules (CO 2 ) split off from the pyruvate . A CO 2 molecule is reduced to carbon monoxide (CO) by an enzyme . The second CO 2 molecule is gradually reduced to a methyl group (-CH 3 ), with the coenzyme tetrahydrofolic acid playing an important role. This methyl group is combined with the carbon monoxide via a cobalamin enzyme and the product is split off as acetyl-CoA. As described above, acetyl-CoA is converted to acetate, with ATP being generated. For reductions that also formed in glycolysis are reducing agent used.

Since the hydrogen previously formed is used up again in this second process of acetate formation, one speaks of a “ syntrophy within a cell”.

## Acetic acid formation from CO 2 and H 2

Homoacetate fermenters which have the enzyme hydrogenase , e.g. B. Clostridium aceticum , can also gain energy from the conversion of carbon dioxide and elemental hydrogen (H 2 ) to acetic acid. The sum equation for this is:

${\ displaystyle \ mathrm {2 \ CO_ {2} +4 \ H_ {2} \ longrightarrow CH_ {3} COOH + 2 \ H_ {2} O}}$

The change in free energy under standard conditions , at pH = 7, is ΔG 0 '= −112 kJ / mol for this reaction .

These bacteria are therefore chemolithotrophic , which means that they can use an inorganic substance (here H 2 ) as an electron donor ( reductant ) . They are also carbon autotrophic , which means that they can meet their carbon needs for growth from carbon dioxide (CO 2 ) alone . This conversion is not fermentation, because an external electron acceptor (CO 2 ) and an external electron donor (H 2 ) are consumed during the conversion .