Mixed acid fermentation
The Mixed acid fermentation is a way of reducing sugars for energy under anoxic conditions, which occurs in some facultative anaerobic bacteria, especially in some genera of Enterobacteriaceae. The degradation of the sugar proceeds in different ways and it will be a number of end-products formed, essentially lactate, acetate, carbon dioxide (CO2), elemental, molecular hydrogen (H2), ethanol, and succinate. A characteristic intermediate is formate (anion of formic acid ), which is almost completely split into H2 and CO2, but sometimes can also be partially or completely eliminated. The Mixed acid fermentation is one of the two forms of formic acid fermentations.
Course of fermentation
Hexoses are typically degraded via the path of glycolysis to pyruvate, which is produced by substrate chain ATP. The small part of this via the Entner- Doudoroff pathway (ED pathway) are converted to pyruvate. In the degradation of NAD is reduced to NADH. So that this is ready for further rounds of glycolysis or the ED pathway, it is reoxidized in the formed in the course of the fermentation intermediates ( pyruvate, acetyl, oxaloacetate and fumarate) back to NAD . In contrast to the homofermentative lactic acid fermentation occur in the Mixed acid fermentation of a mixture of various organic compounds. Depending on the organism and environment situation, the present proportions differ. In Escherichia coli, the amount of produced organic compounds was measured. One mole of glucose is converted to this:
A characteristic key enzyme of this type fermentation is pyruvate formate - lyase, it is formed only under anaerobic conditions.
Biochemistry
Formation of succinate
Oxaloacetate ( OA) is one of the possible acceptors for electrons from NADH. This can be formed by carboxylation of PEP, which catalyzes a phosphoenolpyruvate carboxylase ( PEPC ). OA is then reduced by malate dehydrogenase to L- malate, and this reacted by elimination of water to fumarate, which is a fumarase catalyzed. A fumarate finally reduced fumarate into succinate.
Fumarate reductase is a membrane bound enzyme, which transfers electrons from the menaquinol pool ( MQH2 ) to the fumarate. Menaquinol is either generated by a NADH dehydrogenase, NADH oxidized and menaquinone ( MQ ) is reduced. Alternatively, the electrons could also be of a membrane-bound formate dehydrogenase derived, which oxidizes extracellular formate to carbon dioxide. NADH dehydrogenase ( and also the formate dehydrogenase ) translokieren protons from the cytosol to the outside, thereby forming a proton-motive force Δμ (H ) is established.
In the literature, the use of fumarate is referred to as a pure electron as Fumaratatmung.
The formation of D-lactate
NAD can also be reoxdiert by pyruvate to lactate is reduced. This is catalyzed by a D-lactate dehydrogenase, as opposed to lactic acid fermentation, the D- isomer formed thereby.
Formation of formate, hydrogen and carbon dioxide
Pyruvate can be cleaved under inclusion of coenzyme A by the enzyme pyruvate - formate lyase (PFL) in acetyl-CoA and formate. PFL is the key enzyme formed only under anoxic conditions of this Joint acid fermentation. It replaced under these conditions, the pyruvate dehydrogenase. Formate is largely excreted by the bacteria. If a suitable electron acceptor is present, it is oxidized in the course of Fumaratatmung by a membrane bound dehydrogenase formate to carbon dioxide, wherein the electrons are transferred to menaquinone (see section above). If this possibility no longer exists, and decreases the pH of the medium, formate is not excreted and cleaved by a cytosolic formate hydrogen lyase into CO2 and H2. In this operation, the bound reduction equivalents released as hydrogen gas. Since a strong acid (pKa = 3.7 ) in hydrogen gas ( neutral) and carbon dioxide (pKa = 6.3 ) is implemented, is counteracted by the formate hydrogen lyase of acidification of the medium.
Formation of acetate and ethanol
In the cleavage of pyruvate produced acetyl -CoA. The energy of the high-energy thioester bond can be preserved by coenzyme A is replaced by phosphate. This reaction is catalyzed by a phosphotransacetylase, there arises acetyl. An acetate kinase, finally converts it to acetate, which is generated by Substratkettenphosophorylierung ATP.
Acetyl-CoA may also by Coenzyme A-dependent alcohol dehydrogenase, a bifunctional enzyme, can be reduced to ethanol under consumption of two molecules of NADH. In contrast to other alcohol dehydrogenases acetaldehyde is not released as an intermediate. In this operation, no ATP is generated.
Occurrence
Of the vast majority of each of the following species, facultative anaerobic enterobacteria sugars are broken down on the way the Joint acid fermentation: Citrobacter, Edwarsiella, Escherichia, Proteus, Providencia, Salmonella, Shigella, Yersinia, and several other genres. Also, some facultative anaerobic bacillus species result from this form of fermentation.
Importance for the identification of bacteria
The degradation of sugars in the path of the Joint acid fermentation is a taxonomic feature, which is used for the determination of bacteria, especially Enterobacteriaceae. That, for example, this pathway is present in the enteric bacterium E. coli, by detecting the resulting in a large amount acids ( pH reduction) is detected. Another fermentation which 2,3- Butandiolgärung is excluded due to the absence of the characteristic for this fermentation intermediate acetoin, that is, the detection of acetoin by the Voges - Proskauer reaction is negative. The distribution of acetoin can also be, for example, in E. aerogenes, Enterobacter also detected.