Butanediol fermentation
The 2,3- Butandiolgärung 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 runs in different ways and there are a number of end products formed, mainly carbon dioxide ( CO2), 2,3- butanediol, ethanol and formate ( anion of formic acid). Formate can also be fully or partially in elementary, molecular hydrogen (H2) and carbon dioxide (CO2 ) can be split. In addition, other end products are often formed in lesser amounts, such as lactate and acetate. It is characterized by the formation of the eponymous organic compound 2,3- butanediol in larger quantities. Compared to the mixed acid fermentation, gases in large quantities, but acids are formed only in small amounts. The Butandiolgärung is next to the Mixed acid fermentation of the two forms of formic acid fermentations.
Course of fermentation
Hexoses are typically degraded via the path of glycolysis to pyruvate ( the anion of pyruvic acid (2- pyruvic acid ) ), 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 back to NAD . In contrast to the mixed acid fermentation occur in the 2,3- Butandiolgärung less acids, more carbon dioxide and the eponymous butanediol.
The ratio of the mass flow of the individual channels and so that the mass ratio of the final product may vary. In Enterobacter aerogenes means the amount of organic compounds was measured. One mole of glucose is converted to this:
Biochemistry
Formation of butanediol
Two molecules of pyruvate condense to Acetyllactat, which is released in carbon dioxide. This reaction is catalyzed by a thiamin pyrophosphate - dependent acetolactate synthase. Re decarboxylation is carried out by a Acetyllactatdecarboxylase. The product is acetoin. This is finally reduced to 2,3- butanediol with consumption of NADH, which catalyzes a Butandioldehydrogenase.
Through the two decarboxylation occurs much carbon dioxide. In addition, two molecules of pyruvate, strong acids with a pKa of 3.7 in each case, two molecules of carbon dioxide ( pKa = 6.3 ) and the neutral compound formed butanediol. As a result, the medium is acidified weaker as compared to the mixed acid fermentation.
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 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. 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 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.
The formation of acetate in 2,3- Butandiolgärung but negligible.
The formation of D-lactate
NAD can also be reoxdiert by pyruvate is reduced to D-lactate. This is catalyzed by a D-lactate dehydrogenase, as opposed to lactic acid fermentation, the D- isomer formed thereby. The formation of D-lactate is also negligible.
Occurrence
Of the vast majority of each of the following types of enterobacteria sugars are broken down on the road, the 2.3 - Butandiolgärung: Enterobacter, Klebsiella, Serratia, Erwinia.
Importance
The degradation of sugars on the path of 2,3- Butandiolgärung is a taxonomic feature, which is used for the identification of bacteria, especially Enterobacteriaceae. If this pathway is present, it is determined by checking the intermediate acetoin, because it is characteristic of the 2,3- Butandiolgärung. As proof of the Voges - Proskauer acetoin reaction serves. In contrast to the Entereobakterien that produce many acids by mixed acid fermentation, so that the detection by methyl sample is negative. The increased formation of carbon dioxide gas is noticeable in the volumetric determination.
2,3- Butandiolgärung obtained some importance in the food industry. The acetoin formed from some lactic acid bacteria, is converted by oxidation to diacetyl, which is a main component of the butter flavor.