2,3-butanediol fermentation

Structural formula of 2,3-butanediol, the eponymous end product of 2,3-butanediol fermentation

Gene Ontology
Parent
Glycolytic fermentation D- glucose metabolism
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The 2,3-butanediol fermentation is a way of breaking down sugars for energy production under anoxic conditions, which occurs in some facultative anaerobic bacteria , especially in some genera of enterobacteria . The sugar breaks down in different ways and a number of end products are formed, mainly carbon dioxide (CO ₂ ), 2,3-butanediol , ethanol and formate ( anion of formic acid ). Formate can also be completely or partially split into elemental , molecular hydrogen (H ₂ ) and carbon dioxide (CO ₂ ). In addition, other end products are often formed in smaller quantities, e.g. B. lactate and acetate . Characteristic is the formation of the eponymous organic compound 2,3-butanediol in larger quantities. Compared to mixed acid fermentation , gases are also formed in larger quantities, but acids only in small quantities. The butanediol fermentation is one of the two forms of formic acid fermentation, in addition to the mixed acid fermentation .

Fermentation process

Scheme of the course of 2,3-butanediol fermentation, please see text for details. In contrast to mixed acid fermentation, much fewer acids, especially D- lactate and acetate, are released. Therefore, these products are lightened in the scheme.

Hexoses are typically broken down to pyruvate via glycolysis , with ATP being produced by substrate chain phosphorylation . To a small extent, these are also converted to pyruvate via the Entner-Doudoroff route (ED route). When degraded, NAD ^{+ is} reduced to NADH . So that this is ready for further rounds of glycolysis or the ED pathway, it is reoxidized to NAD ⁺ by the intermediate products formed in the further course of fermentation . In contrast to mixed acid fermentation, 2,3-butanediol fermentation produces fewer acids, more carbon dioxide and the eponymous butanediol.

The ratio of the mass flow of the individual routes and thus the mass ratio of the end products can vary. In Klebsiella aerogenes (formerly Enterobacter aerogenes ), the amount of organic compounds produced was measured. One mole of glucose is converted into:

${\ displaystyle \ mathrm {Glucose \ longrightarrow 0.03 \ Lactate + 0.01 \ Acetate + 0.70 \ Ethanol}}$ ${\ displaystyle \ mathrm {+ \ 0.66 \ butanediol + 0.18 \ formate + 1.72 \ CO_ {2} +0.36 \ H_ {2}}}$

biochemistry

Formation of butanediol

Two molecules of pyruvate condense into acetyl lactate , releasing carbon dioxide. This reaction is catalyzed by a thiamine pyrophosphate- dependent acetolactate synthase. Another decarboxylation is carried out by an acetyl lactate decarboxylase . The product is acetoin . This is finally reduced to 2,3-butanediol with consumption of NADH, which catalyzes a butanediol dehydrogenase .

The two decarboxylations produce a lot of carbon dioxide gas. In addition, strong acids having a are from two molecules of pyruvate, pK _S value of each of 3.7, two molecules of carbon dioxide (pK _S = 6.3) and the neutral compound is formed butanediol. As a result, the medium is less acidic than when compared to mixed acid fermentation.

Formation of formate, hydrogen and carbon dioxide

Pyruvate can be split into acetyl-CoA and formate by the enzyme pyruvate formate lyase (PFL) with the inclusion of coenzyme A. PFL is the key enzyme of this mixed acid fermentation which is only formed under anoxic conditions. Under these conditions it replaces pyruvate dehydrogenase . Most of the formate is excreted by the bacteria. If a suitable electron acceptor is present, it is oxidized to carbon dioxide in the course of fumarate respiration by a membrane-based formate dehydrogenase, with the electrons being transferred to menaquinone. If this possibility no longer exists and the pH value of the medium drops, formate is no longer excreted and is split into CO ₂ and H ₂ by a cytosolic formate hydrogen lyase . During this process, the bound reduction equivalents are released as hydrogen gas. Since a strong acid (pK _S = 3.7) in hydrogen gas (neutral) and carbon dioxide (pK _S is reacted = 6.3), formate hydrogen lyase counteracts the acidification of the medium.

Formation of acetate and ethanol

Acetyl-CoA is formed when pyruvate is broken down. The high-energy thioester bond can be preserved by exchanging coenzyme A for phosphate . This reaction is catalyzed by a phosphotransacetylase , acetyl phosphate is formed . An acetate kinase converts this into acetate, with ATP being generated by substrate chain phosphorylation.

Acetyl-CoA can also be reduced to ethanol by means of a coenzyme A-dependent alcohol dehydrogenase , a bifunctional enzyme, by consuming two molecules of NADH. In contrast to other alcohol dehydrogenases, acetaldehyde is not released as an intermediate product. No ATP is generated during this process.

However, the formation of acetate is negligible in 2,3-butanediol fermentation.

Formation of D -lactate

NAD ⁺ can also be reoxidized by reducing pyruvate to D - lactate . This is catalyzed by a D - lactate dehydrogenase , in contrast to lactic acid fermentation , this creates the D- isomer . The formation of D -lactate is also negligible.

Occurrence

Of the by far most types of the following enterobacteria , sugars are broken down during 2,3-butanediol fermentation: Enterobacter , Klebsiella , Serratia , Erwinia .

meaning

The breakdown of sugars via 2,3-butanediol fermentation is a taxonomic feature that is used to identify bacteria, especially enterobacteria. Whether this degradation path exists is determined by checking for the intermediate product acetoin, because this is characteristic of 2,3-butanediol fermentation. The Voges-Proskauer reaction serves as evidence for acetoin . In contrast to the enterobacteria, which produce many acids through mixed acid fermentation, the detection with the methyl red sample is negative. The increased gas formation of carbon dioxide is noticeable in the volumetric determination.

The 2,3-butanediol fermentation is also gaining some importance in the food industry . The acetoin produced by some lactic acid bacteria is converted to diacetyl through oxidation , which is a main aromatic component in butter .

literature

Georg Fuchs (Ed.): General Microbiology (started by Hans G. Schlegel). 8th edition. Georg Thieme Verlag, Stuttgart, New York 2007, ISBN 978-3-13-444608-1 .

Individual evidence

↑ ^a ^b ^c Garabed Antranikian: Applied Microbiology . Springer, Berlin 2006; ISBN 978-3-540-24083-9 ; P. 65f.
↑ ^a ^b ^c Georg Fuchs (Ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart; 8th edition 2007; ISBN 3-13-444608-1 ; P. 366f.
↑ ^a ^b Katharina Munk (ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 379ff.
↑ Michael T. Madigan and John M. Martinko: Brock Mikrobiologie . Pearson Studies; 11., revised. Edition 2006; ISBN 978-3-8273-7187-4 ; P. 397.
↑ Wolfgang Fritsche: Microbiology . Spectrum Academic Publishing House; 3rd edition 2001; ISBN 978-3827411075 ; P. 243f.

[Antranikian-1] Garabed Antranikian: Applied Microbiology . Springer, Berlin 2006; ISBN 978-3-540-24083-9 ; P. 65f.

[Fuchs-2] Georg Fuchs (Ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart; 8th edition 2007; ISBN 3-13-444608-1 ; P. 366f.

[Munk-3] Katharina Munk (ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 379ff.

[Brock-4] Michael T. Madigan and John M. Martinko: Brock Mikrobiologie . Pearson Studies; 11., revised. Edition 2006; ISBN 978-3-8273-7187-4 ; P. 397.

[5] Wolfgang Fritsche: Microbiology . Spectrum Academic Publishing House; 3rd edition 2001; ISBN 978-3827411075 ; P. 243f.