Mixed acid fermentation

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Glycolytic fermentation D- glucose metabolism
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The mixed acid fermentation is one way of degradation of sugars for energy under anoxic conditions, in some facultative anaerobic bacteria occurs, in particular in some genera of Enterobacteriaceae . The breakdown of sugars takes place in different ways and a number of end products are formed, mainly lactate , acetate , carbon dioxide (CO ₂ ), elemental , molecular hydrogen (H ₂ ), ethanol and succinate . A characteristic intermediate product is formate (anion of formic acid ), which is almost completely split into H ₂ and CO ₂ , but can sometimes also be partially or completely excreted. Mixed acid fermentation is one of the two forms of formic acid fermentation .

Fermentation process

Scheme of the process of mixed acid fermentation, please see text for details. In a typical mixed acid fermentation, very little formate is released, which is indicated in the diagram by the lightening.

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 ⁺ again on intermediate products (pyruvate, acetyl phosphate , oxaloacetate and fumarate ) formed during the further course of fermentation . In contrast to homofermentative lactic acid fermentation , mixed acid fermentation produces a mixture of different organic compounds. The present proportions differ depending on the organism and environmental situation. In the case of Escherichia coli , the amount of organic compounds produced was measured. One mole of glucose is converted into:

${\displaystyle \mathrm {Glucose\longrightarrow 0,84\ Lactat+0,44\ Acetat+0,42\ Ethanol} }$ ${\displaystyle \mathrm {+\ 0,29\ Succinat+0,02\ Formiat+1,88\ H^{+}+0,44\ CO_{2}+0,43\ H_{2}} }$

A characteristic key enzyme of this type of fermentation is pyruvate formate lyase, it is only formed 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 to L - malate by a malate dehydrogenase , and this is converted to fumarate after dehydration , which catalyzes a fumarase . A fumarate finally reduced fumarate to succinate.

Fumarate reductase is a membrane-bound enzyme that transfers electrons from the menaquinol pool (MQH ₂ ) to fumarate. Menaquinol is either produced by a NADH dehydrogenase, which oxidizes NADH and reduces menaquinone (MQ). Alternatively, the electrons could also come from a membrane-based formate dehydrogenase , which oxidizes extracellular formate to carbon dioxide. NADH dehydrogenase (or formate dehydrogenase) translocate protons from the cytosol to the outside, creating a proton motor force Δμ (H ⁺ ).

In the literature, the use of fumarate as a pure electron acceptor is also referred to as fumarate breathing .

Formation of D -lactate

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

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 (see section above). 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 energy of the high-energy thioester bond can be conserved 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.

Occurrence

Sugars are broken down by mixed acid fermentation of the by far most species of the following facultative anaerobic enterobacteria : Citrobacter , Edwarsiella , Escherichia , Proteus , Providencia , Salmonella , Shigella , Yersinia and some other genera. Some facultative anaerobic Bacillus species also carry out this form of fermentation.

Importance for the identification of bacteria

The breakdown of sugars via mixed acid fermentation is a taxonomic characteristic that is used to determine bacteria, especially enterobacteria. The fact that this degradation pathway is present in the enterobacterium E. coli , for example , is determined by the detection of the acids that are produced in large quantities ( lowering of the pH value ). Another fermentation, the 2,3-butanediol fermentation , is precluded by the absence of the intermediate acetoin , which is characteristic of this fermentation , i.e. H. the detection of acetoin by the Voges-Proskauer reaction is negative. The release of acetoin can, for example , be detected in Klebsiella aerogenes , also an enterobacterium.

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 .
• Katharina Munk (ed.): Basic studies in biology . 2. microbiology . Series spectrum textbook. Spectrum Academic Publishing House, Heidelberg, Berlin 2001, ISBN 3-8274-0795-8 .

Individual evidence

1. ↑ ^{a b c d} Katharina Munk (ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 379ff.
2. ↑ ^{a b c} Georg Fuchs (Ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart; 8th edition 2007; ISBN 3-13-444608-1 ; P. 364ff.
3. ↑ Katharina Munk (ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 365.
4. ↑ Garabed Antranikian: Applied Microbiology . Springer, Berlin 2006; ISBN 978-3540240839 ; P. 64ff.
5. ↑ Michael T. Madigan and John M. Martinko: Brock Mikrobiologie . Pearson Studies; 11., revised. Edition 2006; ISBN 978-3-8273-7187-4 ; P. 397.
6. ↑ Katharina Munk: Basic studies in biology - microbiology . Spectrum Academic Publishing House 2000; ISBN 3-8274-0795-8 ; Pp. 3-26.