Butyric fermentation

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Butyric acid fermentation is the microbial breakdown of organic substances, mainly carbohydrates , into butyric acid (butyrate), elemental hydrogen (H 2 ) and carbon dioxide (CO 2 ), and in smaller quantities into acetic acid (acetate) and ethanol . It serves as an energy source for the butyric acid fermenters.

Occurrence

The butyric acid fermentation is carried out by gram-positive , anaerobic endospore formers of the genus Clostridium . The butyric acid fermenters are neutrophilic to alkaliphilic and grow under strictly anaerobic conditions. They occur, for example, in the rumen of ruminants, in water sediments or in the soil. A distinction is made according to the origin of the substrate to be fermented. Carbohydrates such as starch , cellulose and sugar can be fermented by so-called saccharolytic butyric acid fermenters. Peptolytic fermenters use amino acids, peptides or nucleobases .

Asporogenic representatives of the genera Eubacterium , Sarcina , Ruminococcus , Peptococcus , Acidaminococcus or Sporomusa follow similar fermentation routes .

biochemistry

Overview of butyric acid fermentation. Starting from pyruvate, butyrate is mainly formed, with acetate and ethanol in smaller quantities. This fact has been taken into account in the graphic. The processes of glycolysis generate reduction equivalents in the form of NADH. These are reoxidized again in the various branches of butyric acid fermentation.

Saccharolytic butyric acid fermenters break down sugars such as glucose in the course of glycolysis to pyruvate. This creates two molecules each of ATP and NADH . Pyruvate is then oxidatively decarboxylated by pyruvate ferredoxin oxidoreductase with the elimination of CO 2 and incorporation of coenzyme A (CoA) into acetyl-CoA . In contrast to the analogous reaction that is catalyzed by pyruvate dehydrogenase , the electrons are transferred to ferredoxins . A hydrogenase is used to reoxidize reduced ferredoxin (Fd red ) with the formation of elemental hydrogen (H 2 ) (Fd ox ).

Acetyl-CoA can either be reduced via acetaldehyde to ethanol using two molecules of NADH. Alternatively, acetyl-CoA is transesterified with inorganic phosphate , whereby acetyl phosphate is formed. The high-energy anhydride bond is used to form ATP by transferring the phosphate residue to ADP. In the course of this substrate chain phosphorylation , acetate is formed.

In clostridia, acetyl-CoA is mainly converted further via a third metabolic pathway: acetyl-CoA condenses with another molecule of acetyl-CoA to form acetacetyl-CoA, which catalyzes a thiolase . This is reduced to butyryl-CoA in two reduction steps and dehydration via 3-hydroxybutyryl-CoA and crotonyl-CoA . The energy of the high- energy thioester bond of butyryl-CoA is obtained by transesterification using inorganic phosphate, which produces butyryl phosphate. In the last step, a butyrate kinase catalyzes the formation of ATP and butyrate, the anion of butyric acid. Fermentation was named after the latter.

Balance sheet

Assuming that glucose is completely fermented to butyric acid, 247 kJ / mol are released under standard conditions . In Clostridium pasteurianum , for example, this allows the formation of three molecules of ATP:

However, small amounts of acetate and ethanol are also always formed.

Solvent fermentation

In the course of the butyric acid fermentation, the environment is acidified by the acids (acetic acid, butyric acid). Some representatives of the Clostridia change to a so-called solvent fermentation or butanol fermentation when the pH value falls . In this process, neutral fermentation products such as butanol , acetone and 2-propanol are formed instead of acids . These no longer acidify the environment. Fermentation owes its name to the fact that these fermentation products are used as solvents in the chemical industry .

The biochemical processes in Clostridium acetobutylicum are best studied. This quickly changes from butyric acid to solvent fermentation. The butyrate formed during the butyric acid fermentation is converted back into butyryl-CoA. This is finally reduced to butanol by two dehydrogenases . Acetyl-CoA is converted into acetoacetate , which decarboxylase decarboxylates to acetone. Acetone itself can finally be reduced to 2-propanol by a 2-propanol dehydrogenase . This takes place, for example, with Clostridium butyricum .

Solvent fermentation was of economic interest in the 1930s to 1960s because solvents could be made from simple polysaccharides. However, the production quantities are limited in this way.

Individual evidence

  1. ↑ Georg Fuchs (Ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart; 8th edition 2007; ISBN 3-13-444608-1 ; P. 369.

literature

  • Georg Fuchs (ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart; 8th edition 2007; ISBN 3-13-444608-1 ; P. 369ff.
  • Wolfgang Fritsche: Microbiology . Spectrum Academic Publishing House; 3rd edition 2001; ISBN 3-8274-1107-6 ; P. 246ff.
  • Katharina Munk (Ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 383ff.
  • Michael T. Madigan, John M. Martinko, Jack Parker, and Thomas D. Brock: Microbiology . Spectrum Academic Publishing House; ISBN 3-8274-0566-1 ; P. 567

See also