Stickland fermentation

A drive state reaction is the two coupled fermentation of various amino acids with a simultaneous deamination, wherein an amino acid is oxidized and the other amino acid is reduced. This serves Vergärungsart some organisms as an energy source. The Stickland reaction is a special feature of the fermentation of amino acids. The pathway owes the name of its discoverer, Leonard Hubert Stickland.

Occurrence

Organisms that obtain energy by Stickland reactions are typically representative aminosäureverwertender clostridia in the broader sense - a polyphyletic group of anaerobic bacteria within the Firmicutes. Typical representatives are for example Clostridium sticklandii, C. sporogenes and C. botulinum. However, until today only a tiny fraction of all bacteria and archaea is described and classified by isolates of course is not clear whether Sticklandreakionen are limited to only a few phylogenetic groups.

Biochemistry

Stickland first recognized that there are bacteria that ferment, whereas individually, these amino acids are not only utilize combinations of two amino acids. He found that C. sporogenes (NCTC 533) uses six different combinations to any significant extent. Thus was born for this kind of reactions an umbrella term Stickland reaction.

In the Stickland reactions there are two branches:

In the oxidative branch of the amino acid to an α -keto acid is deaminated. This is followed with the incorporation of coenzyme A to an "activated" fatty acid ( acyl -CoA) oxidatively decarboxylated. The high-energy thioester bond of the acyl - CoA is obtained by means of inorganic phosphate to acylphosphate. The phosphate residue of the Acylphosphats is finally transferred to adenosine diphosphate (ADP ) to form ATP. With this substrate chain energy is obtained in the oxidative branch. The original carbon chain of the amino acid after the fermentation is shorter by one carbon atom.

Through the reductive deamination of the second amino acid in the reductive branch of the resulting hydrogen and electron carriers are oxidized and regenerated it. Of the amino acid results in a fatty acid, wherein the number of carbon atoms is retained.

A typical Stickland reaction is shown in the figure using the example of C. sporogenes, the same oxidized D -alanine and two molecules of glycine reduced. By alanine aminotransferase is deaminated to pyruvate and aminated α -ketoglutarate to L- glutamate. The glutamate is then oxidized by a NAD - dependent dehydrogenase again to α -ketoglutarate, which NH4 is released. Some clostridia can directly oxidize pyruvate to alanine by a NAD -dependent alanine dehydrogenase.

The resulting pyruvate is oxidatively decarboxylated by pyruvate - ferredoxin oxidoreductase by coenzyme A into acetyl -CoA. This ferredoxin is reduced. Through a phosphotransacetylase, acetyl - CoA is esterified to acetyl phosphate. This will be implemented as part of the substrate chain to acetate, in this step creates ATP. The enzyme involved in the reaction is an acetate kinase.

In the reductive branch two molecules of glycine are converted by a selenocysteine ​​-containing selenium or Glycinredukatase to two molecules of acetyl phosphate. For these then arises as analogous oxidative branch acetate whereby an additional energy can be recovered.

Substrate diversity

Besides alanine also methionine, leucine, isoleucine, valine, serine, threonine and histidine, can be used both as an electron acceptor as well as an electron donor. In the reductive road act as an electron acceptor in addition to glycine, arginine, leucine, tryptophan, phenylalanine, tyrosine, hydroxyproline and proline.

Even non-protein amino acids, such as D -proline can be used, so that an enormous variety of Stickland reactions is conceivable used by different specialists. However, only in the case of glycine can be formed via acetyl phosphate ATP in the reductive branch. Proline is not deaminated, but reduced by a ring cleavage to δ - Aminovalerat.