Abstract
Anaerobic bacteria ferment carbohydrates and amino acids to obtain energy for growth. Due to the absence of oxygen and other inorganic electron acceptors, the substrate of a fermentation has to serve as electron donor as well as acceptor, which results in low free energies as compared to that of aerobic oxidations. Until about 10 years ago, anaerobes were thought to exclusively use substrate level phosphorylation (SLP), by which only part of the available energy could be conserved. Therefore, anaerobes were regarded as unproductive and inefficient energy conservers. The discovery of electrochemical Na+ gradients generated by biotin-dependent decarboxylations or by reduction of NAD+ with ferredoxin changed this view. Reduced ferredoxin is provided by oxidative decarboxylation of 2-oxoacids and the recently discovered flavin based electron bifurcation (FBEB). In this review, the two different fermentation pathways of glutamate to ammonia, CO2, acetate, butyrate and H2 via 3-methylaspartate or via 2-hydroxyglutarate by members of the Firmicutes are discussed as prototypical examples in which all processes characteristic for fermentations occur. Though the fermentations proceed on two entirely different pathways, the maximum theoretical amount of ATP is conserved in each pathway. The occurrence of the 3-methylaspartate pathway in clostridia from soil and the 2-hydroxyglutarate pathway in the human microbiome of the large intestine is traced back to the oxygen-sensitivity of the radical enzymes. The coenzyme B12-dependent glutamate mutase in the 3-methylaspartate pathway tolerates oxygen, whereas 2-hydroxyglutaryl-CoA dehydratase is extremely oxygen-sensitive and can only survive in the gut, where the combustion of butyrate produced by the microbiome consumes the oxygen and provides a strict anaerobic environment. Examples of coenzyme B12-dependent eliminases are given, which in the gut are replaced by simpler extremely oxygen sensitive glycyl radical enzymes.
Highlights
Fermentation is a chemical reaction catalyzed by living cells with two different meanings
Thereby glutamate is oxidized to CO2 and acetate; the resulting reducing equivalents are used to Abbreviations: FAD−, FAD anionic semiquinone; FADH−, FAD hydroquinone; Fd, ferredoxin; Fd− reduced ferredoxin
Similar to complex III of the mitochondrial respiratory chain, it was postulated that NADH reduced EtfAB, which bifurcated one electron exergonically to δ-FAD of butyryl-CoA dehydrogenase (Bcd) and the other endergonically to ferredoxin (Herrmann et al, 2008)
Summary
Laboratorium für Mikrobiologie, Fachbereich Biologie, Philipps-Universität Marburg, Marburg, Germany. The two different fermentation pathways of glutamate to ammonia, CO2, acetate, butyrate and H2 via 3-methylaspartate or via 2hydroxyglutarate by members of the Firmicutes are discussed as prototypical examples in which all processes characteristic for fermentations occur. The occurrence of the 3-methylaspartate pathway in clostridia from soil and the 2-hydroxyglutarate pathway in the human microbiome of the large intestine is traced back to the oxygen-sensitivity of the radical enzymes. The coenzyme B12-dependent glutamate mutase in the 3-methylaspartate pathway tolerates oxygen, whereas 2-hydroxyglutaryl-CoA dehydratase is extremely oxygensensitive and can only survive in the gut, where the combustion of butyrate produced by the microbiome consumes the oxygen and provides a strict anaerobic environment. Examples of coenzyme B12-dependent eliminases are given, which in the gut are replaced by simpler extremely oxygen sensitive glycyl radical enzymes
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