AUTHOR=Buckel Wolfgang 

TITLE=Energy Conservation in Fermentations of Anaerobic Bacteria

JOURNAL=Frontiers in Microbiology

VOLUME=Volume 12 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.703525

DOI=10.3389/fmicb.2021.703525

ISSN=1664-302X

ABSTRACT=Anaerobic bacteria ferment carbohydrates and amino acids to obtain energy for growth. Due to
8 the absence of oxygen and other inorganic electron acceptors, the substrate of a fermentation
9 has to serve as electron donor as well as acceptor, which results in low free energies as
10 compared to that of aerobic oxidations. Until about 10 years ago, anaerobes were thought to
11 exclusively use substrate level phosphorylation (SLP), by which only part of the available energy
12 could be conserved. Therefore, anaerobes were regarded as unproductive and inefficient
13 energy conservers. The discovery of electrochemical Na+ gradients generated by biotin-
14 dependent decarboxylations or by reduction of NAD+ with ferredoxin changed this view.
15 Reduced ferredoxin is provided by oxidative decarboxylation of 2-oxoacids and the recently
16 discovered flavin based electron bifurcation (FBEB). In this review, the two different
17 fermentation pathways of glutamate to ammonia, CO2, acetate, butyrate and H2 via 3-
18 methylaspartate or via 2-hydroxyglutarate by members of the Firmicutes are discussed as
19 prototypical examples in which all processes characteristic for fermentations occur. Though the
20 fermentations proceed on two entirely different pathways, the maximum theoretical amount of
21 ATP is conserved in each pathway. The occurrence of the 3-methylaspartate pathway in
22 clostridia from soil and the 2-hydroxyglutarate pathway in the human microbiome of the large
23 intestine is traced back to the oxygen-sensitivity of the radical enzymes. The coenzyme B12-
24 dependent glutamate mutase in the 3-methylaspartate pathway tolerates oxygen, whereas 2-
25 hydroxyglutaryl-CoA dehydratase is extremely oxygen-sensitive and can only survive in the gut,
26 where the combustion of butyrate produced by the microbiome consumes the oxygen and
27 provides a strict anaerobic environment. Examples of coenzyme B12-dependent eliminases are
28 given, which in the gut are replaced by simpler extremely oxygen sensitive glycyl radical
29 enzymes.