There are two types of electron bifurcation (EB), either quinone- or flavin-based (QBEB/FBEB), that involve reduction of a quinone or flavin by a two-electron transfer and two reoxidations by a high- and low-potential one-electron acceptor with a reactive semiquinone intermediate. In QBEB, the reduced low-potential acceptor (cytochrome b) is exclusively used to generate ΔμH+. In FBEB, the “energy-rich” low-potential reduced ferredoxin or flavodoxin has dual function. It can give rise to ΔμH+/Na+ via a ferredoxin:NAD reductase (Rnf) or ferredoxin:proton reductase (Ech) or conducts difficult reductions such as CO2 to CO. The QBEB membrane complexes are similar in structure and function and occur in all domains of life. In contrast, FBEB complexes are soluble and occur only in strictly anaerobic bacteria and archaea (FixABCX being an exception). The FBEB complexes constitute a group consisting of four unrelated families that contain (1) electron-transferring flavoproteins (EtfAB), (2) NAD(P)H dehydrogenase (...

Flavin-Based Electron Bifurcation, A New Mechanism of Biological Energy Coupling.

Acryloyl-CoA reductase from Clostridium propionicum catalyses the irreversible NADH-dependent formation of propionyl-CoA from acryloyl-CoA. Purification yielded a heterohexadecameric yellow–greenish enzyme complex [(α2βγ)4; molecular mass 600 ± 50 kDa] composed of a propionyl-CoA dehydrogenase (α2, 2 × 40 kDa) and an electron-transferring flavoprotein (ETF; β, 38 kDa; γ, 29 kDa). A flavin content (90% FAD and 10% FMN) of 2.4 mol per α2βγ subcomplex (149 kDa) was determined. A substrate alternative to acryloyl-CoA (Km = 2 ± 1 µm; kcat = 4.5 s−1 at 100 µm NADH) is 3-buten-2-one (methyl vinyl ketone; Km = 1800 µm; kcat = 29 s−1 at 300 µm NADH). The enzyme complex exhibits acyl-CoA dehydrogenase activity with propionyl-CoA (Km = 50 µm; kcat = 2.0 s−1) or butyryl-CoA (Km = 100 µm; kcat = 3.5 s−1) as electron donor and 200 µm ferricenium hexafluorophosphate as acceptor. The enzyme also catalysed the oxidation of NADH by iodonitrosotetrazolium chloride (diaphorase activity) or by air, which led to the formation of H2O2 (NADH oxidase activity). The N-terminus of the dimeric propionyl-CoA dehydrogenase subunit is similar to those of butyryl-CoA dehydrogenases from several clostridia and related anaerobes (up to 55% sequence identity). The N-termini of the β and γ subunits share 40% and 35% sequence identities with those of the A and B subunits of the ETF from Megasphaera elsdenii, respectively, and up to 60% with those of putative ETFs from other anaerobes. Acryloyl-CoA reductase from C. propionicum has been characterized as a soluble enzyme, with kinetic properties perfectly adapted to the requirements of the organism. The enzyme appears not to be involved in anaerobic respiration with NADH or reduced ferredoxin as electron donors. There is no relationship to the trans-2-enoyl-CoA reductases from various organisms or the recently described acryloyl-CoA reductase activity of propionyl-CoA synthase from Chloroflexus aurantiacus.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1432-1033.2003.03450.x

Acryloyl-CoA reductase from Clostridium propionicum. An enzyme complex of propionyl-CoA dehydrogenase and electron-transferring flavoprotein.

Ample evidence exists that numerous bacteria can use various gaseous hydrocarbons as a sole source of carbon and energy. From the relatively limited information currently available (ZoBell, 1950; Davis and Updegraff, 1954), it is clear that these organisms show differing specificities in their attack on hydrocarbons. Considering that methane is the preponderant gaseous hydrocarbon in nature, surprisingly little work has been published on its utilization by bacteria. Methods for isolation and study of methane-utilizing organisms require development and improvement. As it stands, characterization of these bacteria morphologically, culturally and physiologically to meet standards acceptable to modern systematics is a problem almost entirely for the future. A good illustration is provided by the paucity of information on the prototypic organism among the methane utilizers, Methanomonas methanica. Isolated from aquatic plant material by S6hngen in 1906 and named by him Bacillus methanicus (later amended by Orla-Jensen (1909) to Methanomonas methanica), this is the organism classically associated with methane utilization; its name is perpetuated in most bacteriological textbooks and in all schemes of classification. Curiously enough, the only de-scription available is that furnished 50 years ago by Sohngen. Although some later workers have claimed to have reisolated M. methanica from nature, it is extremely questionable whether their isolates were, in fact, identical with Sohngen's M. methanica.

Studies on pseudomonas methanica (söhngen) nov. comb

Propionate CoA-transferase from Clostridium propionicum has been purified and the gene encoding the enzyme has been cloned and sequenced. The enzyme was rapidly and irreversibly inactivated by sodium borohydride or hydroxylamine in the presence of propionyl-CoA. The reduction of the thiol ester between a catalytic site glutamate and CoA with borohydride and the cleavage by hydroxylamine were used to introduce a site-specific label, which was followed by MALDI-TOF-MS. This allowed the identification of glutamate 324 at the active site. Propionate CoA-transferase and similar proteins deduced from the genomes of Escherichia coli, Staphylococcus aureus, Bacillus halodurans and Aeropyrum pernix are proposed to form a novel subclass of CoA-transferases. Secondary structure element predictions were generated and compared to known crystal structures in the databases. A high degree of structural similarity was observed between the arrangement of secondary structure elements in these proteins and glutaconate CoA-transferase from Acidaminococcus fermentans.

https://febs.onlinelibrary.wiley.com/doi/epdf/10.1046/j.0014-2956.2001.02659.x

B. P. Cardon

Papers

Two New Amino-Acid-Fermenting Bacteria, Clostridium propionicum and Diplococcus glycinophilus.