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Flavin-based electron bifurcation is a newly discovered mechanism, by which a hydride electron pair from NAD(P)H, coenzyme F 420 H 2 , H 2 , or formate is split by flavoproteins into one-electron with a more negative reduction potential and one with a more positive reduction potential than that of the electron pair. Via this mechanism microorganisms generate low- potential electrons for the reduction of ferredoxins (Fd) and flavodoxins (Fld). The first example was described in 2008 when it was found that the butyryl-CoA dehydrogenase-electron-transferring flavoprotein complex (Bcd-EtfAB) of  Clostridium kluyveri  couples the endergonic reduction of ferredoxin (E 0 ′ = −420 mV) with NADH (−320 mV) to the exergonic reduction of crotonyl-CoA to butyryl-CoA (−10 mV) with NADH. The discovery was followed by the finding of an electron-bifurcating Fd- and NAD-dependent [FeFe]-hydrogenase (HydABC) in  Thermotoga maritima  (2009), Fd-dependent transhydrogenase (NfnAB) in various bacteria and archaea (2010), Fd- and H 2 -dependent heterodisulfide reductase (MvhADG-HdrABC) in methanogenic archaea (2011), Fd- and NADH-dependent caffeyl-CoA reductase (CarCDE) in  Acetobacterium woodii  (2013), Fd- and NAD-dependent formate dehydrogenase (HylABC-FdhF2) in  Clostridium acidi-urici  (2013), Fd- and NADP-dependent [FeFe]-hydrogenase (HytA-E) in  Clostridium autoethanogrenum  (2013), Fd(?)- and NADH-dependent methylene-tetrahydrofolate reductase (MetFV-HdrABC-MvhD) in  Moorella thermoacetica  (2014), Fd- and NAD-dependent lactate dehydrogenase (LctBCD) in  A. woodii  (2015), Fd- and F 420 H 2 -dependent heterodisulfide reductase (HdrA2B2C2) in  Methanosarcina acetivorans  (2017), and Fd- and NADH-dependent ubiquinol reductase (FixABCX) in  Azotobacter vinelandii  (2017). The electron-bifurcating flavoprotein complexes known to date fall into four groups that have evolved independently, namely those containing EtfAB (CarED, LctCB, FixBA) with bound FAD, a NuoF homolog (HydB, HytB, or HylB) harboring FMN, NfnB with bound FAD, or HdrA harboring FAD. All these flavoproteins are cytoplasmic except for the membrane-associated protein FixABCX. The organisms—in which they have been found—are strictly anaerobic microorganisms except for the aerobe  A. vinelandii . The electron-bifurcating complexes are involved in a variety of processes such as butyric acid fermentation, methanogenesis, acetogenesis, anaerobic lactate oxidation, dissimilatory sulfate reduction, anaerobic- dearomatization, nitrogen fixation, and CO 2  fixation. They contribute to energy conservation via the energy-converting ferredoxin: NAD +  reductase complex Rnf or the energy-converting ferredoxin-dependent hydrogenase complex Ech. This Review describes how this mechanism was discovered.

frontiers in microbiology

Flavin-Based Electron Bifurcation, Ferredoxin, Flavodoxin, and Anaerobic Respiration With Protons (Ech) or NAD+ (Rnf) as Electron Acceptors: A Historical Review