title: Reduction of ferredoxin or oxygen by flavin-based electron bifurcation in Megasphaera elsdenii.
abstract: Over 50 years ago, it was reported that, in the anaerobic rumen bacterium Megasphaera elsdenii, the reduction of crotonyl-CoA to butyryl-CoA by NADH involved an electron transferring flavoprotein (Etf) as mediator [Baldwin RL, Milligan LP (1964) Biochim Biophys Acta 92, 421-432]. Purification and spectroscopic characterization revealed that this Etf contained 2 FAD, whereas, in the Etfs from aerobic and facultative bacteria, one FAD is replaced by AMP. Recently we detected a similar system in the related anaerobe Acidaminococcus fermentans that differed in the requirement of additional ferredoxin as electron acceptor. The whole process was established as flavin-based electron bifurcation in which the exergonic reduction of crotonyl-CoA by NADH mediated by Etf + butyryl-CoA dehydrogenase (Bcd) was coupled to the endergonic reduction of ferredoxin also by NADH. In the present study, we demonstrate that, under anaerobic conditions, Etf + Bcd from M. elsdenii bifurcate as efficiently as Etf + Bcd from A. fermentans. Under the aerobic conditions used in the study by Baldwin and Milligan and in the presence of catalytic amounts of crotonyl-CoA or butyryl-CoA, however, Etf + Bcd act as NADH oxidase producing superoxide and H2 O2 , whereas ferredoxin is not required. We hypothesize that, during bifurcation, oxygen replaces ferredoxin to yield superoxide. In addition, the formed butyryl-CoA is re-oxidized by a second oxygen molecule to crotonyl-CoA, resulting in a stoichiometry of 2 NADH consumed and 2 H2 O2 formed. As a result of the production of reactive oxygen species, electron bifurcation can be regarded as an Achilles' heel of anaerobes when exposed to air.
authors.affiliation: Laboratorium für Mikrobiologie, Fachbereich Biologie and Synmikro, Philipps-Universität, Marburg, Germany.|Max-Plank-Institut für terrestrische Mikrobiologie, Marburg, Germany.
authors.forename: Nilanjan P
authors.initials: NP
authors.lastname: Chowdhury
authors.affiliation: Max-Plank-Institut für terrestrische Mikrobiologie, Marburg, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Kahnt
authors.affiliation: Laboratorium für Mikrobiologie, Fachbereich Biologie and Synmikro, Philipps-Universität, Marburg, Germany.|Max-Plank-Institut für terrestrische Mikrobiologie, Marburg, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Buckel
mesh_terms: D045850:Acidaminococcus / Q000235:genetics / Q000378:metabolism; D000693:Anaerobiosis; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D044926:Butyryl-CoA Dehydrogenase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004579:Electron Transport; D044927:Electron-Transferring Flavoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005288:Ferredoxins / Q000378:metabolism*; D045851:Megasphaera / Q000235:genetics / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D009243:NAD / Q000378:metabolism; D009247:NADH, NADPH Oxidoreductases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D013053:Spectrophotometry
chemical_list: D001426:Bacterial Proteins; D044927:Electron-Transferring Flavoproteins; D005288:Ferredoxins; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D009243:NAD; D044926:Butyryl-CoA Dehydrogenase; C023107:NADH oxidase; D009247:NADH, NADPH Oxidoreductases; D010100:Oxygen
keywords: FAD semiquinone; NADH; butyryl-CoA dehydrogenase; electron transferring flavoprotein; superoxide