title: Studies on the mechanism of electron bifurcation catalyzed by electron transferring flavoprotein (Etf) and butyryl-CoA dehydrogenase (Bcd) of Acidaminococcus fermentans.
abstract: Electron bifurcation is a fundamental strategy of energy coupling originally discovered in the Q-cycle of many organisms. Recently a flavin-based electron bifurcation has been detected in anaerobes, first in clostridia and later in acetogens and methanogens. It enables anaerobic bacteria and archaea to reduce the low-potential [4Fe-4S] clusters of ferredoxin, which increases the efficiency of the substrate level and electron transport phosphorylations. Here we characterize the bifurcating electron transferring flavoprotein (EtfAf) and butyryl-CoA dehydrogenase (BcdAf) of Acidaminococcus fermentans, which couple the exergonic reduction of crotonyl-CoA to butyryl-CoA to the endergonic reduction of ferredoxin both with NADH. EtfAf contains one FAD (α-FAD) in subunit α and a second FAD (β-FAD) in subunit β. The distance between the two isoalloxazine rings is 18 Å. The EtfAf-NAD(+) complex structure revealed β-FAD as acceptor of the hydride of NADH. The formed β-FADH(-) is considered as the bifurcating electron donor. As a result of a domain movement, α-FAD is able to approach β-FADH(-) by about 4 Å and to take up one electron yielding a stable anionic semiquinone, α-FAD, which donates this electron further to Dh-FAD of BcdAf after a second domain movement. The remaining non-stabilized neutral semiquinone, β-FADH(•), immediately reduces ferredoxin. Repetition of this process affords a second reduced ferredoxin and Dh-FADH(-) that converts crotonyl-CoA to butyryl-CoA.
authors.affiliation: From the Laboratorium für Mikrobiologie, Fachbereich Biologie and SYNMIKRO, Philipps-Universität, 35032 Marburg, Germany.
authors.forename: Nilanjan Pal
authors.initials: NP
authors.lastname: Chowdhury
authors.forename: Amr M
authors.initials: AM
authors.lastname: Mowafy
authors.forename: Julius K
authors.initials: JK
authors.lastname: Demmer
authors.forename: Vikrant
authors.initials: V
authors.lastname: Upadhyay
authors.forename: Sebastian
authors.initials: S
authors.lastname: Koelzer
authors.forename: Elamparithi
authors.initials: E
authors.lastname: Jayamani
authors.forename: Joerg
authors.initials: J
authors.lastname: Kahnt
authors.forename: Marco
authors.initials: M
authors.lastname: Hornung
authors.forename: Ulrike
authors.initials: U
authors.lastname: Demmer
authors.forename: Ulrich
authors.initials: U
authors.lastname: Ermler
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Buckel
mesh_terms: D045850:Acidaminococcus / Q000201:enzymology*; D055162:Biocatalysis; D044926:Butyryl-CoA Dehydrogenase / Q000737:chemistry / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004579:Electron Transport; D044927:Electron-Transferring Flavoproteins / Q000737:chemistry / Q000378:metabolism*; D004583:Electrons; D004591:Electrophoresis, Polyacrylamide Gel; D005288:Ferredoxins / Q000737:chemistry / Q000378:metabolism; D005182:Flavin-Adenine Dinucleotide / Q000737:chemistry / Q000378:metabolism; D005415:Flavins / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D008954:Models, Biological; D062105:Molecular Docking Simulation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013056:Spectrophotometry, Ultraviolet
chemical_list: D044927:Electron-Transferring Flavoproteins; D005288:Ferredoxins; D005415:Flavins; D011994:Recombinant Proteins; D005182:Flavin-Adenine Dinucleotide; C008173:isoalloxazine; D044926:Butyryl-CoA Dehydrogenase
keywords: Bioenergetics/Electron Transfer Complex; Coenzyme A; Crystal Structure; Electron Transfer; Energy Metabolism; Enzyme Mechanisms