title: Reactions of electron-transfer flavoprotein and electron-transfer flavoprotein: ubiquinone oxidoreductase.
abstract: Electron-transfer flavoprotein:ubiquinone oxidoreductase (ETF-Q oxidoreductase) catalyses the re-oxidation of reduced electron-transfer flavoprotein (ETF) with ubiquinone-1 (Q-1) as the electron acceptor. A kinetic assay for the enzyme was devised in which glutaryl-CoA in the presence of glutaryl-CoA dehydrogenase was used to reduce ETFox. and the reduction of Q-1 was monitored at 275 nm. The partial reactions involved in the overall assay system were examined. Glutaryl-CoA dehydrogenase catalyses the rapid reduction of ETFox. to the anionic semiquinone (ETF.-), but reduces ETF.- to the fully reduced form (ETFhq) at a rate that is about 6-fold lower. ETF.-, but not ETFhq, is directly re-oxidized by Q-1 at a rate that, depending on the steady-state concentration of ETF.-, may contribute significantly to the overall reaction. ETF-Q oxidoreductase catalyses rapid disproportionation of ETF.- with an equilibrium constant of about 1.0 at pH 7.8. In the presence of Q-1 it also catalyses the re-oxidation of ETFhq at a rate that is faster than that of the overall reaction. Rapid-scan experiments indicated the formation of ETF.-, but its fractional concentration in the early stages of the re-oxidation of ETFhq is low. The data indicate that the re-oxidation of ETFhq proceeds at a rate that is adequate to account for the overall rate of electron transfer from glutaryl-CoA to Q-1. An unusual property of ETF-Q oxidoreductase seems to be that it not only catalyses the re-oxidation of the reduced forms of ETF but also facilitates the complete reduction of ETFox. to ETFhq by disproportionation of the radical.
mesh_terms: D002384:Catalysis; D044927:Electron-Transferring Flavoproteins; D044943:Fatty Acid Desaturases / Q000378:metabolism*; D005420:Flavoproteins / Q000378:metabolism*; D050770:Glutaryl-CoA Dehydrogenase; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins; D007700:Kinetics; D009097:Multienzyme Complexes / Q000378:metabolism*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism; D044925:Oxidoreductases Acting on CH-CH Group Donors; D000587:Oxidoreductases Acting on CH-NH Group Donors; D013053:Spectrophotometry
chemical_list: D044927:Electron-Transferring Flavoproteins; D005420:Flavoproteins; D007506:Iron-Sulfur Proteins; D009097:Multienzyme Complexes; D010088:Oxidoreductases; D044943:Fatty Acid Desaturases; D044925:Oxidoreductases Acting on CH-CH Group Donors; D050770:Glutaryl-CoA Dehydrogenase; D000587:Oxidoreductases Acting on CH-NH Group Donors; C023120:electron-transferring-flavoprotein dehydrogenase