title: Characterization of two quinone radicals in the NADH:ubiquinone oxidoreductase from Escherichia coli by a combined fluorescence spectroscopic and electrochemical approach.
abstract: The NADH:ubiquinone oxidoreductase (complex I) couples the transfer of electrons from NADH to ubiquinone with the translocation of protons across the membrane. It was proposed that the electron transfer involves quinoid groups localized at the end of the electron transfer chain. To identify these groups, fluorescence excitation and emission spectra of Escherichia coli complex I and its fragments, namely, the NADH dehydrogenase fragment containing the flavin mononucleotide and six iron-sulfur (Fe-S) clusters, and the quinone reductase fragment containing three Fe-S clusters were measured. Signals sensitive to reduction by either NADH or dithionite were detected within the complex and the quinone reductase fragment and attributed to the redox transition of protonated ubiquinone radicals. A fluorescence spectroscopic electrochemical redox titration revealed midpoint potentials of -37 and- 235 mV (vs the standard hydrogen electrode) for the redox transitions of the quinone radicals in complex I at pH 6 with an absorption around 325 nm and a fluorescence emission at 460/475 nm. The role of these cofactor(s) for electron transfer is discussed.
authors.affiliation: Laboratoire de bioelectrochimie et spectroscopie, UMR 7140, CNRS, Université de Strasbourg , 1, rue Blaise Pascal, 67070 Strasbourg, France.
authors.forename: Ruth
authors.initials: R
authors.lastname: Hielscher
authors.forename: Michelle
authors.initials: M
authors.lastname: Yegres
authors.forename: Mariana
authors.initials: M
authors.lastname: Voicescu
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Gnandt
authors.forename: Thorsten
authors.initials: T
authors.lastname: Friedrich
authors.forename: Petra
authors.initials: P
authors.lastname: Hellwig
mesh_terms: D016227:Benzoquinones / Q000737:chemistry* / Q000378:metabolism*; D055664:Electrochemical Techniques; D004579:Electron Transport; D042967:Electron Transport Complex I / Q000737:chemistry* / Q000378:metabolism*; D004926:Escherichia coli / Q000737:chemistry / Q000201:enzymology*; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D013050:Spectrometry, Fluorescence
chemical_list: D016227:Benzoquinones; D005609:Free Radicals; C004532:quinone; D042967:Electron Transport Complex I