Characterization of two novel redox groups in the respiratory NADH : ubiquinone oxidoreductase (complex I)

The proton-pumping NADH:ubiquinone oxidoreductase is the first of the respi ratory chain complexes in many bacteria and mitochondria of most eukaryotes , The bacterial complex consists of 14 different subunits. Seven peripheral subunits bear all known redox groups of complex I, namely one FMN and five EPR-detectable iron-sulfur (FeS) clusters. The remaining seven subunits ar e hydrophobic proteins predicted to fold into 54 alpha-helices across the m embrane. Little is known about their function, but they are most likely inv olved in proton translocation. The mitochondrial complex contains in additi on to the homologues of these 14 subunits at least 29 additional proteins t hat do not directly participate in electron transfer and proton translocati on. A novel redox group has been detected in the Neurospora crassa complex, in an amphipathic fragment of the Escherichia coli complex I and in a rela ted hydrogenase and ferredoxin by means of UV/Vis spectroscopy. This group is made up by the two tetranuclear FeS clusters located on NuoI (the bovine TYKY) which have not been detected by EPR spectroscopy yet. Furthermore, w e present evidence for the existence of a novel redox group located in the membrane arm of the complex. Partly reduced complex I equilibrated to a red ox potential of -150 mV gives a UVNis redox difference spectrum that cannot be attributed to the known cofactors. Electrochemical titration of this ab sorption reveals a midpoint potential of -80 mV. This group is believed to transfer electrons from the high potential FeS cluster to ubiquinone. (C) 2 000 Elsevier Science B.V. All rights reserved.