Characterization of the complex I-associated ubisemiquinone species: toward the understanding of their functional roles in the electron/proton transfer reaction

NADH-ubiquinone oxidoreductase (called complex I for mitochondrial enzyme a nd NDH-1 for bacterial counterparts) is an energy transducer, which utilize s the redox energy derived from the oxidation of NADH with ubiquinone to ge nerate an electrochemical proton gradient (Delta<(mu)over tilde>(H+)) acros s the membrane. The complex I/NDH-1 contain one non-covalently bound flavin mononucleotide and as many as eight iron-sulfur clusters as electron trans fer components in common. In addition, electron paramagnetic resonance (EPR ) spectroscopic studies have revealed that three ubisemiquinone (SQ) specie s with distinct spectroscopic and thermodynamic properties are detectable i n complex I and function as electron/proton translocators. Thus, the unders tanding of molecular properties of the individual quinone species is prereq uisite to elucidate the energy-coupling mechanism of complex I. We have inv estigated these SQ species using EPR spectroscopy and found that the three SQ species have strikingly different properties. We will report characteris tics of these SQ species and discuss possible functional roles of individua l quinone species in the electron/proton transfer reaction of complex I/NDH -1. (C) 2000 Elsevier Science B.V. All rights reserved.