Reduction of hydrophilic ubiquinones by the flavin in mitochondrial NADH:ubiquinone oxidoreductase (Complex I) and production of reactive oxygen species.

TitleReduction of hydrophilic ubiquinones by the flavin in mitochondrial NADH:ubiquinone oxidoreductase (Complex I) and production of reactive oxygen species.
Publication TypeJournal Article
Year of Publication2009
AuthorsKing, MS, Sharpley, MS, Hirst, J
JournalBiochemistry
Volume48
Issue9
Pagination2053-62
Date Published2009 Mar 10
ISSN1520-4995
KeywordsAlgorithms, Animals, Cattle, Cytochromes c, Electron Transport Complex I, Flavins, Hydrogen Peroxide, Kinetics, Mitochondria, Liver, NAD, Oxidation-Reduction, Oxygen, Reactive Oxygen Species, Rotenone, Ubiquinone, Uncoupling Agents, Water
Abstract

NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria is a complicated, energy-transducing, membrane-bound enzyme that contains 45 different subunits, a non-covalently bound flavin mononucleotide, and eight iron-sulfur clusters. The mechanisms of NADH oxidation and intramolecular electron transfer by complex I are gradually being defined, but the mechanism linking ubiquinone reduction to proton translocation remains unknown. Studies of ubiquinone reduction by isolated complex I are problematic because the extremely hydrophobic natural substrate, ubiquinone-10, must be substituted with a relatively hydrophilic analogue (such as ubiquinone-1). Hydrophilic ubiquinones are reduced by an additional, non-energy-transducing pathway (which is insensitive to inhibitors such as rotenone and piericidin A). Here, we show that inhibitor-insensitive ubiquinone reduction occurs by a ping-pong type mechanism, catalyzed by the flavin mononucleotide cofactor in the active site for NADH oxidation. Moreover, semiquinones produced at the flavin site initiate redox cycling reactions with molecular oxygen, producing superoxide radicals and hydrogen peroxide. The ubiquinone reactant is regenerated, so the NADH:Q reaction becomes superstoichiometric. Idebenone, an artificial ubiquinone showing promise in the treatment of Friedreich's Ataxia, reacts at the flavin site. The factors which determine the balance of reactivity between the two sites of ubiquinone reduction (the energy-transducing site and the flavin site) and the implications for mechanistic studies of ubiquinone reduction by complex I are discussed. Finally, the possibility that the flavin site in complex I catalyzes redox cycling reactions with a wide range of compounds, some of which are important in pharmacology and toxicology, is discussed.

DOI10.1021/bi802282h
Alternate JournalBiochemistry
Citation Key10.1021/bi802282h
PubMed ID19220002
PubMed Central IDPMC2651670
Grant ListMC_U105663141 / / Medical Research Council / United Kingdom