Composition and Enzymatic Properties of the Mitochondrial NADH- and NADPH-Ubiquinone Reductase (Complex I)
Complex I is one of the five component enzyme complexes of the mitochondrial electron transport-oxidative phosphorylation system (Fig. 1) (1–3). The enzymatic reactions catalyzed by complex I are summarized in Table I and the composition of the preparation in terms of flavin, nonheme iron (Fe), acid-labile sulfide (S*), ubi quinone, and phospholipids is given in Table II. The resolution of complex I by chaotropic agents (4–6) results in solubilization of ~ 20% protein containing flavin and approximately 50% of the Fe and S* content of complex I. The remainder of Fe and S* plus lipids are associated with the water-insoluble fraction (Fig. 2). Upon fractionation with ammonium sulfate, the soluble fraction yields an iron-sulfur protein with epr characteristics of FeS* center 2 of complex I (see below), plus a flavoprotein containing approximately 4 g atoms of Fe and 4 moles of S* per mole of FMN. The latter catalyzes the oxidation of NADH by various quinones and ferric compounds, including cytochrome c (Table III).
KeywordsElectron Paramagnetic Resonance Enzymatic Property Nonheme Iron Submitochondrial Particle Chaotropic Agent
electron paramagnetic resonance
reduced 3-acetylpyridine adenine dinucleotide
iron-sulfur centers 1,2,3 and 4, respectively
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