title: Semiquinone intermediates are involved in the energy coupling mechanism of E. coli complex I.
abstract: Complex I (NADH:quinone oxidoreductase) is central to cellular aerobic energy metabolism, and its deficiency is involved in many human mitochondrial diseases. Complex I translocates protons across the membrane using electron transfer energy. Semiquinone (SQ) intermediates appearing during catalysis are suggested to be key for the coupling mechanism in complex I. However, the existence of SQ has remained controversial due to the extreme difficulty in detecting unstable and low intensity SQ signals. Here, for the first time with Escherichia coli complex I reconstituted in proteoliposomes, we successfully resolved and characterized three distinct SQ species by EPR. These species include: fast-relaxing SQ (SQNf) with P1/2 (half-saturation power level)>50mW and a wider linewidth (12.8 G); slow-relaxing SQ (SQNs) with P1/2=2-3mW and a 10G linewidth; and very slow-relaxing SQ (SQNvs) with P1/2= ~0.1mW and a 7.5G linewidth. The SQNf signals completely disappeared in the presence of the uncoupler gramicidin D or squamotacin, a potent E. coli complex I inhibitor. The pH dependency of the SQNf signals correlated with the proton-pumping activities of complex I. The SQNs signals were insensitive to gramicidin D, but sensitive to squamotacin. The SQNvs signals were insensitive to both gramicidin D and squamotacin. Our deuterium exchange experiments suggested that SQNf is neutral, while SQNs and SQNvs are anion radicals. The SQNs signals were lost in the ΔNuoL mutant missing transporter module subunits NuoL and NuoM. The roles and relationships of the SQ intermediates in the coupling mechanism are discussed.
authors: Johnson Research Foundation, Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, United States.
authors: Madhavan
authors: M
authors: Narayanan
authors: Johnson Research Foundation, Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, United States.
authors: Steven A
authors: SA
authors: Leung
authors: Johnson Research Foundation, Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, United States.
authors: Yuta
authors: Y
authors: Inaba
authors: Johnson Research Foundation, Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, United States.
authors: Mahmoud M
authors: MM
authors: Elguindy
authors: Johnson Research Foundation, Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, United States. Electronic address: eikoo@mail.med.upenn.edu.
authors: Eiko
authors: E
authors: Nakamaru-Ogiso
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives / Q000494:pharmacology; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D042967:Electron Transport Complex I / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006096:Gramicidin / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009245:NADH Dehydrogenase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010084:Oxidation-Reduction; D011510:Proteolipids; D011522:Protons; D014451:Ubiquinone / Q000737:chemistry* / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D029968:Escherichia coli Proteins; D011510:Proteolipids; D011522:Protons; C020485:proteoliposomes; C102771:squamotacin; D014451:Ubiquinone; D006096:Gramicidin; D009245:NADH Dehydrogenase; C479743:NuoL protein, E coli; C476247:NuoM protein, E coli; D042967:Electron Transport Complex I; D015107:4-Butyrolactone
keywords: Complex I; EPR; Energy coupling; Iron–sulfur cluster; Proton pumping; Semiquinone