title: Characterization of the delta muH+-sensitive ubisemiquinone species (SQ(Nf)) and the interaction with cluster N2: new insight into the energy-coupled electron transfer in complex I.
abstract: In this report, we describe the electron paramagnetic resonance (EPR) spectroscopic characterizations of the fast-relaxing ubisemiquinone (SQ(Nf)) species associated with NADH-ubiquinone oxidoreductase (complex I) detected in tightly coupled submitochondrial particles (SMP). The signals of SQ(Nf) are observed only in the presence of delta muH+, whereas other slowly relaxing SQ species, SQ(Ns) and SQ(Nx), are not sensitive to delta muH+. In this study, we resolved the EPR spectrum of the delta muH+-sensitive SQ(Nf), which was trapped during the steady-state NADH-Q1 oxidoreductase reaction, as the difference between coupled and uncoupled SMP. Thorough analyses of the temperature profile of the resolved SQ(Nf) signals have revealed previously unrecognized spectra from delta muH+-sensitive SQ(Nf) species. This newly detected SQ(Nf) signals are observable only below 25 K, similar to the cluster N2 signals, and exhibit a doublet signal with a peak-to-peak separation (deltaB) of 56 G. In this work, we identify the partner to the interacting cluster N2. We have analyzed the g = 2.00 and g = 2.05 splittings using a computer simulation program that includes both exchange and dipolar interactions as well as the g-strain effect. Computer simulation of these interaction spectra showed that cluster N2 and fast-relaxing SQ(Nf) species undergo a spin-spin interaction, which contains both exchange (55 MHz) and dipolar interaction (16 MHz) with an estimated center-to-center distance of 12 A. This finding delineates an important functional role for this coupled [(N2)(red)-SQ(Nf)] structure in complex I, which is discussed in connection with electron transfer and energy coupling.
authors.affiliation: Johnson Research Foundation, Department of Biochemistry and Biophysics, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. yano@mail.med.upenn.edu
authors.forename: Takahiro
authors.initials: T
authors.lastname: Yano
authors.forename: William R
authors.initials: WR
authors.lastname: Dunham
authors.forename: Tomoko
authors.initials: T
authors.lastname: Ohnishi
mesh_terms: D000818:Animals; D020134:Catalytic Domain; D002417:Cattle; D003067:Coenzymes; D003080:Cold Temperature; D003198:Computer Simulation; D041961:Deuterium Exchange Measurement; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D042967:Electron Transport Complex I / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008929:Mitochondria, Heart / Q000201:enzymology; D008956:Models, Chemical; D009245:NADH Dehydrogenase / Q000737:chemistry; D010084:Oxidation-Reduction; D011522:Protons; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000737:chemistry*
chemical_list: D003067:Coenzymes; D007506:Iron-Sulfur Proteins; D011522:Protons; D014451:Ubiquinone; D009245:NADH Dehydrogenase; D042967:Electron Transport Complex I; C024989:coenzyme Q10