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要旨

Quinone molecules are ubiquitous in living organisms. They are found either within the lipid phase of the biological membrane (quinone pool) or are bound in specific binding sites within membrane-bound protein complexes. The biological function of such bound quinones is determined by their ability to be reduced and/or oxidized in two successive one-electron steps. As a result, quinones are involved as one- or two-electron donors or acceptors in a large number of biological electron-transfer steps occurring during respiratory or photosynthetic processes. The intermediate formed by a one-electron reduction step is a semiquinone, which is paramagnetic and can be studied by electron paramagnetic resonance (EPR) spectroscopy. Detailed studies of such states can provide important structural information on these intermediates in such electron-transfer processes. In this study, we focus on the redox-active ubiquinone-6 of the yeast cytochrome bc₁ complex (QCR, ubiquinol: cytochrome c oxidoreductase) from Saccharomyces cerevisiae at the so-called Q_i site. Although the location of the Q_i binding pocket is quite well known, details about its exact binding are less clear. Currently, three different X-ray crystallographic studies suggest three different binding geometries for Q_i. Recent studies in the bacterial system (Rhodobacter sphaeroides) have suggested a direct coordination to histidine as proposed in the chicken heart crystal structure model. Using the yeast system we apply EPR and especially relaxation filtered hyperfine (REFINE) spectroscopy to study the Q_i binding site. ¹⁴N-electron spin-echo envelope modulation spectroscopy together with an inversion-recovery filter (REFINE) is applied to resolve the question of whether ¹⁴N modulations arise from interactions to Q_i^·- or to the Rieske iron–sulphur center. These results are discussed with regard to the location and potential function of Q_i in the enzyme.