What information do inhibitors provide about the structure of the hydroquinone oxidation site of ubihydroquinone: Cytochromec oxidoreductase?

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Abstract
The Q cycle mechanism of thebc 1 complex requires two quinone reaction centers, the hydroquinone oxidation (QP) and the quinone reduction (QN) center. These sites can be distinguished by the specific binding of inhibitors to either of them. A substantial body of information about the hydroquinone oxidation site has been provided by the analysis of the binding of QP site inhibitors to thebc 1 complex in different redox states and to preparations depleted of lipid or protein components as well as by functional studies with mutantbc 1 complexes selected for resistance toward the inhibitors. The reaction site is formed by at least five protein segments of cytochromeb and parts of the iron-sulfur protein. At least two different binding sites for QP site inhibitors could be detected, one for the methoxyacrylate-type inhibitors binding predominantly to cytochromeb, the other for the chromone-type inhibitors and hydroxyquinones binding predominantly to the iron-sulfur protein. The interactions with the protein segments, between different protein segments, and between protein and ligands (substrate, inhibitors) are discussed in detail and a working model of the QP pocket is proposed.

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