X-ray crystallographic studies on an inhibitor-induced conformational change of the iron-sulfur protein in mitochondrial BC1 complex

As one of universal components, the be1 complex catalyzes proton transport across the membrane coupled to electron transfer through its four redox centers in a complicated, but very ordered manner. The Q cycle hypothesis features two separated ubiquinone binding sites (Qo and Qi) and a bifurcated electron path; It is strongly supported by the effects of different types of specific inhibitors on each electron path. Our X-ray crystallographic structure of the native and inhibitor-bound bcl complex showed two distinct cavities (Qo and Qi) in the structure of the cytochrome b subunit. The electron density of different Qo inhibitors largely overlapped each other, indicating that they are mutually exclusive; they appear to be independent of the binding of Antimyein, one of Qi inhibitors. One unexpected finding in this study is a conformational change of the iron-sulfur protein (ISP) induced by binding of certain inhibitors. This conformational change was detected by a peak height change and/or a positional shift of the anomalous signal of the protein's iron-sulfur center, as well as features in difference Fourier maps. In one extreme case, the conformational change of the ISP causes a t5 A positional shift of iron-sulfur center, which reduces its center-to-center distance to cytochrome cl from 31 A to 21 A. The ability of the ISP to adopt at least two different conformations in the be1 complex makes us speculate that electron transfer between ubiquinone and cytochrome cl may be mediated by a hinged motion of the ISP.