On the mechanism of ubiquinol oxidation by the bc1-complex: the role of the Rieske subunit, and its mobility.

Edward A. Berry#, Zhaolei Zhang#, LiShar Huang#, Richard Kuras*, Mariana Guergova-Kuras* and Antony R. Crofts*, Center for Biophysics and Computational Biology, U. of Illinois at Urbana-Champaign*, and Lawrence Berkeley National Laboratory, U. C. Berkeley#.
The positions of Fe-centers in the mitocondrial bc1-complex of beef heart have been made available by Xia, Deisenhofer, Yu and colleagues. We have compared these positions with those from our own work on the complex from chicken heart, beef heart (in hexagonal crystals) and from rabbit. We have noted that, although the heme Fe-atoms appear to occupy equivalent positions in all the structures, the positions of the 2Fe2S centers of the Rieske subunit (ISP, or iron sulfur protein) are different. There is a displacement of ~10 between the position in beef enzyme as solved by Xia et al., and that in the chicken, the rabbit, and the hexagonal form of the beef enzyme. We suggest that the different ISP conformations reflect a displacement of the subunit which is a necessary part of the catalytic mechanism of the Qo-site of the complex. We note that in none of the crystal forms is the complex in a form in which the static configuration could be competent in all partial reactions of quinol oxidation. However, in each of the two main positions, the configuration could function for a part of the reaction cycle. We suggest that the ISP must move between the positions seen in different crystal forms. In confirmation of this possibility, we have observed that in the chicken enzyme, stigmatellin induces a displacement of the ISP from its position close to cyt c1, to a position close to that inferred for the Xia et al. structure. The ISP binds at a concave interface on cyt b, which brings the 2Fe2S center close to a buried myxothiazol binding site (as observed from the electron density difference in myxothiazol-containing crystals), which is likely the quinol binding pocket. We discuss the mechanism of quinol oxidation in the context of such a movement. We note that movement of the ISP as a necessary part of turn-over throws new light on many anomalous points from the literature.