W0244

Mobility of the Iron-Sulfur Protein Subunit in the Cytochrome Bc1 Complex Revealed by X-Ray Crystallography. Hoeon Kim, Di Xia, Johann Deisenhofer, Chang-An Yu, Anatoly Kachurin, Li Zhang, Linda Yu, HHMI and Department of Biochemistry, Univeristy of Texas Southwestern Medical Center at Dallas, Dallas, TX 75235 USA

Early studies (1,2) of the cytochrome bc1 complex with some typical inhibitors showed that the two separate pockets in the structure of cytochrome b provide binding sites for two major types of electron transfer inhibitors (Qo and Qi), and that Qo inhibitors bind to slightly different sectors in the Qo pocket, depending on their functionally characterized subtypes. Furthermore, some of Qo inhibitors could induce the structural change of the iron-sulfur protein (ISP) subunit. This finding led to the speculation that the mobility of the ISP is essential in electron transfer between ubiquinone and cytochrome c1.

In order to confirm the Qo inhibitor-induced structral effect on the ISP, crystallographic studies were continued with several new Qo inhibitors in either absence or presence of antimycin A, a Qi inhibitor. Co-crystals were grown and X-ray data were collected at various synchrotron sources. All the data were isomorphous to the native data and difference Fourier maps showed clear density for bound inhibitors. It was found that all Qo-I inhibitors bind to the same sector of the Qo pocket and exert the similar structural effect on the ISP, as the anomalous signal for the iron-sulfur center of the ISP constantly disappears. It indicates that binding of the Qo-I inhibitors to that region promotes the mobility of the ISP. On the other hand, all Qo-II and -III inhibitors that bind to other sector, showed the opposite structural effect; depression of the ISP mobility.

Taking the ISP mobility into an account, we proposed a catalytic model in the Qo pocket of the cytochrome bc1 complex.

(1) J. Deisenhofer et al. (1997) FASEB J. 11, A1279

(2) H. Kim et al. (1997) FASEB J. 11, A1084