Mechanistic Aspects of the Q0 -Site of the bc1 -Complex As Revealed by Mutagenesis Studies, and the Crystallographic Structure

Ubiquinolxytochrome c oxidoreductase (bc1 -complex) is a central component of the electron transfer system in almost all organisms, occurring ubiquitously in respiratory and photosynthetic chains of mitochondria and bacteria, and (as the b6 f-complex) in the photo-synthetic chain of oxygenic photosynthesis (see refs. 1-3 for recent reviews). A modified Q-cycle (4—9) accounts well for a large body of experimental data from studies in which the function has been explored. The mechanism involves two catalytic sites for oxidation or reduction of the quinone couple, and a third site for electron transfer to cytochrome c. The function of the two quinone reactive sites has been explored biochemically, and characterized using kinetic spectroscopy, and their activities differentiated through use of inhibitors that act specifically at one site or the other (1-13). The quinol oxidizing site (Qo -site) catalyzes a unique reaction in which the electrons from QH2 are passed to separate electron transport chains in the complex through a bifurcated reaction. Further information on the occupancy of the Qo -site has been obtained though study of the interaction of the occupant with the 2Fe2S center of the reduced ISP, observed through changes in the EPR spectrum of this center (33-40). In addition to the effects observed with stigmatellin and UHDBT (10-13, 33), a well-defined band at gx = 1. 800 has been observed when quinone occupies the pocket. The dependence of these spectral changes on ambient redox potential, and on the extent of extraction of quinone, has lead Ding (34, 35) and colleagues to suggest a double occupancy of the site by weakly (Qow) and strongly (Qos) binding quinone species.