Mechanistic aspects of the Qo-site of the bc1-complex as revealed by mutagenesis studies, and the crystallographic structure.

 A.R. Crofts*, Blanca Barquera*, R.B. Gennis*, R. Kuras*, Mariana Guergova-Kuras* and E.A. Berry#
Center for Biophysics and Computational Biology, U. of Illinois at Urbana-Champaign*, and Lawrence Berkeley National Laboratory, U. C. Berkeley#.


Solution of the X-ray crystallographic structure of the ubiquinol:cytochrome c oxidoreductase (the bc1-complex) from several animal sources has provide a basis for understanding the mechanism, and the effects of mutation on the function of the complex. In this paper, we review the biophysical information on the mechanism of the ubihydroquinone (quinol) oxidizing site (the Qo-site), the information from analysis of mutant strains which effect function or inhibitor binding at the quinol oxidizing site, and details of the structure of the complex from avian heart mitochondria as it pertains to this catalytic site. A paradoxical feature of the quinol oxidation reaction is the mechanism that ensures that the two electrons from quinol enter separate electron transfer chains. The crystallographic data show several features of interest in this mechanism. The iron sulfur protein is found in several different positions in different structures, and in structures with inhibitors bound, stigmatellin and myxothiazol occupy different domains in the Qo-binding pocket, with the former distal and the latter proximal to heme bL. We suggest a mechanism with the following novel features: i) movement of the iron sulfur protein between two separate reaction interfaces on cytochrome c1 and cytochrome b, and ii) movement of the occupant of the Qo-binding pocket between a domain distal from the heme of cyt bL, but close to the iron sulfur protein docking interface, and a domain proximal to heme bL. We suggest that all interactions with the iron sulfur protein occur from the distal position. These include a) the binding of stigmatellin and UHDBT, which lead to a change in mid-point potential and a shift in spectrum of the reduced 2Fe2S center, b) the formation of a complex between ubiquinone and the reduced iron sulfur protein which gives rise to the gx=1.800 EPR band, and c) the oxidation of quinol to ubisemiquinone by the oxidized 2Fe2S center. We suggest that after formation, the ubisemiquinone moves to the proximal position before transferring an electron to heme bL, and that this movement, together with a change in conformation of the site through displacement of a few  residues, prevents further interaction with the 2Fe2S center.

Crofts, A.R., Barquera, B., Gennis, R.B., Kuras, R., Guergova-Kuras, M. and Berry, E.A. (1997) Mechanistic aspects of the Qo-site of the bc1-complex as revealed by mutagenesis studies, and the crystallographic structure. Proceedings of the IXth. International Symposium on Phototrophic Prokaryotes, Vienna, Sept. 1997, (Peschek et al., eds.). In press