ELECTRON-TRANSFER BY DOMAIN MOVEMENT IN STOCKBROKER BC(1)

The cytochrome bc(1) is one of the three major respiratory enzyme comp lexes residing in the inner mitochondrial membrane. Cytochrome bc(1) t ransfers electrons from ubiquinol to cytochrome c and uses the energy thus released to form an electrochemical gradient across the inner mem brane. Our X-ray crystal structures of the complex from chicken, cow a nd rabbit In both the presence and absence of inhibitors of quinone ox idation, reveal two different locations for the extrinsic domain of on e component of the enzyme, an Iron-sulphur protein. One location Is cl ose enough to the supposed quinol oxidation site to allow reduction of the Fe-S protein by ubiquinol. The other site is close enough to cyto chrome cl to allow oxidation of the Fe-S protein by the cytochrome. As neither location will allow both reactions to proceed at a suitable r ate, the reaction mechanism must involve movement of the extrinsic dom ain of the Fe-S component In order to shuttle electrons from ubiquinol to cytochrome c(1). Such a mechanism has not previously been observed in redox protein complexes.