Electron transfer by domain movement in cytochrome bc1

Abstract

The cytochrome bc1 is one of the three major respiratory enzyme complexes residing in the inner mitochondrial membrane. Cytochrome bc1 transfers electrons from ubiquinol to cytochrome c and uses the energy thus released to form an electrochemical gradient across the inner membrane. Our X-ray crystal structures of the complex from chicken, cow and rabbit in both the presence and absence of inhibitors of quinone oxidation, reveal two different locations for the extrinsic domain of one component of the enzyme, an iron-sulphur protein. One location is close enough to the supposed quinol oxidation site to allow reduction of the Fe-S protein by ubiquinol. The other site is close enough to cytochrome c1 to allow oxidation of the Fe-S protein by the cytochrome. As neither location will allow both reactions to proceed at a suitable rate, the reaction mechanism must involve movement of the extrinsic domain of the Fe-S component in order to shuttle electrons from ubiquinol to cytochrome c1. Such a mechanism has not previously been observed in redox protein complexes.

X-Ray Structure of Rhodobacter Capsulatus Cytochrome bc1: Comparison with its Mitochondrial and Chloroplast Counterparts

“…Bacterial cyt bc 1 is smaller (a total of 886 amino acid residues in Rb. capsulatus) than the mitochondrial cyt bc 1 (2166 amino acid residues in beef (Schagger et al 1995) or 2079 in Saccharomyces cereviciae) (Zhang et al 1998) or even the chloroplast cyt b 6 f (975 amino acid residues in…”

“…These cyt bc complexes contain eight, seven and four additional non-catalytic subunits, respectively Zhang et al 1998;Stroebel et al 2003). Otherwise, the overall shape of the catalytic subunits and the relative positions of their redox cofactors are highly conserved and the secondary structural elements are very similar between the three classes of bc/bf complexes.…”

“…Indeed, a great deal of information is available concerning the effects of single, as well as compensatory pairs of point mutations on the assembly and function of Rhodobacter cyt bc 1 (Brasseur et al 1996). While the interpretation of these data has been greatly aided by the availability of the mitochondrial structures (Xia et al 1997;Iwata et al 1998;Zhang et al 1998;Hunte et al 2000;Gao et al 2003;Palsdottir et al 2003), there are also important differences due to specific insertions and deletions that are uniquely present in the bacterial or mitochondrial enzymes.…”

“…Various three dimensional structures of the mitochondrial cyt bc 1 , which have been accumulated during the last several years (Xia et al 1997;Iwata et al 1998;Zhang et al 1998;Hunte et al 2000;Gao et al 2003;Palsdottir et al 2003), have recently been supplemented by the structures of the cyt b 6 f from cyanobacteria and chloroplast (Kurisu et al 2003;Stroebel et al 2003). Crystallization of the simpler bacterial cyt bc 1 has been 6 considerably more challenging, and the crystals obtained to date are not particularly satisfactory for structure determination due to high mosaicity as well as poor and anisotropic crystalline order.…”

“…As in the mitochondrial enzyme the membrane extrinsic domain of bacterial cyt c 1 sits atop the A, B, C, D helices and their connecting loops of cyt b, while its membrane anchor runs parallel to the α-helix E of cyt b (Iwata et al 1998;Zhang et al 1998;Hunte et al 2000). Even between the closely related proteobacterial and mitochondrial cyt c 1 there are major differences that will be discussed below.…”