Orientation of the g-tensor axes of the Rieske subunit in the cytochrome bc1 complex

The orientation of the g-tensors of the Rieske iron-sulfur protein subunit was determined in a single crystal of the bovine mitochondrial cytochrome bc(1) complex with stigmatellin in the Q(0) quinol binding site. The g-tensor principal axes are skewed with respect to the Fe-Fe and S-S atom direction in the 2Fe2S cluster, which is allowed by the lack of rigorous symmetry of the cluster. The asymmetric unit in the crystal is the active dimer, and the g-tensor axes have slightly different orientations relative to the iron-sulfur cluster in the two halves of the dimer. The g similar to 1.79 axis makes an average angle of 30degrees with respect to the Fe-Fe direction and the g similar to 2.024 axis an average angle of 26degrees with respect to the S-S direction. This assignment of the g-tensor axis directions indicates that conformations of the Rieske protein are likely the same in the cytochrome bc(1) and b(6f) complexes and that the extent of motion of the Rieske head domain during the catalytic cycle has been highly conserved during evolution of these distantly related complexes.