Structural studies on cytochrome bc1 complex from bovine heart mitochondria

Abstract: The cytochrome bc1 complex (Ubiquinol:Cytochrome c oxidoreductase or Complex III) is an enzyme of the mitochondrial respiratory chain, crucial for aerobic metabolism. This enzyme catalyses the electron transfer from membrane-bound ubiquinol to soluble cytochrome c. Coupled with this reaction, protons are translocated from the matrix side to the mitochondrial intermembrane space. This proton translocation mechanism is known as the proton motive Qcycle, originally proposed by Peter Mitchell. The generated proton gradient across the membrane is utilised by the FoF1-ATPase for ATP synthesis. Bovine heart mitochondrial bc1 complex is composed of 11 polypeptide subunits with a total combined weight of 240 kDa. This thesis presents structural studies on the cytochrome bc1 complex from bovine heart mitochondria using X-ray crystallography. Structures of the complex were solved in two different crystal forms, P65 and P6522 at 2.5 and 3.0 Å, respectively. The structures of the protein with three different inhibitors and a substrate were also examined. The high resolution of the P65 form crystals, provided detailed structural information for the complex, including the associated lipid and solvent structures. Further comparative studies of the structures from different crystal forms revealed a domain motion of the Rieske FeS protein. Using this domain motion and the structures of the quinone binding sites, the electron bifurcation mechanism at the QP binding pocket, the key step of the proton motive Qcycle, has been clearly explained. The structure of the cytochrome bc1 complex also revealed the cause of a mitochondrial myopathy. The disorder, in which exercise intolerance is the predominant symptom, is associated with mutations in the cytochrome b subunit. The positions and physiological implications of these mutations were identified using the structure of the cytochrome bc1 complex. The results clearly indicated that the molecular cause of this myopathy is either dysfunctional quinone binding sites or impaired electron transfer from the QP site to the Rieske FeS protein.