Crystallization and preliminary structure of beef heart mitochondrial cytochrome-bc₁ complex

The method reported for isolation of ubiquinol-cytochrome-c reductase complex from submitochondrial particles was modified to yield a preparation for crystallization. The cytochrome be, complex was first crystallized in large thin plate form and diffracts X-rays to 7 A resolution in the presence of mother liquor. This crystalline complex was enzymatically active and contains ten protein subunits. It had 33 mol phospholipid and 0.6 mol ubiquinone per mol protein. With slightly modified crystallization conditions, different crystal forms were obtained. Crystals grown in the presence of 20% glycerol diffracted X-rays up to 2.9 Å resolution using a synchrotron source. Four heavy atom derivatives have been obtained. The 3-D structure of the cytochrome bc₁ complex was solved to 3.4 Å resolution. Crystalline cytochrome bc₁ complex is a dimer: most of the masses of core proteins I and II protrudes from the matrix side of the membrane, whereas the cytochrome b protein is located mainly within the membrane. There are 13 transmembrane helices in each monomer. Most of the mass of cytochrome c₁ and iron-sulfur protein including their redox centers are located on the cytoplasmic side of the membrane. The distances between these redox centers have been determined, and several electron transfer inhibitor binding sites in the complex have been located.