CRYSTAL STRUCTURE OF BEEF HEART MlTOCHONDRIAL CYTOCHROME BC1 COMPLEX. Di Xia, Hocon Kim, Johann Deisenhofer¹. Chang-An Yu, Jia-Zhi Xia, Linda Yu^2, HHMI and Department of Biochemistry, University of Texas Sourthwestern Medical Center at Dallas TX 75235^1, Oklahoma State University, Stillwater, OK 74078²

The cytochrome bcl complex from bovine mitochondria plays a central role in cell respiration. It is a membrane protein complex that consists of ten protein subunits and four redox centers with a molecular weight of 240,000 Daltons. Its crytallization has been reported by several laboratories. Our crystals, grown in the presence of glycerol, diffract Xray to better than 3 Å resolution under cryogenic conditions. They have the symmetry of the space group I4₁22 with unit cell dimensions of a=b=153.5 Å and c=597.7 Å, and one bcl complex in the crystallographic asymmetric unit. Phases have been determined to 3.3 Å resolution by the MIR method with four heavy atom derivatives. The solvent flattened electron density clearly shows the transmemebrane region with thirteen transmembrane helices. Four high peaks in the electron density and in maps calculated using anomalous scattering data measumd near the iron absorption edge are interpreted as the redox centers of the bc1 complex. Two of these sites are 20 Å apart in the transmembrane region and most likely represent the heme irons of cytochromes b₅₆₂ and b₅₆₆. Another site near the membrane surface and 26 Å away from the nearest b-heme could be the iron-sulfur center, the fourth site, presumably the cytochrome c1 heme is 31 Å apart from this center. The majority of the molecular mass outside the membrane is located on the side of the membrane opposite from the redox centers, presumably the matrix side of the mitochondrial membrane. The electron density map also reveals a very tight association of two monomers related by a two-fold symmetry. The overall dimensions of the dimer are about 130 Å in diameter and 151 Å in height, with the heights for the inter-membrane space region, the transmembrane region and the matrix region 41 Å, 35 Å and 75 Å, respectively. The distances between the symmetry-related irons of b₅₆₂, b₅₆₆, FeS and cl are 33 Å, 21 Å, 63 Å and 53 Å, respectively.