Refined Structure of Cytochromeb562fromEscherichia coliat 1.4 Å Resolution

Abstract The structure of cytochrome b 5 6 2 from Escherichia coli has been refined at 1.4 A resolution against X-ray data collected on a Picker four-circle diffractometer. The triclinic unit cell parameters are a =33.68 A, b =50.48 A, c =32.67 A, α=102.51°, β=86.56° and γ=107.01° and there are two molecules in the asymmetric unit. A total of 138 cycles of restrained crystallographic refinement using the program PROLSQ were augmented at intermediate stages by two cycles of simulated annealing refinement using X-PLOR. The final crystallographic R -factor is 16.4% for data in the resolution range 6.0 A to 1.4 for a model containing 1650 protein atoms, 86 heme atoms, 165 water molecules and four sulfate anions. The root-mean-square deviations from ideal bond lengths and angles are 0.012 A and 2.0°, respectively. Each molecule consists of a bundle of four α-helices arranged in a simple up-dooown-up-down manner with a non-covalently bound heme group inserted between the first and fourth helices. In addition, there is a very short 3 1 0 helix in the 15-residue loop connecting the first and second pairs of helices. The two independent molecules show r.m.s. differences of 0.30 A for main-chain atoms and 0.88 A for all atoms. A detailed comparison with the structurally similar cytochrome c ° from Rhodospppirulum molishianum is presented. In addition, the titration behavior of cytochrome b 5 6 2 in solution is discussed in terms of its molecular structure.