Structure Determination and Refinement of Pseudomonas aeruginosa Cytochrome c₅₅₁ at 2.0 A Resolution

The crystal structure of the protein cytochrome c 5 5 1 from Pseudomonas aeruginosa has been solved by the method of multiple isomorphous replacement and refined by constrained difference Fourier 0 methods to an R-factor of 16.2% at 2.0 Å resolution. Crystallization conditions were found at pH 5.6. Three heavy atom derivatives (K2 PtCl4, UO2(NO3)2, and NaAu(CN)2) were used in the multiple isomorphous replacement x-ray diffraction phase analysis. A low resolution protein electron density map was calculated. The protein main chain was continuous and easily interpreted. The location of the heme and a few side chains were also determined. The resolution was extended to 2.4 Å and a wire model was built to the electron density. The map quality was good and almost all atoms fit into strong density. These coordinates were improved by refinement at 2.0 Å resolution to an R-factor of 16.2%. Additional structural information was obtained, including individual isotropic temperature factors, estimates of coordinate errors, and solvent positions. c 5 5 1 has major changes, compared to eukaryotic c, in the number (82 compared to 103) and the sequence of amino acids. Residues which have been highly conserved in c-type cytochrome structures are altered or deleted in c 5 5 1. Homology alignment predictions based on sequence are discussed. Despite these differences, the overall main chain fold is conserved, with only one major deletion and some minor deletions and additions.