CRYSTAL STRUCTURES OF CYTOCHROME c' AND ITS n-BUTYL-ISOCYANIDE-BOUND FORM FROM PURPLE PHOTOTROPHIC BACTERIUM. Tahir H. Tahirov, Shintaro Misaki, Yoshiki Higuchi and Noritake Yasuoka, Department of Life Science, Faculty of Science, Himeji Institute of Technology, Hyogo 678-12, Japan and Terry E. Meyer, and Michael A. Cusanovich, Department of Biochemistry, University of Arizona, Tucson, AZ 85721, U.S.A.

The structures of cytochrome c' from Rhodobacter capsulatus (RCCP) strain M110 have been determined by the molecular replacement method. Iron anomalous scattering data were also used to confirm the molecular replacement solution. The structures are refined at 1.72 Å and 2.0 Å resolutions to R values of 15.0% and 16.3% respectively. The RCCP molecule is a dimer and each of the identical 129 residue subunits folds as a four-helical bundle with a covalently bound heme group in the center. This structural motif resembles that of cytochromes c' reported from Rhodospirillum molischianum (RMCP), Rhodospirillum rubrum (RRCP), and Chromatium vinosum (CVCP). However, the architecture of the RCCP dimer, that is, the mode of association of subunits, differs substantially from that of the other cytochromes c'. In the RCCP dimer, the subunits are roughly parallel to each other and only helix B of each subunit participates in formation of the dimer interface. In RMCP, CVCP and RRCP the subunits cross each other to form an X shape, and two helices, A and B, of each subunit are involved in the interactions across the dimer interface. Structural comparison of four cytochromes c' reveal that they can be divided into two groups. In group 1 cytochromes c', CVCP and RCCP, the amino acid sequences and the folding of subunits are arranged in such a way to allow the formation of a large groove between helices B and C with direct solvent accessibility to the heme sixth ligand position. There is no such groove in group 2 cytochromes c', RMCP and RRCP. This may account in part for the differences in carbon monoxide binding.

An X-ray structure analysis of n-butylisocyanide-bound Rhodobacter capsulatus cytochrome c' was also carried out at 2.4 Å resolution. A comparison of the ligand bound structure with that of the native protein reveals that a significant conformational change of amino acid residues occurs in the heme vicinity, accompanied by a rearrangement of the hydrogen bonding pattern. The results suggest that heme puckering in cytochrome c' is a unique mechanical mechanism which may control the dissociation of the dimer to monomers upon ligand binding.