CRYSTAL-STRUCTURE OF CYTOCHROME C' FROM RHODOCYCLUS-GELATINOSUS AND COMPARISON WITH OTHER CYTOCHROMES C'

Cytochromes c' are heme proteins found in photosynthetic and denitrify ing bacteria, where they are presumably involved in electron transport . The cytochrome c' isolated from the bacterium Rhodocyclus gelatinosu s (RGCP) forms a homodimer with each polypeptide containing 129 residu es. It has been crystallised in ammonium sulfate at pH 6. Crystals bel ong to space group P3(1)21 with cell parameters a = 70.2 Angstrom and c = 126.8 Angstrom, which corresponds to a dimer in the asymmetric uni t (VM = 3.5 Angstrom(3) / Da). The crystal structure of RGCP was solve d by the molecular replacement method and refined using data to 2.5-An gstrom resolution. The final crystallographic R factor was 17.9% for a ll reflections (above 2 sigma) in the resolution range 27.4 to 2.5 Ang strom. The refined model includes 1876 non-hydrogen protein atoms and 56 water molecules. As typical of c-type cytochromes, the heme group i s covalently bound to Cys-X-Y-Cys-His through thio-ether bonds, and Hi s123 oc occupies the fifth axial coordination position. On the vacant ''distal'' site, Phe16 blocks the direct access to the sixth coordinat ion site, which is in a predominantly hydrophobic environment. In spit e of the low sequence homology among cytochromes c' the overall fold i s similar. The monomer structure consists of 4 anti-parallel a-helices and has random coils in the loops between the helices, and at the N- and C-termini. The subunits cross each other to form an X shape.