ECTOTHIORHODOSPIRA-HALOPHILA FERROCYTOCHROME C-551 - SOLUTION STRUCTURE AND COMPARISON WITH BACTERIAL CYTOCHROME-C

The solution structure of the Ectothiorhodospira halophila ferrocytoch rome c(551) has been determined. This molecule belongs to a separate c lass of small bacterial cytochromes c for which no 3D structure has be en reported so far. It is characterized by a very low redox potential (58 mV) and is isolated from the periplasm of halophilic purple photot rophic bacteria. For the 78 residue protein, 1445 NOE derived distance constraints were used in a combined simulated annealing/restrained mo lecular dynamics calculation. The final ensemble of 37 structures pres ents a backbone r.m.s.d. of less than 0.5 Angstrom compared to the mea n structure. The physical viability of these structures was investigat ed by subjecting eight of them to a constraint free molecular dynamics simulation. No systematic conformational change was observed and the average backbone r.m.s.d. compared to the initial structures was less than 1.5 Angstrom. The structure of the E. halophila cytochrome c(551) shows a striking resemblance to Azotobacter vinelandii cytochrome c(5 ). Significant differences in backbone conformations occur in three sm all regions which are implicated in solvent protection of the heme pro pionates and thiomethyl-8(1). Comparison with Pseudomonas aeruginosa c ytochrome c(551) reveals that only the common cytochrome c core, i.e. three helices, is conserved. The folding of the protein chain around t he heme propionates is very different and results in more efficient so lvent protection in Ps. aeruginosa. The electrostatic surface of E. ha lophila cytochrome c(551) was found to be significantly different from mitochondrial cytochromes c and bacterial cytochromes c(2) but simila r to that of Ps. aeruginosa cytochrome c(551). (C) 1996 Academic Press Limited