Structure of cytochrome c₇ from Desulfuromonas acetoxidans at 1.9 Å resolution

Multihaem cytochromes play a key role in electron-transport reactions in the periplasm of sulfate- and sulfur-reducing bacteria. The redox proteins grouped in the c₃ superfamily also display metal-reducing activities, which make them interesting biotechnological tools. The crystal structure of the fully oxidized cytochrome c₇ from Desulfuromonas acetoxidans has been solved by combined molecular-replacement and MAD methods. The structure has been refined at 1.9 Å resolution to an R value of 19.1% (R_free = 24.3%) and includes three haems and 116 water molecules. The protein displays the cytochrome c₃ fold in a highly minimized form, while haem 2 and the surrounding protein environment are missing. The geometry of haem packing and of the haem axial ligands and propionates are described and compared with that of c₃ cytochromes. The crystal structure is compared with the solution structure recently obtained by NMR methods and with its homologue cytochromes of the c₃ superfamily. Comparison of the high number of available structures makes it possible to analyze the structural role of the few highly conserved residues, in addition to the cysteines and histidines that link the porphyrin rings and the Fe atoms to the protein chain.