Structure of cytochrome c₂ from Rhodospirillum centenum

Cytochrome c₂ from the purple photosynthetic bacterium Rhodospirillum centenum has been crystallized by the sitting-drop vapour-diffusion method. The crystals belong to the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a = 29.7, b = 59.9, c = 65.4 Å, and diffract to a resolution limit of 1.7 Å. The Fe-atom position was determined from its anomalous scattering contribution and a molecular-replacement solution was calculated. The correctness of the solution was confirmed by parallel isomorphous replacement studies. The resulting model has a type I cytochrome fold with two features, an extended α-helix and a surface-charge distribution, that are distinctive to this protein. The implications of these structural features for the ability of the cytochrome to serve as an electron carrier are discussed.