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Abstract

The crystal structure of the soluble domain of the membrane bound cytochrome c₅₅₂ (cytochrome c₅₅₂′) from Paracoccus denitrificans was determined using the multiwavelength anomalous diffraction technique and refined at 1.5 Å resolution for the oxidized and at 1.4 Å for the reduced state. This is the first high-resolution crystal structure of a cytochrome c at low ionic strength in both redox states. The atomic model allowed for a detailed assessment of the structural properties including the secondary structure, the heme geometry and interactions, and the redox-coupled structural changes. In general, the structure has the same features as that of known eukaryotic cytochromes c. However, the surface properties are very different. Cytochrome c₅₅₂′ has a large strongly negatively charged surface part and a smaller positively charged area around the solvent-exposed heme atoms. One of the internal water molecules conserved in all structures of eukaryotic cytochromes c is also present in this bacterial cytochrome c. It contributes to the interactions between the side-chain of Arg36 and the heme propionate connected to pyrrole ring A. Reduction of the oxidized crystals does not influence the conformation of cytochrome c₅₅₂′ in contrast to eukaryotic cytochromes c. The oxidized cytochrome c₅₅₂′, especially the region of amino acid residues 40 to 56, appears to be more flexible than the reduced one.