Abstact

Cytochrome c6from the unicellular green alga Scenedesmus obliquus was sequenced, crystallized in its reduced and oxidized state and the three-dimensional structure of the protein in both redox states was determined by X-ray crystallography. Reduced cytochrome c6crystallized as a monomer in the space group P 21212, whereas the oxidized protein crystallized as a dimer in the space group P 3121. The structures were solved by molecular replacement and refined to 1. 9 and 2.0 A, respectively. Comparison of the structures of both redox states revealed only slight differences on the protein surface, whereas a distortion along the axis between the heme iron and its coordinating Met61 residue was observed. No redox-dependent movement of internal water molecules could be detected. The high degree of similarity of the surfaces and charge distributions of both redox states, as well as the dimerization of cytochrome c6as observed in the oxidized crystal, is discussed with respect to its biological relevance and its implications for the reaction mechanisms between cytochrome c6and its redox partners. The dimer of oxidized cytochrome c6may represent a molecular structure occurring in a binary complex with cytochrome b6f. This assembly might be required for the correct orientation of cytochrome c6with respect to its redox partner cytochrome b6f, facilitating the electron transfer within the complex. If the dimerization is not redox-dependent in vivo, the almost identical surfaces of both redox states do not support a long range differentiation between reduced and oxidized cyt c6, i.e. a random collision model for the formation of an electron transfer complex must be assumed.