Amino acid sequence, crystallization and structure determination of reduced and oxidized cytochrome c(6) from the green alga Scenedesmus obliquus

Cytochrome c(6) from the unicellular green alga Scenedesmus obliquus was se quenced, crystallized in its reduced and oxidized state and the three-dimen sional structure of the protein in both redox states was determined by X-ra y crystallography. Reduced cytochrome c(6) crystallized as a monomer in the space group P2(1)2(1)2, whereas the oxidized protein crystallized as a dim er in the space group P3(1)21. The structures were solved by molecular repl acement and refined to 1.9 and 2.0 Angstrom, respectively. Comparison of th e structures of both redox states revealed only slight differences on the p rotein surface, whereas a distortion along the axis between the heme iron a nd its coordinating Met61 residue was observed. No redox-dependent movement of internal water molecules could be detected. The high degree of similari ty of the surfaces and charge distributions of both redox states, as well a s the dimerization of cytochrome c(6) as observed in the oxidized crystal, is discussed with respect to its biological relevance and its implications for the reaction mechanisms between cytochrome c(6) and its redox partners. The dimer of oxidized cytochrome c(6) may represent a molecular structure occurring in a binary complex with cytochrome b(6)f. This assembly might be required for the correct orientation of cytochrome c(6) with respect to it s redox partner cytochrome b(6)f, facilitating the electron transfer within the complex. If the dimerization is not redox-dependent in vivo, the almos t identical surfaces of both redox states do not support a long range diffe rentiation between reduced and oxidized cyt c(6), i.e. a random collision m odel for the formation of an electron transfer complex must be assumed. (C) 1999 Academic Press.