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A Brownian Dynamics Study of the Interactions of the Luminal Domains of the Cytochrome b₆f complex with Plastocyanin and Cytochrome c₆: The Effects of the Rieske FeS Protein on the Interactions

Biophysics Program and Department of Biochemistry, The Ohio State University, Columbus, Ohio 43210

Correspondence: Address reprint requests to Elizabeth L. Gross, Dept. of Biochemistry, The Ohio State University, Columbus, OH 43210. E-mail: gross.3{at}osu.edu.

The availability of the structures of the cytochrome b₆f complex (cyt b₆f), plastocyanin (PC), and cytochrome c₆ (cyt c₆) from Chlamydomonas reinhardtii allowed us, for the first time, to model electron transfer interactions between the luminal domains of this complex (including cyt f and the Rieske FeS protein) and its redox partners in the same species. We also generated a model structure in which the FeS center of the Rieske protein was positioned closer to the heme of cyt f than observed in the crystal structure and studied its interactions with both PC and cyt c₆. Our data showed that the Rieske protein in both the original crystal structure and in our modeled structure of the cyt b₆f complex did not physically interfere with binding position or orientation of PC or cyt c₆ on cyt f. PC docked on cyt f with the same orientation in the presence or the absence of the Rieske protein, which matched well with the previously reported NMR structures of complexes between cyt f and PC. When the FeS center of the Rieske protein was moved close to the heme of cyt f, it even enhanced the interaction rates. Studies using a cyt f modified in the 184–191 loop showed that the cyt f structure is a more important factor in determining the rate of complex formations than is the presence or the absence of the Rieske protein or its position with respect to cyt f.