THE CYTOCHROME B(6)F COMPLEX

The cytochrome b(6)f complex in oxygenic photosynthetic membranes is believed to have many structure-function analogies to the cytochrome bc(1) complex found in the mitochondrial respiratory chain and in chromatophore membranes of purple photosynthetic bacteria. Three polypeptides in the complex, cytochrome f,cytochrome b(6) and the Rieske iron-sulfur protein contain redox prosthetic groups. Cytochromes f and b(6) are anchored to the membrane by one transmembrane helix and four transmembrane helices respectively. Recent studies on the Rieske high-potential iron-sulfur protein suggest that it may be an extrinsic membrane protein. As in mitochondria, the complex that is active in vitro is predominantly dimeric and it has not yet been possible to obtain a monomeric complex that has significant activity. The crystal structure of the major (252 residues, 87% of the protein) lumenal domain of cytochrome f has been solved to high (2.3 Angstrom) resolution. The elongate predominantly beta-strand structure, reminiscent of the motif of animal cell surface proteins, and the amino-terminal alpha-amino group acting as an axial heme ligand are unique features of the structure. This ligation provided an unexpected insight into the mechanism of translocation of the protein into the membrane. The structure makes specific predictions about the location of the positively charged docking site for the electron acceptor, plastocyanin.