THE CYTOCHROME B(6)F COMPLEX

The cytochrome b(6)f complex in oxygenic photosynthetic membranes is b elieved to have many structure-function analogies to the cytochrome bc (1) complex found in the mitochondrial respiratory chain and in chroma tophore membranes of purple photosynthetic bacteria. Three polypeptide s in the complex, cytochrome f,cytochrome b(6) and the Rieske iron-sul fur protein contain redox prosthetic groups. Cytochromes f and b(6) ar e anchored to the membrane by one transmembrane helix and four transme mbrane helices respectively. Recent studies on the Rieske high-potenti al iron-sulfur protein suggest that it may be an extrinsic membrane pr otein. As in mitochondria, the complex that is active in vitro is pred ominantly dimeric and it has not yet been possible to obtain a monomer ic 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 pre dominantly beta-strand structure, reminiscent of the motif of animal c ell surface proteins, and the amino-terminal alpha-amino group acting as an axial heme ligand are unique features of the structure. This lig ation provided an unexpected insight into the mechanism of translocati on of the protein into the membrane. The structure makes specific pred ictions about the location of the positively charged docking site for the electron acceptor, plastocyanin.