Transmembrane traffic in the cytochrome b6f complex

Crystal structures and their implications for function are described for the energy transducing hetero-oligomeric dimeric cytochrome b(6)f complex of oxygenic photosynthesis from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonds reinbardtii. The complex has a cytochrome b core and a central quinone exchange cavity, defined by the two monomers that are very similar to those in the respiratory cytochrome bc(1) complex. The pathway of quinol/quinone (Q/QH(2)) transfer emphasizes the labyrinthine internal structure of the complex, including an 11 x 12 A portal through which Q/QH2, containing a 45-carbon isoprenoid chain, must pass. Three prosthetic groups are present in the b(6)f complex that are not found in the related bc(1) complex: a chlorophyll (Chl) a, a beta-carotene, and a structurally unique covalently bound heme that does not possess amino acid side chains as axial ligands. It is hypothesized that this heme, exposed to the cavity and a neighboring plastoquinone and close to the positive surface potential of the complex, can function in cyclic electron transport via anionic ferredoxin.