Subunits of the cytochrome b6f complex from plants and cyanobacteria: Towards a model of the cytochrome b6f complex.

摘要

The cytochrome b₆f complex is the central electron transfer complex of oxygenic photosynthesis. It is analogous to the cytochrome bc₁ complex of respiration. The soluble fragments of two subunits of the cytochrome b₆f complex were crystallized, and the structures of these subunits were obtained. The structure of one of these subunits, the high-potential Rieske protein was compared with the Rieske protein found in the cytochrome bc ₁ complex. The comparison of these two structures showed that the structures of the cluster-binding domains of the b₆f and bc₁ Rieske proteins were virtually identical, while the large domain structures were divergent. The structures of cytochrome f from the cyanobacterium Phormidium laminosum and from the green alga Chlamydomonas reinhardtii were compared with cytochrome f from turnip. The turnip cytochrome f structure was also re-refined in both the reduced and oxidized states. Cytochrome f from Cyanobacteria, from higher plants and from green algae have one common surface with a conserved neutral electrostatic profile. It is suggested that this conserved surface should contact lumen-side loops of the membrane-spanning cytochrome b₆ and subunit IV, which are homologous to the N-terminal and C-terminal regions of cytochrome b, respectively. A hypothetical model of a b₆f complex was constructed, using the membrane-spanning subunits of the bc₁ complex as a model for cytochrome b₆ and subunit IV and the b₆f Rieske protein substituted for the bc₁ Rieske protein. Cytochrome f was placed in accordance with an electron density map obtained from electron diffraction studies and an angle of 30° between the heme group and the membrane obtained from EPR studies. Finally, the conserved neutral surface was placed against the membrane-spanning subunits. Possible hypotheses regarding the origins of the large subunits of the b₆f and bc₁ complexes are also discussed.