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

Purdue University
Publication Date
  • Biophysics
  • General
  • Biology


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. ^

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