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An atypical haem in the cytochrome b6f complex


Photosystems I and II (PSI and II) are reaction centres that capture light energy in order to drive oxygenic photosynthesis; however, they can only do so by interacting with the multisubunit cytochrome b6f complex. This complex receives electrons from PSII and passes them to PSI, pumping protons across the membrane and powering the Q-cycle. Unlike the mitochondrial and bacterial homologue cytochrome bc1, cytochrome b6f can switch to a cyclic mode of electron transfer around PSI using an unknown pathway. Here we present the X-ray structure at 3.1 Å of cytochrome b6f from the alga Chlamydomonas reinhardtii. The structure bears similarities to cytochrome bc1 but also exhibits some unique features, such as binding chlorophyll, β-carotene and an unexpected haem sharing a quinone site. This haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand. This haem may be the missing link in oxygenic photosynthesis.

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We thank the local contacts of beamline BM30 (ESRF, CRG FIP) J.-L. Ferrer, R. Kahn and P. Carpentier for assistance in data collection; C. Lebreton for membrane preparations; P. Hervé and P. Fellmann for the synthesis of TDS; Y. Pierre, F. Zito and C. Breyton for biochemical expertise; P. Joliot, J. Lavergne, F. Daldal, F. Baymann, W. Nitschke and G. Ajlani for discussions; and C. Breyton, G. Finazzi and A. A. Pascal for discussions and critical reading of the manuscript. This work was supported by the CNRS and Paris-7 University.

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Correspondence to Jean-Luc Popot or Daniel Picot.

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Figure 1: Crystallography of b6f complex.
Figure 2: Three views of b6f normal to the membrane.
Figure 3: Cofactors in the dimer.
Figure 4: Qo site with inhibitor.
Figure 5: Qi site.


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