The photosynthetic unit of oxygenic photosynthesis is organized as two large multimolecular membrane complexes, photosystem II (PSII) that extracts low- energy electrons from water and photosystem I (PSI) that raises the energy level of such electrons using light energy to produce a strong reductant, NADPH. The two photosystems operate in a series linked by a third multiprotein complex called the cytochrome b 6 f complex ( F ig. 1 ). The cytochrome b 6 f complex is a membrane-spanning protein complex embedded in the thylakoid membrane of photosynthetic organisms. The molecular weight of the complex is 220 , 000 as a dimer with 26 transmembrane helices. The b 6 f complex controls the electron transfer between the plasto q uinol reduced by PSII and the electron carrier protein plastocyanin that associate with PSI. C oupled with the electron transfer, the b 6 f complex also generates a transmembrane proton gradient for ATP synthesis. The crystal structures of the cytochrome b 6 f complex [1 , 2] complete the description of the architecture of the oxygenic photosynthetic electron transport chain, since three-dimensional structures have been provided for PSI and PSII [3 - 5] . The structure of the b 6 f complex from cyanobacterium M. laminosus was determined by the isomorphous replacement method using Pb and Pt derivatives and multiwavelength anomalous diffraction from native iron atoms. X -ray diffraction data from native crystals and complex crystals with the q uinone-analogue inhibitor D BM I B were collected at the O saka U niversity beamline BL44XU of SPring- 8 . The highest resolution data of 3 . 0 Å from the complex crystal with another analogue inhibitor, TDS, was collected at the S BC beamline 19 ID, APS. The initial model was developed into a 3 . 4 Å map of the native complex. F inal refinement was carried out with a dataset from a co- crystal with TDS ( F igs. 2 , 3 ). V iewed along the membrane normal, the b 6 f complex is 90 Å × 55 Å within the membrane side, and 120 Å × 75 Å on the lumen ( p ) side ( F ig. 2 ). A prominent feature of this structure is an extended q uinone exchange cavity between the monomers, which exchanges lipophilic plasto q uinone in the bilayer center, and also mediates the electron and proton transfer across the complex. The heme-binding 4 transmembrane helices core of the b 6 f complex is almost identical to that of the analogous bc 1 complex in the respiration chain of the mitochondrial membrane. However, there are three prosthetic groups recently found in the b 6 f complex that are not present in the bc 1 complex : a high spin heme x covalently bound to the cyt b 6 polypeptide by one thioether bond, and the pigment molecules, chlorophyll a and β - carotene. Heme x occupies the binding site of the n - side bound q uinone in the bc 1 complex. The presence of heme x in contact with heme b n and a plasto q uinone in the cavity suggests the mechanism of ferredoxin-mediated cyclic electron transfer (dotted line in F ig. 1 ) that uses classical elements of the Q - cycle mechanism [1 , 2] . The q uinone-mediated redox connection between the ( p ) and ( n ) sides of the complex can be visualized Fig. 1. Integral membrane protein complexes and electron carrier proteins responsible for electron transport and proton translocation in oxygenic photosynthesis. t h r o u g h ( a ) a p l a s t o q u i n o n e molecule close to heme x on the n - s i d e , a n d ( b ) a q u i n o n e analogue inhibitor, TDS, on the p - side of the other monomer that surrounds the cavity ( F ig. 3 ). The position of TDS in the b 6 f complex is similar to that of the p -side inhibitor myxothiazol in the bc 1 complex. Another p -side inhibitor, D BM I B , is bound near G lu in the conserved se q uence in the p -side peripheral loop. B oth of these inhibitors are > 10 Å from the closest histidine ligand of the [2F e- 2 S ] cluster of R ieske ISP , Structure of the Cytochrome b 6 f Complex of Oxygenic Photosynthesis 8 Genji Kurisu* Institute for Protein Research, Osaka University E-mail: GKurisu@bio.c.u-tokyo.ac.jp * Present address : D epartment of L ife S ciences, T he University of T okyo an d cannot form an H -bon d w ith the histi d ine l i g a n d a s d o e s s t i g m a t e l l i n e i n t h e b c 1 comple x . T he [2F e- 2S] cluster is 29 Å from the heme F e of its electron acceptor, cyt f . In the case of the bc 1 comple x , the positional chan g e of the [2F e- 2S] cluster to a more c 1 -pro x imal position in d ifferent crystal forms su gg ests that Rieske I S P me d iates the electron transfer bet w een the membrane-boun d q uinol an d cyt c 1 by shuttlin g bet w een the membrane-pro x imal an d c 1 -pro x imal states. H o w ever, only a sin g le membrane-pro x imal position for the Rieske I S P has been observe d in the b 6 f comple x . T o g ether w ith the d ifferent positions of cytochrome f an d its heme relative to cyt c 1 , this implies the d ifference in trajectory bet w een Rieske I S P an d cyt f . Fig. 2. Side view of eight-subunit dimeric cytochrome b 6 f complex. Hemes bn , bp and f (grey), heme x (dark brown), chlorophyll a (dark green), β -carotene (orange), cyt b 6 (blue), subunit IV (red), cyt f (purple), iron-sulfur protein (orange), and small subunits (light green). Fig. 3. Molecular surface of the complex mapping the electrostatic potential. Plastoquinone (PQ) and Q-analogue inhibitor (TDS) are drawn in the cavity as a cpk model, and the chemical formulas of inhibitors are shown on the right. References [1] G. Kurisu, H . Z han g , J . L . S mith an d W . A . C ramer: S cience 302 (2003) 1009 . [2] D . S troebel et al. : N ature 426 (2003) 413 . [3] P. J or d an et al. : N ature 411 (2001) 909 . [4] A . Z ouni et al. : N ature 409 (2001) 739 [5] N . Kamiya an d J .-R. S hen: Proc. N atl. A ca d . S ci. U SA 100 (2003) 98 . 9
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