The Cytochrome b6f Complex

In 2003, the structure of the cytochrome b<f complex was solved both in the thermophilic cyanobacterium Mastigocladus laminosus [2, 18] and in the green algae Chlamydomonas rheinhardtii [19]. It is described in Chapter 7. The structures from both organisms are extremely similar which shows that the structure of the

Figure 1.1 The components of the light reactions of cyanobacterial oxygenic photosynthesis. (a) Structural models of the protein components of the electron transport chain and the ATP synthase in the order they appear in the electron transport chain. The proteins shown and the pdb files used are a dimer of Photosystem II from Thermosynechococcus elongatus (2AXT, [1 ], the dimer of cytochrome bf from Mastigocladus laminosus (1VF5, [2], the soluble protein plastocyanin from Synechococcus sp. Pcc7942 (1BXV, [3], cytochrome c6 from Arthrospira maxima (1F1F, [4], the trimeric complex of Photosystem I from Thermosynechococcus elongatus (1JB0, [5], the soluble protein ferredoxin from Anabaena Pcc7119 (1CZP, [6], and the ferredoxin:NADP+ reductase from Anabaena Pcc7TT9 (1QUE, [7, 8]. The final protein, the ATP synthase, is not directly part of the electron transfer chain, but produces ATP from ADP and phosphate by using the electrochemical proton gradient generated during electron transfer. Whereas all other models have been derived from cyanobacterial sources, we used the model of the bovine mitochondrial F1-ATPase (1H8E, [9] as placeholder for the membrane-extrinsic CF1 part of the protein. The membrane-intrinsic F0 part is even less well characterized, which is indicated by the cartoons. A glimpse at how the rotor ring composed of subunits c might look is given by the structure derived from a similar enzyme, the F-Type Na+ ATPase from Ilyobacter tartaricus (1YCE, [10]). Color coding of cofactors: All chlorophylls are depicted in green, pheophytins in yellow, carotenoids in orange, hemes in red, lipids in brown, FeS clusters, S in yellow, Fe in red. Color coding of the individual protein subunits: Photosystem II: D1, yellow; D2 orange, CP47, red; CP 43, blue; PsbO, green; all other subunits are depicted in gray. Cytochrome b6f complex: heme b, grey; Rieske FeS protein, deep purple; cytochrome f green, subunit IV, blue; PetG, light grey; PetM, turquoise; PetL, gold; and PetN, pink. Photosystem I: PsaA, blue; PsaB, red; PsaC, magenta; PsaD, cyan; PsaE, green; PsaF yellow; PsaL orange; all other small subunits are depicted in grey. ATP synthase: F1 subunits subunits a and subunits P, orange and grey; subunit y, yellow; subunit e, cyan. The ring of subunits III is represented by the structural model of the c-ring from I. tartaricus in a blue box. Subunit IV is depicted as a purple box.

Subunits I and II that form the peripheral stalk are shown as orange cartoons. (b) The functional components of the electron transfer chain. Here, the same structural files as in panel A were used. The multimeric complexes of Photosystems I and II are represented by one monomer each, whereas the dimeric cytochrome bfis reduced to one functional half. The way of the electrons through the system can be seen by following the arrows. The magenta arrows represent direct electron transport, whereas the green arrows indicate the transport of an electron together with a proton, that is, in the form of a reduced plastoquinone (plastohydroquinone). Oxidized plastoquinones follow those arrows in the opposite direction. The blue molecule in the cytochrome bf complex labeled Qp is a specific inhibitor sitting in the luminal plastoquinol binding site of the structure. For a detailed description of the processes in the electron transfer chain, see text. Photosystem II transport electrons from light-induced charge separation from the luminal (lower) side of the membrane to the stromal (upper) side, where the electrons, together with protons from the stroma, are used to reduce plastoquinone to plastoquinol. These plastoquinol exchanges with plastoquinones in the adjacent quinone pool. Photosystem II is re-reduced with electrons derived from water by the action of the oxygen-evolving complex (OEC). Plastoquinol binds to the luminal binding pocket of the cytochrome bf complex. One electron is cycling through the q-cycle which involves three hemes and the interaction with an (oxidized) plastoquinone in the stromal binding pocket, which can take up protons from the stroma. The other electron is given via several mediators to the soluble carrier plastocyanin, and the protons are released into the lumen. Cytochrome c6 can replace plastocyanin in cyanobacteria. In the meanwhile, Photosystem I transports an electron by light-induced charge separation to the stromal side of the membrane, where it is used to reduce the soluble carrier ferredoxin. Ferredoxin can be replaced by flavodoxin under iron deficiency. The primary electron donor of Photosystem I is re-reduced from the luminal side by the reduced plastocyanin or cytochrome c6. Ferredoxin delivers the electrons to the ferredoxin:NADP+ reductase, where they are finally used to produce the reduction equivalent utilized in the fixation of carbon.

Figure 1.2 Structure of the light-harvesting complex II and the PSI-LHC-I supercomplex from higher plants. Left: structure of the trimeric LHC-II complex from spinach (1RWT, [11]). The individual subunits are colored blue, brown, and silver. The chlorophylls are shown in green, the carotenoids in orange. Right: structure of the plant PSI-LHC-I supercomplex from pea (2001, [12]). The structure contains a monomeric photosystem that is connected to a half-moon belt of 4 LHC-I proteins. The view direction is along

Figure 1.2 Structure of the light-harvesting complex II and the PSI-LHC-I supercomplex from higher plants. Left: structure of the trimeric LHC-II complex from spinach (1RWT, [11]). The individual subunits are colored blue, brown, and silver. The chlorophylls are shown in green, the carotenoids in orange. Right: structure of the plant PSI-LHC-I supercomplex from pea (2001, [12]). The structure contains a monomeric photosystem that is connected to a half-moon belt of 4 LHC-I proteins. The view direction is along the membrane plane, from the side of the complex where the LHC-I belt is located. The LHC-I subunits are color-coded as follows: Lhcal, cyan; Lhca4, yellow; Lhca2, orange; and Lhca3 pink. The subunits of the reaction center core have the same color coding as shown in Figure 1.1 for the cyanobacterial PSI, except that all small membrane intrinsic subunits that are not plant specific are shown in grey. The plant specific subunits are color-coded as follows: PsaG, deep pink; PsaN, light green; PsaH, gold.

Figure 1.3 Structure of the purple bacterial reaction center with the peripheral Lh1 and Lh2 antenna rings. Left: Structure of the purple bacterial reaction center (PBRC). There are three proteins. The L subunit is depicted in blue, the M subunit in red, and the H subunit in cyan. Only the L-branch is involved in the first steps of the electron transfer. The cofactors of the electron transport chain consist of the special pair of bacteriochlorophylls (P) (green), the L-branch accessory chlorophylls (acc Chl) (green), the L-branch peophytin (Pheo) (yellow), and two quinones, QA and QB (purple). Center: Structure of the PbRC in complex with the LH1 ring from Rps. palustris (1PYH, [16]. The reaction center has the same color coding as on the left panel of the figure. The PbRC is surrounded by an open elipsoidal ring of 15 aP pairs of the LH1 proteins. The alpha chains are depicted in yellow and form the inner ring. The outer ring is formed by the P subunits, depicted in deep purple. The ring of LH1 proteins is broken by the helix W, which is depicted in dark yellow in the center of the picture. The bacteriochlorophyll molecules form a ring structure close to the luminal side of the membrane and are depicted as B880, as they have an absorption maximum at 880nm. Right: Structure of the LH2 complex from Rps. acidophila (1 NKZ, [17]). Nine pairs of the a and P subunits form a symmetric ring structure. The inner ring is formed by the a chain, depicted in yellow, while the outer ring consists of the P chain of the protein. Nine monomeric chlorophylls, which are oriented parallel to the membrane plane, form the B800 ring, while 18 chlorophylls, which are oriented perpendicular to the membrane plane and closely interact, form the B850 ring.

cytochrome bf complex has been conserved over 1.5 billion years of evolution. The cytochrome bf complex is a dimer. In contrast to Photosystems I and II, where the monomer is the functional unit, the dimerization of the bf complex is essential for the function, as there is a crossover of one essential subunit, the Rieske-iron sulfur protein which carries the 2Fe2S cluster between the two monomers. The monomer of the cytochrome b6f complex consists ofeight protein sub-units. Three of them coordinate the cofactors of the electron transport chain. Cytochrome b6 is the membrane integral core subunit of the complex and contains three heme groups (two b-type hemes and one covalentiy bound c-type heme) that form a major part of the electron transport chain. The Rieske-FeS-protein contains one transmembrane helix and a larger luminal extension that harbors a 2Fe2S cluster. The extrinsic subunit cytochrome f contains a c-type heme and mediates electron transfer from the cytochrome b6f complex to plastocyanin. In addition, two quinone binding sites have been identified in the structure. A surprise was the presence of one chlorophyll and one P-carotene in the structures of the cytochrome b6f complex. The function of the chlorophyll and the carotenoid is still under debate and is discussed in Chapter 7 - The remaining five subunits of the complex are all membrane integral and stabilize the complex.

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