Cytochrome b,f complex

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Because photosystem 11 and the cytochrome b/f complex release protons from reduced plastoquinone into the lumen (via a Q. cycle), photosynthetic electron transport establishes an electrochemical gradient across the thylakoid membrane (see p. 126), which is used for ATP synthesis by an ATP synthase. ATP and NADPH+H", which are both needed for the dark reactions, are formed in the stroma. 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

If the reaction centers of photosystem I and photosystem II are segregated into separate regions of the thylakoid membrane, how can electrons move from photosystem I to photosystem II Evidently the plastoquinone that is reduced in photosystem II can diffuse rapidly in the membrane, just as ubiquinone does in the mitochondrial inner membrane. Plastoquinone thus carries electrons from photosystem II to the cytochrome b6f complex. Plastocyanin acts similarly as a mobile electron carrier from the cytochrome b f complex to the reaction center of photosystem I, just as cytochrome c carries electrons from the mitochondrial cytochrome bct complex to cytochrome oxidase and as a c-type cytochrome provides electrons to the reaction centers of purple bacteria (see fig. 15.13). 

Joliot, P. and Joliot, A. 1987. The low potential electron transfer chain in the cytochrome b/f complex. Biochim. Biophys. Acta, in press. 

Reduction of plastoquinone Qb by QA- and protonation at the acceptor side of PSII. The Qa is tightly bound to the protein, acting as a one electron acceptor. It passes electrons to a second plastoquinone, Qb, which can accept two electrons and two protons and acts as a mobile electron carrier connecting PSII to the next complex of the photosynthetic apparatus (i.e. the cytochrome b(f complex). After two electron-reductions and two protonation events, QbH2 leaves the reaction center and is replaced by an oxidized quinone from the pool in the membrane. 

Thus, the concentration necessary for 50% displacement roughly corresponds to the PIjq value. It is possible, therefore, to assay the pl value of a new compound just by examination of its displacement behaviour. It is no longer necessary to determine the pi value by testing the inhibition of a light-driven photoreduction. Another very potent inhibitor of photosynthetic electron transport, DBMIB (2,5-dibromo-3-methyl-6-isopropyl-l,4-benzoquinone) (13), almost completely fails to displace metribuzin from the membrane (Figure 3). This is due to the fact that DBMIB has a completely different site of action as compared to the photosystem II herbicides, i. e. it inhibits plastohydroquinone oxidation by acting at the cytochrome b /f-complex (13). 6... 

Like chlorophyll, plastoquinone A has a nonpolar terpenoid or isoprenoid tail, which can stabilize the molecule at the proper location in the lamellar membranes of chloroplasts via hydrophobic reactions with other membrane components. When donating or accepting electrons, plastoquinones have characteristic absorption changes in the UV near 250 to 260, 290, and 320 nm that can be monitored to study their electron transfer reactions. (Plastoquinone refers to a quinone found in a plastid such as a chloroplast these quinones have various numbers of isoprenoid residues, such as nine for plastoquinone A, the most common plastoquinone in higher plants see above.) The plastoquinones involved in photosynthetic electron transport are divided into two categories (1) the two plastoquinones that rapidly receive single electrons from Peso (Qa and Qb) and (2) a mobile group or pool of about 10 plastoquinones that subsequently receives two electrons (plus two H+ s) from QB (all of these quinones occur in the lamellar membranes see Table 5-3). From the plastoquinone pool, electrons move to the cytochrome b f complex. 

Cramer, W.A., Soriano, G.M., Ponomarey, M., Huang, D., Zhang, H., Martinez, S.E., and Smith, J.L. 1996. Some new structural aspects and old controversies concerning the cytochrome b(f complex of oxygenic photosynthesis. Annu. Rev. Plant Physiol. Plant Mol. Biol. 47 477-508. Deisenhofer, J., and Norxns,XR. (Eds.) 1993. The Photosynthetic Reaction Center Vols. 1 and 2. Academic Press, San Diego, CA. 

Stlamatellln. A qyxobacterlales species, Stiamatella aurontlaca. produces an antibiotic stigmatellln [76] which also has herbicldal properties (2721. This dimethoxychromone alters photosynthetic electron transport at the herbicide-binding protein and the cytochrome b f complex. 

All photosynthetic organisms that produce O2 contain two photosystems connected in series, as shown in Figure 16. Fom main protein complexes are embedded in the membrane. Photosystems I and II, the cytochrome b(,f complex and the ATP synthase or coupling factor. The two photosystems are entirely distinct entities, with different pigment and protein compositions and different genes coding for the proteins. All fom of these complexes have been pmified, extensively studied, and crystallized (see more below). 

Figure 16 Membrane organization in oxygen-evolving photo synthetic systems. The four main complexes present in the membrane are (from left to right) Photosystem 11, the Cytochrome b f complex. Photosystem I, and ATP synthase. All complexes now have published crystal structures, ribbon diagrams of which are shown in the Figure. Coordinates taken from the Protein Data Bank and visualized using Accelrys DSViewer Pro, or 2-D pictures were taken from the PDB website and modified...

A number of other electron carriers are present in various photosynthetic systems. These include soluble carriers such as the blue copper protein plastocyanin and auracyanin see Copper Proteins with Type 1 Sites) and soluble cytochromes and ferredoxins see Iron-Sulfur Proteins), as well as additional membrane-bound complexes. The membrane-bound multisubunit cytochrome b f complex is discussed is Section 7. 

Water is present in protein cavities as individual molecules, water chains, and clusters. Indeed, tightly bound waters can be resolved in X-ray crystallography experiments. Water molecules in larger cavities, especially those with a hydrophobic surface, are mobile and less readily resolved. In some proteins, such as the cytochrome b(f complex or cytochrome c oxidase, bound water molecules tend to form water chains. These water molecules provide hydrogen-bonded relays for proton transfer, and they may mediate donor-to-acceptor electronic coupling (2-6). 

Kurisu, G., Zhang, H., Smith, J., Cramer, W. (2003). Structure of the cytochrome b f complex of oxygenic photosynthesis tuning the cavity. Science 302,1009-1014. 

Fig. 12.1. Relative sizes of mitochondrial and chloroplast chromosomes and location of protein structural genes. The figure was constructed from published data [5,15,17,22,26-28]. The structural genes are marked by wide sections. Black areas code for proteins. White areas are introns. 0x1, OxII and OxIII are subunits I, II and III of cytochrome c oxidase. Cyt b, cytochrome b. Fo and Fo, are subunits 6 and 9 of the proton ATPase complex. In the chloroplast chromosome the arrows indicate the transcription direction and the size of the transcripts. CF,a, CFj/8, CFjc and CFoIII are subunits a, /S, t and III of the chloroplast proton ATPase complex [30]. PSII5], PSII44, and PSII34 are subunits of photosystem II reaction center with the corresponding molecular weights of 51000, 44000 and 34000. PSI70 is subunit I of photosystem I reaction center. Cyt /is cytochrome/ cyt is cytochrome b and b -flV is subunit IV of cytochrome b(,-f complex.

Partial reactions take place in which cytochrome b /f complex and partially assembled photosystem I reaction center are involved. 

As indicated, above, the two Z -hemes of the Cyt b f complex provide a pair of reacting sites spanning the thylakoid membrane, one near the stromal side and the other near the lumenal side of the thylakoid membrane. The plastohydroquinone is first oxidized by the Rieske FeS to a semiquinone, which is then oxidized by cytochrome/, which then releases the electron to the copper protein plastocyanin. After loss of one electron by the plastohydroquinone, the resulting semiquinone loses an electron to the two fc-hemes in series. The Z -hemes operate in the so-called Q-cycle, similar to that in the mitochondrial or bacterial cytochrome bc complex, and provide a translocation of additional protons across the membrane into the lumenal space. Discussion of the cytochrome b(,f complex and the Q-cycle will be presented in Chapter 35. 

These mobile electron carriers are relatively low-molecular-weight proteins and are both linked to the PS-1 reaction-center complex by electrostatic forces. Plastocyanin is located on the lumen side and is the electron donor to P700, while ferredoxin is located on the stroma side and receives electrons from the terminal, membrane-bound iron-sulfur proteins FeS-A/B. Water is the ultimate source of electrons for plastocyanin reduction. The electron from water is generated by its oxidation by photosystem 11, and transferred by way of cytochrome/in the cytochrome b(f complex. Reduction of NADP by ferredoxin requires mediation by the enzyme ferredoxin-NADP -reductase orFNR. Figure 1, right is a schematic representation of the relationship of the PS-1 reaction center to the peripheral electron carriers plastocyanin and ferredoxin as well as the protein catalyst ferredoxin-NADP -reductase. 

Cytochrome b /also serves as an intermediate in a non-linear, or so-called cyclic, electron-transport pathway around PS I, as formulated in Fig. 1 (B). A third function of Cyt b /is translocation of protons across the thylakoid membrane during electron transfer from plastoquinol to plastocyanin [Fig. 1 (C)]. The unique effects resulting from electron transport and proton translocation in the cytochrome b(f complex are the production of an electrochemical potential and a pH gradient across the thylakoid membrane to provide energy in a form needed for ATP synthesis (see the following chapter). 

Fig. 2. (A) A model for b/in chloroplast thylakoid membrane (B) Topological arrangement of the four subunits of the purified Cyt b/complex (C) A densitometric scan of an SDS-PAGE gel for b/ (B) from Hauska, Schiitz and Biittner (1996) The cytochrome b/ complex - composition, structure and function. In DR Ort and CF Yocum (eds) Oxygenic Photosynthesis - The Light Reactions, p 384. Kluwer (C) from Black, Widger and Cramer (1987) Large-scale purification of active cytochrome b f complex from spinach chloroplasts. Arch Biochem Biophys 252 657.

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