Plastoquinols

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,... 

The cyt b6f complex (/ stands for feuille, the French word for "leaf") consists of four subunits—one molecule of cyt/, one heme-containing cyt b6, one iron-sulfur protein, and one bound plastoquinol. The cyt b6f complex transports electrons from the outside to the inside of the thylakoid membrane. 

The cytochrome b(6)f complex mediates electron transfer between the PSI and PSII reaction centers by oxidizing hpophUic plastoquinol (PQH2) (see Figure 7.24) and reducing the enzymes plastocyanin or cytochrome Ce. The electronic connection also generates a transmembrane electrochemical proton gradient that can support adenosine triphosphate (ATP) synthesis instead of electron transport. 

Electrons temporarily stored in plastoquinol as a result of the excitation of P680 in PSII are carried to P700 of PSI via the cytochrome b6/complex and the soluble protein plastocyanin (Fig. 19-49, center). Like Complex III... 

PSI and PSII. PSII contains the site of water cleavage, and utilizes the electrons extracted from water to reduce plastoquinone to plastoquinol. The latter diffuses through the membrane until it is reoxidized by another membrane protein, the cytochrome bf complex, which transfers the electrons to a water-soluble electron carrier (plastocyanin or cytochrome c6). This carrier in turn is oxidized by PSI, which delivers the electrons via ferredoxin to the enzymes that produce NADPH (Figure 11.6) [12],... 

That hole in I we now negate, plastoquinol moves in With b and/to mediate, and plastocyanin. 

Cytochrome b f is used in place of cytochrome bc in cyanobacteria and chloroplasts. Light energy utilized by PSII results in plastoquinol production from plastoquinone. Reducing equivalents carried by the plastoquinol are then transferred across b(,f, through cytochromes bound to b(,f, to plastocyanin or cytochrome c. Reduced plastocyanin or cytochrome ce is then used to rereduce PSI. The crystal structure of cytochrome b(,f at 3.0 A resolution is shown... 

Additional polypeptides ascribed to the Cyt b-f complex are a bound form of ferredoxin-NADP reductase (FNR) [99] and one or more smaller polypeptides [100]. An association of the complex with ferredoxin-NADP reductase may be expected in view of the reported role of FNR in cyclic electron flow from PS I to the Cyt complex [101]. FNR remains associated with the complex during the early stages of the purification of the complex but there is no evidence that it is an intrinsic component of the complex necessary for plastoquinol-plastocyanin oxido-reductase. The presence of small polypeptides in the complex requires further investigation. Polypeptides of about 5 kDa have been reported to be associated with the spinach complex [100]. 

The plastoquinol (QH2) produced by photosystem II contributes its electrons to continue the electron chain that terminates at photosystem I. These electrons are transferred, one at a time, to plastocyanin (Pc), a copper protein in the thylakoid lumen. 

The two protons from plastoquinol are released into the thylakoid lumen. This reaction is reminiscent of that catalyzed by ubiquinol cytochrome c oxidoreductase in oxidative phosphorylation. Indeed, most components of the enzyme complex that catalyzes the reaction, the cytochrome bf complex, are homologous to those of ubiquinol cytochrome c oxidoreductase. The cytochrome hf complex includes four subunits a 23-kd cytochrome with two Z>-type hemes, a 20-kd Rieske-type Fe-S protein, a 33-kd cytochrome/with a c-type cytochrome, and a 17-kd chain. 

This complex catalyzes the reaction through the Q cycle (Section 18.3.4). In the first half of the Q cycle, plastoquinol is oxidized to plastoquinone, one electron at a time. The electrons from plastoquinol flow through the Fe-S protein to convert oxidized plastocyanin into its reduced form. Plastocyanin is a small, soluble protein with a single copper ion bound by a cysteine residue, two histidine residues, and a methionine residue in a distorted tetrahedral arrangement (Figure 19.17). This geometry facilitates the interconversion between the Cu2+ and the Cu+ states and sets the reduction potential at an appropriate value relative to that of plastoquinol. Plastocyanin is intensely blue in color in its oxidized form, marking it as a member of the "blue copper protein," or type I copper protein family. 

The oxidation of plastoquinol results in the release of two protons into the thylakoid lumen. In the second half of the Q cycle (Section 18.3.4). cytochrome hf reduces a second molecule of plastoquinone from the Q pool to plastoquinol, taking up two protons from one side of the membrane, and then reoxidizes plastoquinol to release these protons on the other side. The enzyme is oriented so that protons are released into the thylakoid lumen and taken up from the stroma, contributing further to the proton gradient across the thylakoid membrane (Figure 19.18). 

We can now estimate the overall stoichiometry for the light reactions. The absorption of 4 photons hy photosystem II generates 1 molecule of O2 and releases 4 protons into the thylakoid lumen. The 2 molecules of plastoquinol are oxidized hy the Q cycle of the cytochrome bf complex to release 8 protons into the lumen. Finally, the electrons from 4 molecules of reduced plastocyanin are driven to ferredoxin by the absorption of 4 additional photons. The 4 molecules of reduced ferredoxin generate 2 molecules of NADPH. Thus, the overall reaction is ... 

Many commercial herbicides kill weeds by interfering with the action of photosystem II or photosystem I. Inhibitors of photosystem II block electron flow, whereas inhibitors of photosystem I divert electrons from the terminal part of this photosystem. Photosystem II inhibitors include urea derivatives such as diuron and triazine derivatives such as atrazine. These chemicals bind to the Qg site of the D1 subunit of photosystem II and block the formation of plastoquinol (QH2). 

The isolation of an active, structurally intact complex was obtained using an association of cholate and octylglucoside and sucrose gradient centrifugation [111]. This preparation did not contain cytochrome 6-559 and possessed a plastoquinol-plastocyanine oxidoreductase activity, inhibited by specific inhibitors (DBMIB, UHDBT). The complex was essentially free of chlorophyll and contaminations by other membrane components, specifically of the ATPase complex. 

The association of cyt. 6-559 with intramembrane complexes is also obscure this cytochrome has been found associated with the b /f complex and with PSII-RC preparations [70]. After the purification of an intact active b /f complex, however, the lack of involvement of cyt. 6-559 in the plastoquinol-plastocyanine oxidoreduc-... 

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