Plastohydroquinone oxidation

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... 

Fig. 6 (B) shows a recording of neutral-red absorbance changes in chloroplasts, with the initial population present in the Si-state, in response to a flash train. The chloroplasts also contained the inhibitor DNP-INT (dinitrorphenolether of iodonitrotoluol) to suppress plastohydroquinone oxidation and the attendant proton release. The recording is obtained by averaging the responses of 100 fresh chloroplast samples. With a 200-/ -per-channel time resolution, the rapid rise is only partially resolved. A plot of the relative yield ofthe released protons in Fig. 6 (B) actually shows a proton-release pattern of [0,1,2,1 ] for the transition sequence [Si->S2->S3->(S4)->So->S,]. Since the initial, dark-adapted state was 100% in the S, state, the proton release pattern in the [So->S,->S2->S3->(S4)->So] transition is thus [1,0,1,2].

The principles of electric potential generation and proton pumping in thylakoids are understood. There is one electrogenic reaction in each of the two photosystems and a third one associated with cyclic electron transfer through the cytochrcme-b -segment of the electron transfer chain. Proton deposition into thylakoids occurs at the level of water oxidation and of plastohydroquinone oxidation. Proton uptake from outside is initiated idien plastoquinone is reduced by photosystem II (or during cyclic electron transfer) and upon reduction of the terminal electron acceptor. Broadly... 

Tris-treated chloroplasts were prepared from broken pea chloroplasts according to standard procedures. pH changes in the internal phase of thylakoids were monitored flash photometrically via absorption changes of the m brane-soluble pH-indicator dye neutral red at 548nm (for details see V.Forster et al., 1981). Proton release due to plastohydroquinone oxidation by PSI was inhibited by DBMIB (2,5-dibromo-3-methyl- 6-isopropyl- 1, 4-benzoquinone). 

The Cyt f complex lying between PS II and PS I in the electron-transport system resembles the Cyt be complex of mitochondria and photosynthetic bacteria. These cytochrome complexes possess one Rieske iron-sulfur protein R-FeS (a [2Fe-2S] protein discovered by John Rieske) and a so-called subunit IV. The two fc-hemes of Cyt b(, and the subunit IV span the thylakoid membrane, while the R-FeS and Cyt/ are located near the lumen side. As previously noted, the placement of the i>-hemes across the thylakoid membrane helps form a redox chain across the membrane. The function of the Cyt complex in green-plant thylakoids is to oxidize the plastohydroquinone formed by PS II and to transfer these electrons to plastocyanin. Accordingly, the Cyt ig/ complex has therefore also been called the plastohydroquinone-plastocyanin-oxidoreductase. ... 

When the plastohydroquinone becomes fully oxidized at the lumenal surface ofthe membrane, it loses two electrons and also releases two protons to the (inside) lumen phase. Thus accompanying the reduction of one plastoquinone molecule by PS 11 and its subsequent reoxidation, there is a net transfer of two protons from the (outside) stromal phase into the (inside) lumen phase. Thus with the splitting of two water molecules by PS 11 to form one oxygen molecule, four protons are translocated across the membrane. As mentioned above, oxidation of two water molecules also releases an additional four protons into the lumen space. Thus water splitting and plastoquinone reduction/re-oxidation result in the generation of eight protons and the creation of a proton gradient across the thylakoid membrane. 

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. 

Photosystem II uses the energy from absorbed photons to remove electrons from water on the inside of the photosynthetic membrane and give them to plastoquinone. PS I removes electrons from plastohydroquinone and donates them to NADP+. Thus, overall, water is oxidized (O2 is evolved) and NADP" is reduced by the cooperative action of PS I and PS II. Each photosystem is a complex of protein with many pigments such as chlorophylls (Chls) and carotenoids as well as electron transfer components such as quinones, cytochromes, and iron-sulfur centers. The most... 

Photosynthetic electron transport is under control of the intrathyla-koid proton potential, pHj. (1,2). When pH. increases in the light, with the build-up of a transthylakoid proton gradient (ApH), electron flow is decelerated. Two major sites of pH. -dependent feed-back control have been discussed the reduction of the primary photosystem (PS) II acceptor (2) and the oxidation of plastohydroquinone at the cytochrome... 

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