Electron transport system, thylakoid

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. ... 

Foyer CH and Harbinson J (1997) The photosynthetic electron transport system efficiency and control. In Foyer CH, Quick WP (eds), A molecular approach to primary metabolism in higher plants, pp 3-39. Taylor and Francis, London, UK Foyer CH and Lelandais M (1995) Ascorbate transport into protoplasts, chloroplasts and thylakoid membranes of pea leaves. In Mathis P (ed), Photosynthesis From light to Biosphere, Vol V,pp511-514. Kluwer Academic Publishers, Dordrecht... 

Plastoquinones QH is a molecule in the electron transfer system betweeen photosystem II (PSII) and photosystem I. The sequence of carriers is as follows Pheophytin -> QA -> QB -> QH -> Cytochrome b6f. Plastoquinone QH2 is the photo synthetic equivalent of coenzyme Q of the electron transport system. It moves freely in the thylakoid membrane, carry electrons between the photo system II complex and the cytochrome b6f complex (Figure 17.12). Electron transfer involving plastoquinones is shown here. Reduction of a plastoquinone creates a plastoquinol. 

The measuring principle of a biosensor designed for pesticide determination is mainly enzyme inhibition. Cyanobacteria, thylakoid membranes, protoplasts, immobilized enzymes, and labeled enzymes are receptor components. The analyte blocks the enzyme, enzyme systems, or electron transport systems of intact cells. 

Desaturation in chloroplasts requires oxygen, and probably an electron donor as well. By analogy with animal systems, the electrons would be transferred from the donor to the desaturase via electron transport components. In fact, NAD(P)H can serve as a donor for "reverse" electron transport In thylakoids. In Chlamydomonas reduction of PQ by NADH through a flavoprotein has been reported (Godde and Trebst, 1980). In a similar fashion, added NADPH Is able to reduce the primary acceptor Q of PS II In lysed spinach chloroplasts (Mills et al, 1979). 

Table 1 shows that EDTA treatment alone, as well as the subsequent lysozyme treatment depletes the cytosol from cations. This depletion engenders irreversibly changes in the thylakoid embedded electron transport systems. 

Fig. 10. Redox systems of the photosynthetic electron transport chain incorporated in the thylakoid membrane. Irradiation causes the generation of a proton gradient (after Trebst and Hauska135))...

For the formation of one 02 molecule four electrons have to be transferred. This requires a "quantum storage device". In the photosynthetic system of green plants this is achieved with two photosystems that are linked through an electron transport chain, Fig. 10.2, and by means of the thylakoid-membrane that enables the separation of the photoproducts 02 and the reduced form of nicotinamide adenine dinucleotide phosphate, NADPH. 

The proton gradient drives ATP synthesis via an ATP synthase located in the thylakoid membrane (photophosphorylation). Since the electron transport involves a linear array of electron carriers, the system is called noncyclic photophosphorylation. 

The functional and morphological heterogeneity of a lamellar system of chloroplasts indicates that pH values in different compartments (in granal and intergranal thylakoids) differ. This type of structure makes it difficult to measure local pH values at different sites. Therefore, mathematical models taking into account the spatial structure of chloroplasts provide a tool for studying the effect of diffusion restrictions on pH distributions over the thy lakoid on the rates of electron transport, proton transport, and ATP synthesis. The rate of ATP synthesis depends on the osmotic properties of a chloroplast-incubation medium and, therefore, on topological factors. 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

The ability of membranes to compartmentalize reagents and control the permeation of chemical species may also allow the control of electron transfer in a more sophisticated way within the aggregate bilayer [86]. Photosynthetic processes occur specifically in membranes [87] (thylakoid membranes) so there is continuous interest in mimicking these phenomena with synthetic vesicles [86]. Though a large amount of information is available on the components of biological systems that operate electron transport, the actual mechanism of the process is far from being understood in detail. 

As mentioned in Chapter 35, the Cyt b(Jcomplex is involved not only in noncyclic, or linear, electron transport but also in cyclic transfer around PS I. In the latter case, the electrons received from photosystem I by Fd, instead of going to reduce NADP, are transferred to the plastoquinone pool via b f. During this cyclic process, protons are translocated across the thylakoid membrane, contributing to the transmembrane proton gradient. This cyclic electron-transfer pathway, which is independent of PS II, functionally resembles that of the bacterial photosynthetic system. The existence of a cyclic electron-transfer pathway also helps to account for the observation that chloroplasts often require more than 8 photons for the evolution of one O2 molecule. The physiological function of the cyclic pathway, just as it is for the Q-cycle, is to increase the amount of ATP produced relative to the amount of NADPH formed, and thus provide a mechanism for the cell to adjust the relative amounts of the two substances according to its needs. 

Figure 8a. Photosystem-l-dependent phosphorylation (CPP), photosystem-I-dependent electron transport (MV, methylviologen reduction in the presence of an electron donor system), and electron transport through photosystems II and I (NADP and ferricyaniae reduction) in thylakoids after freezing for 3 hours to —25°C in solutions containing different ratios of sucrose to NaCl...

The above results imply that DCMU-type inhibitors (Group 1), have one site of action on or near the B-protein" complex that is located upon the external surface of the photosynthetic membrane. They specifically bind (non-specific binding is not taken into account) to this protein. On the other hand, ioxynil and i-dinoseb (Group 2) seem to affect another site of action on the O2 evolving system. This is located presumably on the inside of the thylakoid membrane. These two inhibitors do not lose their inhibitory potency towards electron transport because a part of their activity lies in an area that is not easily accessible to trypsin. The Group 2 inhibitors also inhibit silicomolybdate-mediated O2 evolution (data not shown). This reaction is essentially insensitive to DCMU (18) and DCMU-like inhibitors (, 16). 

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