Photosystem with photosynthetic electron

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

Triazines inhibit photosynthesis in all organisms with oxygen-evolving photosystems. They block photosynthetic electron transport by displacing plastoquinone from a specific-binding site on the D1 protein subunit of photosystem II (PS II). This mode of action is shared with several structurally different groups of other herbicides. The elucidation of the mechanism of the inhibitory action is followed in this review. 

Triazines are selective herbicides used to control a wide spectrum of grass and broadleaf weeds in cereal, oilseed, and horticultural crops. Triazine herbicides kill weeds by interfering with the electron transport chain in photosystem II (PS II). These herbicides bind to the QB protein in the PS II reaction center and block the flow of electrons through the photosynthetic electron transport chain. 

In-vitro approach Data are available in abundance concerning metal effects on isolated chloroplasts (for a review, see Clijsters and Van Assche, 1985). All the metals studied were found to be potential inhibitors of photosystem 2 (PS 2) photosystem 1 (PS 1) was reported to be less sensitive. From the in-vitro experiments, at least two potential metal-sensitive sites can be derived in the photosynthetic electron transport chain the water-splitting enzyme at the oxidising side of PS 2, and the NADPH-oxido-reductase (an enzyme with functional SH-groups) at the reducing side of PS 1 (Clijsters and Van Assche, 1985). Moreover, in vitro, non cyclic photophosphorylation was very sensitive to lead (Hampp et al., 1973 b) and mercury (Honeycutt and Korgmann, 1972). Both cyclic and non-cyclic photophosphorylation were proven to be inhibited by excess of copper (Uribe and Stark, 1982) and cadmium (Lucero et al, 1976). 

Herbicides that inhibit photosynthetic electron flow prevent reduction of plastoquinone by the photosystem II acceptor complex. The properties of the photosystem II herbicide receptor proteins have been investigated by binding and displacement studies with radiolabeled herbicides. The herbicide receptor proteins have been identified with herbicide-derived photoaffinity labels. Herbicides, similar in their mode of action to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) bind to a 34 kDa protein, whereas phenolic herbicides bind to the 43-51 kDa photosystem II reaction center proteins. At these receptor proteins, plastoquinone/herbicide interactions and plastoquinone binding sites have been studied, the latter by means of a plastoquinone-deriv-ed photoaffinity label. For the 34 kDa herbicide binding protein, whose amino acid sequence is known, herbicide and plastoquinone binding are discussed at the molecular level. 

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. 

Figure 6-4. Energy aspects of photosynthetic electron flow. The lengths of the arrows emanating from the trap chi s of Photosystems I and II represent the increases in chemical potential of the electrons that occur upon absorption of red light near the Xmax s of the trap Chip s. The diagram shows the various midpoint redox potentials of the couples involved (data from Table 5-3) and the three types of election flow mediated by ferredoxin. Spontaneous electron flow occurs toward couples with higher (more positive) redox potentials, which is downward in the figure.

Spectroscopic and crystallographic studies have identified four Fe S clusters in the membrane-bound photosynthetic electron transport chain of plant and cyanobacterial chloro-plasts. One is the Rieske-type [2Fe-2S] + + center in the cyt b(,f complex, which catalyzes electron transfer from plasto-quinol to plastocyanin with concomitant proton translocation, and is functionally analogous to the cyt bc complex, with cyt / in place of cyt The remainder are low-potential [4Fe 4S] + + centers in Photosystem I which constitute the terminal part of the electron transfer chain that is initiated by the primary donor chlorophyll. One is a very low-potential [4Fe S] + + center, Fx (Em =-705 mV), that bridges two similar subunits (PsaA and PsaB) and is coordinated by two cysteines from each subunit in a C-Xg-C arrangement. This cluster transfers electrons to the 2Fe-Fd acceptor via an electron transfer chain composed of Fa, a [4Fe S] + + cluster with Em = -530 mV, and Fb, a [4Fe S] + + clusters with Em = -580 mV. Fa and Fb are in a low-molecular weight subunit (PsaC, 9 kDa) that shows strong sequence and structural homology with bacterial 8Fe-Fds. The center-to-center distance between Fx and Fa and between Fa and Fb are 14.9 A and 12.3 A, respectively, well... 

Carotenoids function in photosynthesis as quenchers of chlorophyll triplet states to prevent their harmful reaction with oxygen. Current research has mainly focused on their detection and identification, the determination of kinetic parameters, and the elucidation of the triplet energy transfer pathways in both photosynthetic antenna and reaction centers. Since carotenoids do not take part in the photosynthetic electron transfer reactions, their paramagnetic radical species occur to a lesser extent in vivo, although they may play arole in the photoprotection of Photosystem II. 

Photosystem II (PSII) is a large, heteromeric enzyme complex with more than twenty different protein subunits and an array of cofactors, that participate in the photosynthetic electron transport in the thylakoid membranes of chloroplasts and cyanobaaeria. PSII demonstrates the oxido-reductase aaivity and couples the oxidation of H2O with the reduction of plastoquinones through a series of intermediate redox reactions. The central core of the PSII reaction center is composed of D1 and D2 proteins associated >vith redox active components. These compounds include a tetra-manganese cluster, two redox-active residues, four to six chlorophyll a molecules, two pheophytins, and plastoquinones and The secondary plastoquinone Qg is a two-electron carrier which... 

The toxic effect of nickel (Ni) on photosynthetic electron transport was investigated by monitoring the Hill activity, fluorescence, oxygen evolution and thermo-luminescence properties in the green algae Scenedesmus obliquus. The results obtained surest that Ni modify the Qp site or interact with the nonheme iron between the Qa and Qp inhibiting the photosystem II functions. ... 

In higher plants altered photosynthetic electron transport in the triazine-resistant mutants (2, 3) has been correlated with slower growth and lower yield (jT). Experiments with Chlamydomonas, a unicellular alga, suggest that trlazine resistance is not necessarily associated with an alteration of the photosystem II electron transport kinetics ( 5). Selection of mutants resistant to other classes of photosystem II inhibiting herbicides, (e.g. dluron, bromacll) should also be feasible based on successful Isolation of such mutants in Chlamydomonas (16). 

Although p (J) values are slightly smaller than pI q s obtained for the Hill reaction, they remain very similar except for ioxynil. Generally speaking, quantitative measurements of the fluorescence parameters described herein showed that DCMU-type inhibitors disconnect the two photosystems progressively as concentrations of the inhibitors are increased. Ioxynil does not behave in this manner. Ioxynil may not interfere at exactly the same site as the DCMU-type inhibitors, or it may have a secondary point of interference with the photosynthetic electron transport chain. 

Approximately half of all commercial herbicides act by inhibiting photosynthesis by interacting with specific sites along the photosynthetic electron transport chain. A number of diverse chemicals including the ureas, amides, triazines, triazinones, uracils, pyridazinones, quinazolines, thiadiazoles, and certain phenols are thought to act specifically at a common inhibitory site at the reducing side of photosystem II (PS II) (U ). 

Photoaffinity labels are an efficient tool for identification of inhibitor binding proteins in the photosynthetic electron transport chain. [ H]Azido-dinoseb, an azido-deri-vative of the phenolic herbicide dinoseb, was synthesized almost a decade ago and was shown to bind primarily to a 41 kDa protein (1,2). Contrary, labeling with azido-deri-vates of diuron-type herbicides revealed that these herbicides bind to a 32 kDa protein, which has now been recognized as the D-1 protein of the photosystem II reaction center core complex (see references in (3)). Tyrosine residues in positions 237 and 254 of the D-1 sequence were demonstrated to be the primary target of [ CJazido-monuron (3). The phenolic herbicide [ I]azido-ioxynil also labels predominantly the D-1 protein in position of Val249 and only in trace amounts a 41 kDa protein (4). 

Incubation of leaf discs in LY181977 causes an increase in chlorophyll fluorescence similar to that caused by atrazine and diuron (Figure 2). Similarly the IC50 for DCPIP reduction by isolated spinach thylakoids is similar for LY181977 (0.73 pM) and atrazine (0.36 pM). This indicates that LY181977 inhibits photosynthetic electron transport through photosystem II with efficacy similar to that of atrazine. 

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