Photophosphorylation, inhibition

Comparative responses obtained with quercetin and narlngenin are shown in Traces D and E, respectively. Quercetin and narlngenin, at concentrations that completely inhibited photophosphorylation, inhibited the ADP-stimulated rate of 0 utilization 70 and 82%, respectively. FCCP restored the rate to 12 and 42% of the original rate for quercetin and narlngenin, respectively. The failure to obtain complete recovery can be attributed to interference with a component of the electron-transport pathway, at these same concentrations, as evidenced by data presented in Table II. Similar results were obtained with the remainder of the representative compounds. The results reinforce the postulate that the primary effect Imposed by the allelochemlcals on thylakolds is energy transfer inhibition with a secondary effect imposed on the electron-transport pathway. 

The different results of the photophosphorylation-inhibiting experiments performed by Bishop (1962) indicate a difference in the action mechanism of triazine herbicides and urea herbicides (Ashton and Crafts, 1973). According to Avron, diuron inhibits photophosphorylation catalysed both by FMN and by vitamin Kj, whereas Bishop demonstrate that simazine inhibits only photophosphorylation catalised by FMN. 

McCarty, R. E., P. R. Pittman, and Y. Tsuchiya. Light-dependent inhibition of photophosphorylation by Af-ethylmaleimide. J. Biol. Chem. 247 304B-3051, 1972. 

In a study designed to determine the mode of action of atrazine in higher plants, Shimabukuro and Swanson (1969) concluded that atrazine inhibits the Hill reaction and its noncyclic phosphorylation, while being ineffective against cyclic photophosphorylation. Atrazine readily penetrated the chloroplast of resistant as well as susceptible plants. In tolerant plants such as sorghum, the metabolism of atrazine was postulated to occur outside the chloroplasts to form water-soluble and insoluble residues that reduced the concentration of photosynthetic inhibitors in the chloroplasts. 

Shimabukuro et al. (1973) identified 2-chloro-4,6-diamino-.v-triazinc (G-28273), which represented complete dealkylation of the triazine ring, as an organosoluble metabolite in sorghum. This metabolite did not inhibit the Hill reaction and cyclic and noncyclic photophosphorylation in isolated pea chloroplasts. 

In soybean seedlings in-vivo supply of cadmium and lead was reported to stimulate the respiration rate (Lee et al., 1976a, b) this effect was ascribed to a demand for ATP production through oxidative phosphorylation because photophosphorylation was reduced. In vitro, several metal ions were effective inhibitors of the mitochondrial respiratory electron transport chain (Kleiner, 1974 Koeppe, 1977) Koeppe (1977) considered the inhibition of the electron transfer at the terminal NADH-oxidase to be specific for cadmium. 

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). 

In contrast to the previous results, Weigel (1985 a, b) reported that in mesophyll protoplasts of Valerianella locusta and in intact chloroplasts of Spinacea oleraceae cadmium affects photosynthesis by inhibition of several reaction steps of the Calvin cycle and not by interaction with the electron transport or photophosphorylation (cf. section on photosynthetic C02 fixation). 

In-vivo metal application Reports of metal effects on electron transport and photophosphorylation after application of toxic amounts of metals to intact plants are less frequent. Cadmium and zinc inhibited PS 2 activity in Lycopersicon esculen-tum (Bazinsky et al, 1980) and Phaseolus vulgaris, respectively (Van Assche and Clijsters, 1983). In the green alga Euglena gracilis (De Filippis et al, 1981 b), PS 2 was sensitive to cadmium, copper and zinc. In the three species mentioned above, the water-splitting enzyme was the site of action. 

Although photophosphorylation was reported to be very sensitive to metals in vitro, the reduction of photosynthetic ATP production was shown to be related to an inhibition of the electron flow rate in Euglena gracilis treated with toxic concentrations of zinc, cadmium and mercury (De Filippis et al., 1981 b), in cadmium-treated Lycoper-sicon esculentum (Bazinsky et al., 1980) and in zinc-treated Phaseolus vulgaris (Van Assche and Clijsters, 1986a). 

Focusing on the mechanisms of action of BOA into the plant cell, Barnes et al.7 suggested that the chlorotic seedlings observed in the presence of BOA and DIBOA could be the consequence of a benzoxazinone effect on the photophosphorylation and electron transport into the plant metabolism. In this way, Niemeyer et al.28 studied the effects of BOA on energy-linked reactions in mitochondria and reported an inhibition of the electron transfer between flavin and ubiquinone in Complex I, with complete inhibition of electron transport from NADH to oxygen in SMP. They could also detect an inhibition of BOA on ATP synthesis by acting directly on the ATPase complex at the F1 moiety. 

Uncouplers. Uncouplers dissociate electron transport from photophosphorylation. Both noncyclic and cyclic phosphorylation are inhibited, but electron transport reactions are either unaffected or stimulated. Because uncouplers relieve the inhibition of electron transport imposed by energy transfer inhibitors, they are considered to act at a site closer to the electron transport chain than the site of phosphate uptake. In Figure 2, they are shown (site 2) as dissipating some form of conserved energy represented as on the noncyclic and cyclic ATP-gener-ating pathways. Perfluidone is the only herbicide identified to date that functions as a pure uncoupler at pH 8.0 (2). Compounds that uncouple photophosphorylation also uncouple mitochondrial oxidative phosphorylation. 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

The bipyridyliums support both noncyclic and cyclic photophosphorylation, are photoreduced by illuminated chloroplasts under anaerobic conditions, and inhibit the photoreduction of NADP. 

Answer Neither 02 nor NADPH is produced. At high [NADPH]/[NADP + ] ratios, electron transfer from reduced ferredoxin to NADP+ is inhibited and the electrons are diverted into the cytochrome b6f complex. These electrons return to P700 and ATP is synthesized by photophosphorylation. Because electrons are not lost from P700, none are needed from PSII. 

Tricolorin A (46) and G (47) are prototype members of this class of amphiphilic glycoconjugates (24). They constitute the allelochemical principles of Ipomoea tricolor Cav., a plant used in traditional agriculture in Mexico as a cover crop to protect sugar cane against invasive weeds. Their molecular mechanism of action likely involves the inhibition of the FT-ATPase of the plasma membrane, an enzyme that plays a crucial role in plant cell physiology. Moreover, 46 acts as a natural uncoupler of photophosphorylation in spinach chloroplasts. This compound also displays general cytotoxicity against several... 

The study of transport by isolated chloroplasts requires the use of good organelles, and the criteria for this are photosynthetic rate and chloroplast intactness. If isolated chloroplasts are capable of rapid electron transport and photophosphorylation, but have lost the ability to assimilate C02 when illuminated in a suitable reaction mixture, they have then been damaged or have been irreversibly inhibited during isolation. Because there is often a clear correlation between envelope integrity and function, results obtained with relatively inactive chloroplasts are unlikely to reflect the behavior of chloroplasts in situ. 

As pointed out previously, most herbicides have been discovered using random screening programs rather than from an applied rational approach to herbicide design, target, and synthesis. A few attempts to rationally design herbicides chose Inhibition of sites of photophosphorylation uncouplers (284). glycol ate oxidase (285). oxidation of Indoleacetic acid (lAA) by peroxidase (286). and secondary plant metabolism, I.e., phenlyalanine ammonia-lyase (2SZ)... 

The participation of FNR in cyclic photophosphorylation has been suggested on the basis of inhibition of cyclic phosphorylation by antibodies raised against FNR [60,61] and more recently on the basis of inhibitor studies [62]. Studies on isolated FNR have shown that this enzyme can reduce Cyt / [63] and the enzyme has recently been extracted from thylakoids together with Cyt/and Cyt by a procedure involving the use of detergents [64]. Whether the catalytic activity of FNR as Cyt / reductase and its possible association with the Cyt f-bf, complex have any relation to its participation in cyclic photophosphorylation remains to be established. 

The diphenyl ether herbicides are active only in the presence of light and canse chlorosis of leaf tissue. They inhibit the Hill reaction in photosynthesis and photophosphorylation. However, the primary mode of action probably involves the photosynthetic rednction to form radicals, which initiate destructive reactions in lipid membranes leading to cell leakage. 

The mode of action of the substituted ureas is relatively well known. It results in an inhibition of photosynthesis by blocking photosynthetic electron transport and photophosphorylation. 

The halftime for the PPj-induced changes is around 0.4 s which is one-tenth of the half-time when ATP is used as the energy donor [4]. The extent of the changes obtained with PPj is usually about two to three times that obtained with ATP. The effects of PPj and ATP are additive [2], The reoxidation of h-type cytochrome starts when the concentration of PP falls below 25 juM [4], Uncouplers of photophosphorylation and electron transport inhibitors abolish both the PPj and the ATP-in-duced reactions. In line with the results with PP synthesis and hydrolysis, the PPj-induced reactions are not inhibited by oligomycin. 

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