Plastoquinone reduction inhibition

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. 

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. 

Other functions of cytochrome b PSI cyclic electron transport. The rate of reoxidation of cyt after a flash is increased by a factor of 4-5 if heme b is reduced prior to the flash (57). This suggests a mechanism for PSI cyclic phosphorylation that incorporates the oxidant-induced reduction of heme bp concomitant with reduction of heme b by PSI and ferredoxin, possibly through a quinone site (62). The oxidation of the two hemes by plastoquinone would be cooperative, as implied by the effect of b reduction on the reoxidation rate. The proposal of a quinone niche near the center of the bilayer that could oxidize both hemes resembles the semiquinone cycle model (63). One confusing aspect is the action of antimycin A which inhibits PSI cyclic phosphorylation (64). Because this compound is a classic n-side inhibitor of the mitochondrial cyt 6, it would be expected to act on cyt b y but there is no clear spectrophotometric effect. The slow reduction of cyt b mediated by ferredoxin in the dark (65) may be a problem for this model, although this would be explained if the reduction of cyt b as well as its oxidation is cooperative. 

The effects of the electron inhibitor DCMU, the uncoupler S-13 and the energ transfer inhibitor DCCD on electron transfer and photophosphorylation in membrane vesicles of Syneohocoocus 6716 are shown in Fig. 1. DCMU behaves as expected, fully blocking electron transfer and the resulting phosphorylation at 1 to 10 yM. S-13 is a potent uncoupler full inhibition of ATP synthesis occurs at 1 yM however, electron transfer is not enhanced. In linear electron transfer DCCD seems to act as an electron transfer inhibitor at lower concentrations as was found before in chloroplasts, where DCCD bloc) the reduction site of plastoquinone (Sane et al. 1979). Only at higher concentrations (10 M) DCCD also inhibits energy transfer as can be seen in th< PMS-mediated cyclic photophosphorylation. 

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