Inhibitors of photosystem

Achnrne L, Pereda-Miranda R, Iglesias-Prieto R, Moreno-Sanchez R, Lotina-Hennsen B (1999) Tricolorin A, a Potent Natural Uncoupler and Inhibitor of Photosystem II Acceptor Side of Spinach Chloroplasts. Physiol Plant 106 246... 

Arrhenius A, Gronvall F, Scholze M, Backhaus T, Blanck H. 2004. Predictability of the mixture toxicity of 12 similarly acting congeneric inhibitors of photosystem II in marine periphyton and epipsammon communities. Aquat Toxicol 86 351-367. 

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). 

Figure 4. Interaction of structural elements of herbicidal inhibitors of photosystem 11 with a postulated receptor...

Figure 6. Structural elements of some heterocyclic inhibitors of photosystem II [adapted from Trebst and Harth (16)], Numbers...

Many commercial herbicides kill weeds by interfering with the action of photosystem II or photosystem I. Inhibitors of photosystem II block electron flow, whereas inhibitors of photosystem I divert electrons from the terminal part of this photosystem. Photosystem II inhibitors include urea derivatives such as diuron and triazine derivatives such as atrazine. These chemicals bind to the Qg site of the D1 subunit of photosystem II and block the formation of plastoquinol (QH2). 

Paraquat (1,1 -dimethyl-4-4 -bipyridinium) is an inhibitor of photosystem 1. Paraquat, a dication, can accept electrons from photosystem I to become a radical. This radical reacts with O2 to produce reactive oxygen species such as superoxide and hydroxyl radical (OH ). Such reactive oxygen species react with double bonds in membrane lipids, damaging the membrane (Section 18.3.6). 

Compounds of non-adenine structure were sought based on the considerations of steric similarities between A -substituted adenine and phenylurea and also between phenylurea, A-arylcarbamates and s-triazine herbicides. Many of the compounds found are known as inhibitors of photosystem 11 electron flow and some also inhibit the cytokinin-stimulated callus growth [36,37]. It has been concluded that phenylureas, s-triazines and V-arylcarbamates share the same place of action, which is the cytokinin receptor localized in chloroplasts [28]. 

Soskic, M. and Plavsic, D. (2001) QSAR study of 1,8-naphthyridin-4-ones as inhibitors of photosystem II./. Chem. Inf. Comput. Sci, 41, 1316-1321. 

Bromoxynil is a special case because the parent compound bromoxynil-octanoate, although being a baseline toxicant, is highly toxic due to its high hydrophobicity (Fig. 10). The hydrolysis product bromoxynil is the active ingredient and has a specific mode of action Bromoxynil is a potent inhibitor of Photosystem II and is also an uncoupler of photophosphorylation (i.e. destroys the electrochemical proton gradient formed in the electron transport... 

Amicarbazone is a new herbicide for broad spectrum weed control in com and sugarcane. It belongs to the chemical class of carbamoyl triazolinones and acts as an inhibitor of photosystem II. It was discovered 1988 by the former Plant Protection Division of Bayer AG (now Bayer Crop Science) and developed under the internal code no. BAY MKH 3586 (Pig. 10.3). 

W. Draber, 2. Naturforsch., Sect. C, 42, 713 (1987). Can Quantitative Structure Activity Analyses and Molecular Graphics Assist in Designing New Inhibitors of Photosystem-II ... 

---