Photosystem herbicide binding site

Figure 4. Proposed plastoquinine (QB) and herbicide binding site on the 32 kDalton D-1 polypeptide of photosystem II. The quinone is bound through an iron-complexed histidine residue (his 215) and hydrogen bonding to ser 264. Further interactions occur with arg 269 and phe 255 lying above and below the binding site. Amino acid substitutions in herbicide-tolerant mutants have been identified at the residues numbered 219. 255, 264 and 275. Reproduced with permission from Ref. 57. Copyright 1986 Verlag der Zeitschrift fur Naturforschung.

Ohad N, Hirschberg J. Mutations in the D1 subunit of photosystem II distinguish between quinonc and herbicides binding sites. Plant Cell 1992 4 273-282. 

In order to study the herbicide binding sites and the structure-function relationship of the D protein in photosystem II we have selected different mutants resistant to DCMU, Atrazine and loxynil in a unicellular cyanobacteria Syneohooyst is 6714. We have determined the Dj sequence in each mutant and analyzed by different techniques (fluorescence, oxygen, thermoluminescence) the electron transfer in photosystem II of the different strains. We have also studied the electron transfer in resistant and susceptible Chenoipodium album. 

The data presented here indicates that LY181977 inhibits photosynthetic electron transport through photosystem II. LY181977 has some interaction with that portion of the herbicide binding site which confers atrazine resistance in mutant DCMU4. 

In the reaction centers of photosystem II or purple bacteria, two quinones function in series as the primary (Q ) and the secondary (Qg) electron acceptors [1], The Qg sites are known to be the herbicide binding sites [2]. On the other hand, binding of herbicide to PS I reaction center has never been reported. 

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. 

Shortly after the introduction of the triazine herbicides, it was confirmed that their target site in the photosystem II (PS II) complex was in the thylakoid membranes. Triazines displace plastoquinone at the QB-binding site on the D1 protein, thereby blocking electron flow from QA to QB. This in turn inhibits NADPH2 and ATP synthesis, preventing C02 fixation. 

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. 

In 1979, the concept of a photosystem II herbicide binding protein with different but overlapping binding sites for the various photosystem II herbicides was simultaneously established by Trebst and Draber ( 5) and Pfister and Arntzen (6). This idea of a herbicide receptor protein proved to be extremely fruitful because the techniques of receptor biochemistry were now applicable. Tischer and Strotmann (7) were the first investigators to study binding of radiolabeled herbicides in isolated thylakoids. 

As already stressed, photosystem II herbicides bind reversibly to their binding site. A1tough radiolabeled herbicides are available, it is impossible to identify the herbicide receptor protein without a chemical modification of the herbicide that allows for covalent... 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

Herbicides inhibit photosynthesis by interrupting electron flow on the reducing side of the reaction center of photosystem 11. As originally proposed by Wraight and by Velthuy , based on several lines of evidence, that inhibition by a herbicide occurs in the Qe-binding site in D1 protein and that the action arises from the ability of the herbicide molecule to compete with Qb forthebinding site, thus resulting in the disruption of electron transfer from to Qb-... 

The present paper is a discussion of the photosystem II herbicides and their mechanisms of action. Among the topics covered are the green plant photosystems, photochemistry and electron transfers within photosystem II, requirements for herbicidal activity, mechanisms of action, herbicide selectivity and resistance, herbicide-binding proteins, and theoretical studies of herbicidebinding site interactions. 

The reaction center of photosystem II (PSII) consists of three proteins The 32-kDa protein (32K, also referred to as D,), Dj, and cytochrome bjs, (1,2). It has several features in common with the reaction center from purple bacteria (3,4), including amino acid sequence homology in functional regions (3,5), arrangement of the transmembrane helices (6,7,8), and conservation of the binding sites for chlorophylls, pheophytins, quinones and a non-heme iron (3,4,6,7,9). Furthermore, 32K and the L-subunit of the bacterial reaction center are the site of triazine herbicide action (10-12), and point mutations at conserved residues in these proteins can confer herbicide resistance (3,13-15). 

COMPETITIVE BINDINGS OF HERBICIDE AND QUINONE TO PHOTOSYSTEM I PHYLLOQUINONE (A-1) BINDING SITE. 

Photosystem II herbicides inhibit electron transfer from to Qg by binding to the pi protein thus causing a displacement of the plastoquinone Qg [1,2]. Competition between the herbicides and Qg for the same binding site has been demonstrated [2,3]. Different amino acid substitutions in the D1 protein have been previously found to reduce herbicide binding thereby conferring herbicide-resistance [4,5]. 

Sorgoleone was initially found to inhibit mitochondrial respiration, but it was later found to be a more potent inhibitor of photosyndietic electron transport of photosystem II (PSII) IS, 16). Sorgoleone is structurally similar to plastoquinone (PQ), a benzoquinone involved in photosyndietic electron transport. Sorgoleone competes for the PQ binding site of die D-1 protein in a manner similar to most commercial photosynthetic inhibitors IT). The in vitro PSII inhibiting activity of sorgoleone is similar to some of the commercial herbicides targeting this site e.g., atrazine and diuron). 

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). 

The negative cross-resistances in atrazine-resistant weeds include herbicides that act at or near the same site in photosystem II (DNOC and dinoseb) as well as herbicides acting on other photosystems (paraquat) or at totally different sites. There was negative cross-resistance to other tubulin binding herbicides in dinitroaniline resistant Eleucine indica (Table II), but not to six commercial herbicides on this weed (12). The negative cross-resistance to imazaquin (Table II) occurred in only one of 21 chlorsulfuron resistant mutants. The other mutants had varying levels of co-resistance to imazaquin. 

Photosystem II (PSII) is part of the photosynthetic apparatus in cyanobacteria, algae and higher plants and catalyzes the light-induced transfer of electrons from water to plastoquinone via a set of delicately arranged cofiictors. It has a well known bindii site for diverse chemical compounds in its so-called D1 subunit and the ability to convert such a binding event into signals which can be easily detected by optical, potentiometric or amperometric systems. Due to these inherent properties PSII can be considered as a natural biosensor and has consequentially been used for the detection of herbicides and other pollutants in pilot studies. ... 

Different classes of herbicides block the electron transfer at the level of photosystem II. The herbicides and Qg bind in the same region of the D protein which belongs to the PSII core complex (1). The resistance to an herbicide is due to a decrease in the affinity constant of the herbicide for its site. 

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