Plastoquinone binding site

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 an attempt to learn more about the nature of the plastoquinone binding site, we have analyzed the displacement behaviour of 25 different 1,4-benzoquinones to DCMU. A quantitative structure activity relationship revealed that the displacing activity of a quinone toward DCMU is governed by the redox potential and the geometrical conformation of the quinone (30). 

Compounds that bind to the D, protein at (or near) the plastoquinone-binding site... 

Explain the significance of the two plastoquinone binding sites, Qa and Qb, in the bacterial reaction center. 

Eventually, the electrons in PQBH2 pass through the cytochrome b6f complex (Fig. 19-49). The electron initially removed from P680 is replaced with an electron obtained from the oxidation of water, as described below. The binding site for plastoquinone is the point of action of many commercial herbicides that kill plants by blocking electron transfer through the cytochrome b6f complex and preventing photosynthetic ATP production. 

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. 

Compounds with the same mode of action interact with the same binding site on the protein. Triazines and ureas, as well as the other compounds listed in Figure 8.1, displace plastoquinone QB. Therefore, they also displace each other from the target site in PS II, and their inhibitory potency can be evaluated by the procedure introduced by Tischer and Strotmann (1977). This is experimentally followed with a radioactive derivative in which a 14C labeled triazine is bound to the target. The radioactivity will be diluted out of this site by an unlabeled compound of similar potency and mode of action. This method does not require measuring photosynthetic activity, but does require a structurally and functionally intact PS II because binding efficiency is easily lost by improper handling of the membrane. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

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. 

It is worthy of special interest to study directly the displacement of a herbicide by plastoquinone or its analogues. In normal thy-lakoids, almost no displacement of DCMU even by a million-fold excess of the short-chain plastoquinone analogue plastoquinone-1 can be observed (28). This may be due to the high endogenous plastoquinone content of the thylakoid membrane. If the thylakoids are depleted of plastoquinone by means of n-hexane extraction, a competitive displacement of DCMU by plastoquinone-1 is observed (28). This result establishes a direct interaction between herbicide and plastoquinone, though not necessarily at an identical binding site. From the displacement experiments, a binding constant for plastoquinone-1 of 51 19 jiM in plastoquinone-depleted thylakoids can be calculated (28). ... 

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. 

The doubly reduced Qb can take up two protons from the surrounding medium, forming plastohydro-quinone, QbH2. Eventually QbH21s released from its binding site to the thylakoid matrix where it can interact with the cytochrome Z>6/complex and thus transfer its two electrons to photosystem I eventually. The site vacated by QbH2 is then replenished with an unreduced PQ molecule from the plastoquinone pool, this replacement plastoquinone now ready to function as Qb and accept electron from newly formed Qa . The plastoquinone pool contains 5-10 PQ molecules per reaction center, with each PQ able to store two electrons. This series of reactions is summarized in Pig. 6. 

The SPR response was proportional to the concentration of herbicide in solution (Fig. 11). The sensitivity increased (with shift 110-170 s for concentration 0.1 pg/ml) if the protein was adsorbed from the solution with pH 2.2-5.2 probably due to partial denaturation of the protein-plastoquinone complex which facilitates the access of the atrazine molecules to binding sites. 

A large number of commercial herbicides such as arylureas, triazines, triazinones and phenolic compounds act as competitors to plastoquinones (Fig. 1). They occupy the Qp-binding site of the D1 protein, thereby displacing from its binding niche and prevent the oxidation of reduced Q/v. The displacement of electron mediator Qp from the D1 protein leads to interruption of the electron flow and, consequently, results in plant s death. 

Mechanism 1. Inhibitory herbicides displace plastoquinone B from its proteinaceous binding site on the reducing site of PS II. 

Two families of inhibitors interfere with the plastoquinone or herbicide binding site on the D-1 polypeptide, i.e. on one of the reaction center subunits of PS II. The phenol and urea/triazinone family of PS II inhibitors are different in their functional inhibitory pattern (reviewed in [1]), although they both bind to the D-1 polypeptide and displace each other from the binding site (1). Both QSAR studies (2) and - more refined - quantum mechanical calculations... 

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