Plastoquinone binding proteins

Figure 19.22. Pathway of Electron Flow From H2O to NADP in Photosynthesis. This endergonic reaction is made possible by the absorption of light by photosystem II (P680) and photosystem I (P700). Abbreviations Ph, pheophytin Qa and Qg, plastoquinone-binding proteins Pc, plastocyanin Aq and Aj, acceptors of electrons from P700 Fd, ferredoxin Mn, manganese.

Qb. plastoquinone-binding proteins Pc, plastocyanin Aq and A. acceptors of electrons from P700 Fd, ferredoxin Mn. manganese. 

The 1950s saw the development of triazine-based herbicides such as atrazine, 24.7, prepared from amines and the cyanuric chloride, 24.8. These compounds bind to the plastoquinone-binding protein in photosystem II. They are effective and inexpensive. They are largely banned in the EU (where relatively little corn is grown) as estrogen disruptors but are widely used for corn in the United States. 

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. 

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

Presented experimental results suggest that application of herbicide-binding protein in sensor technology has a high potential. Several detection systems were tested in combination with D1 protein electrochemical (amperometry and cyclic voltammetry), optical (surface plasmon resonance and ellipsometty) and assays (ELISA and D1 protein- containing liposomes and DELFIA fluori-metric assay). The main mechanisms of D1 action are either on the ability of herbicides to replace the plastoquinone molecule in D1 protein and in this way change the electrochemical and optical... 

In thylakoid membranes (TM) diuron-type herbicides block electron flow between the primary and the secondary plastoquinone 1. Since, the binding site is located on the 32 Kd herbicide-binding protein (32 Kd H-B protein), which is embedded in both thylakoid monolayers 2, one can ask whether acyl lipids play a particular role in the herbicide-protein interaction, as suggested elsewhere 3. ... 

PS II absorbs more light than PS I. As such PS I cannot take electrons as fast as PS II can supply, leaving plastoquinone in its reduced state. This reduced plastoquinone activates a protein kinase that phosphorylates the threonine (Thr) residue of the LHCs that in turn migrate to the unstacked portion of the thylakoid membrane where it binds to PS I. As a result, a large portion of incident light is funneled to PS I. 

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 was originally assumed that the herbicides bind to a protein component of photosystem II (named "B" or R") ( 1, 2J. This protein component was assumed to contain a special bound plastoquinone whose midpoint potential is lowered due to herbicide binding. Consequently, electron flow is interrupted ( 1,2). The photosystem II... 

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