Triazines binding sites

There is generally a lack of fitness or ability in the triazine-resistant biotypes to compete with the crop or with other nontriazine-resistant weeds as a result of the altered triazine binding site at the D1 protein in PS II. 

In this paper, we have summarized our current understanding of the biochemical nature of the triazine binding site within the PS II complex. Studies using the proteolytic enzyme trypsin as a selective, surface-specific modifier of membrane polypeptides and the use of a photoaffinity triazine have been utilized separately to identify the triazine receptor protein as a 32-34 kilodalton (kDal) polypeptide of the PS II complex in peas (Pisum sativum L.). The nature of the covalent attachment of the photoaffinity probe has also enabled us to identify the triazine receptor protein as a product of chloroplast-directed protein synthesis this implies that the structural gene for the triazine receptor polypeptide is encoded on chloroplast DNA. 

It was of interest to determine if the chloroplast membrane protein of 32-34 kDal that binds the photoaffinity triazine, and which appears to be required for triazine binding in isolated PS II particles, is a chloroplast gene product. In developing chloroplasts, in parallel to the appearance of functional activities, there is rapid synthesis and accumulation of a major thylakoid protein of 34 kDal ( ). This rapidly synthesized chloroplast protein has been shown to be encoded by the chloroplast genome in Z. mays (2Z.) This section outlines experiments that were carried out to determine if the chloroplast polypeptide, which serves as the triazine binding site, is identical to the chloroplast-encoded protein of the same molecular weight. 

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. 

As stressed above, two PS II classes of herbicides with the same mode of action (e.g., triazines and ureas) share the same binding site and replace each other from that site on the D1 protein (Tischer and Strotmann, 1977). From this it... 

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. 

The high efficacy of triazine herbicides and their repetitive use in crops and noncrop situations has resulted in the selection of weeds that are resistant to these herbicides or are not well controlled at the lower rates now being used. In most instances, triazine resistance is due to an alteration in the herbicide-binding site in PS II. Despite the widespread occurrence of triazine resistance, these herbicides are still widely used, even in fields in which triazine-resistant biotypes are known to occur. The rate of increase in the selection for triazine-resistant weed species depends in part on the integration of alternative weed control strategies, in addition to the use of triazine herbicides, for control of these weed species. Due to their resistance mechanism, many triazine-resistant weeds are less competitive than their susceptible counterparts. 

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. 

Smeda et al. (1993) reported that in a mutation of the psb A gene in a photoautotropic potato, atrazine resistance was attributable to a mutation from AGT (ser) to ACT (threonine) in codon 264 of the psb A gene that encodes the Qb protein. Although the mutant cells exhibited extreme levels of resistance to atrazine, no concomitant reductions in photosynthetic electron transport or cell growth rates were detected compared to the unselected cells. This is in contrast with the losses in productivity observed in atrazine-resistant mutants that contain a Ser to Gly 264 alteration. Research has shown that triazine resistance by various algae and photosynthetic bacteria has been due to changes in many different binding sites (Oettmeier, 1999). 

A precise description of bonding between triazines and humic substances is complicated by the extreme heterogeneity of humic substances. However, it is clear that all of the above mechanisms contribute to the sorption of triazines and that two or more mechanisms may contribute to the interaction energy for a given molecule. The stereo chemistry of each potential binding site determines which mechanisms are involved. Figures 21.2 and 21.3 summarize the types of interactions that may contribute to the retention of chloro-.v-triazines and protonated-keto-triazines, respectively. 

Michel et al. (1986b) describe two X-ray stmctures of Rp. viridis RCs where the competitive inhibitors terbutryn [2-(methylthio)-4-(ethylamino)-6-(fert-butylamino)-5-triazine] and o-phenanthroline are bound into the Qg pocket. Terbutryn interacts with the protein via hydrogen bonds to Ser L223 and He L224 and is located close to the hydrophobic entrance of the Qg binding site. o-Phenanthrolin is situated nearer the bottom of the Qg pocket and... 

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