Herbicide Binding Niche

Many commercial herbicides inhibit photosynthesis by displacing Qg from its binding site in D1 and thus block electron transport from to They belong to various chemical classes like triazines, ureas or phenols (For a review see ref. 29) and pollute soil and water due to their massive use in agriculture. This in turn can be harmful for human and animal health and necessitates the development of fast and sensitive detection methods. Coincidentally, the herbicidal target itself is part of the PSII complex, which represents a reporter system directly coupled to an analyte binding site. Thus the most obvious application of the Dl protein in association with other central PSII proteins is its use as a biosensor for herbicides. 

As already stressed, the mutation Set264 Gly is one which has been observed in nature. It is found by now in all countries and in a variety of weeds which are rendered resistant t ainst triazines and triazinones (see Table 1). It should be noted, that atrazine-resistant rape with a modified Dl protein (Ser264 Gly) is used as a crop in Canada. Resistance against ureas is comparably small and against phenolic herbicides like i-dinoseb, ioxynil and DNOC negative cross resistance is observed. 

Changes in binding affinities for herbicides in the Leu275 Phe mutant are marginal. It should only be stressed that triazinones are rendered resistant in this mutant and supersensitivity is observed against the phenolic herbicide ioxynil. 

Biotechnological Applications of Photosynthetic Proteins Biochips, Biosensors and Biodevices 

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Figure 8.2 Folding of the amino acid sequence of the herbicide-binding niche in the D1 protein of PS II.

Trebst, A. (1987). The three dimensional structure of the herbicide binding niche on the reaction center polypeptide of photosystem II. 

In 1999 Masabni and Zandstra reported on a mutant of Portulaca oleracea with a resistance pattern to PS II inhibitors that was different to most triazine resistant weeds [28], This mutant was resistant to the phenylureas linuron and diuron, but also cross-resistant to atrazine and other triazines. Sequencing of the D1 protein revealed that in the resistant biotype the serine 264 was replaced by threonine and not by glycine. This was the first report on a serine 264 to threonine mutation on a whole plant level. It was proposed that the serine-to-threonine mutation modified the conformation of the herbicide binding niche at the D1 protein in a way, which resulted in reduced binding of phenylureas and triazines as well. 

A. Trebst (1987) The Three-Dimensional Structure of the Herbicide Binding Niche on the Reaction Center Polypeptide of Photosystem II. Z. Naturforsch. 42c, 272-750... 

The phenolic derivatives indicated in Figure 8.1 are also bound to the same binding niche on PS II as the triazines (Oettmeier, 1992). However, they have a somewhat different inhibition pattern than the classical family of PS II herbicides (e.g., triazines and ureas) and, therefore, were regarded as a separate family with a somewhat different mode of action (Van Rensen et al., 1978 Trebst and Draber, 1986). It is now clear that they just orient somewhat differently in the same binding niche, as discussed below. Although the phenolics are photosynthesis inhibitors, dinoseb and the halogenated benzonitriles also inhibit respiration. 

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. 

Certain hydroxyquinolines have already been reported to be herbicides in the patent literature (33,34), although their mode of action had not been established. The well known inhibitor (35,36) of electron flow systems, hydroxyquinoline-N-oxide, is, of course, also a hydroxyquinoline derivative, although with a different substitution pattern to those reported here. Therefore the compound may be oriented in a turned around way in the binding niche (see 37), placing it in the serine family. 

Quantitative Structure Activity Relationships. The goal of our structure activity studies was not the prediction of more active compounds in order to finally get new hints for the development of a herbicide. Instead, we wanted to corroborate our concept that phenols with the appropriate substitution enter the binding niche in the D1 protein just as the classical herbicidal inhibitors like atrazine and diuron do. 

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