Inhibition acetolactate synthase ALS

What are the criteria for regarding a compound as a TS analog The observation that the binding affinity of an inhibitor is greater than that of a substrate, i.e., X, < XM, is insufficient as many potent inhibitors bind differently to an enzyme than the substrate examples are methotraxate, inhibiting dihydrofolate reductase (DHFR) X] = 0.15 pM (Werkheiser, 1961), and sulfonyl urea herbicides, inhibiting acetolactate synthase (ALS) at picomolar levels. 

The sulfonylureas, an extremely potent class of herbicides, act by inhibiting acetolactate synthase (ALS), which is the first common enzyme in the biosynthetic pathways leading to the branched chain amino acids. Two other unrelated classes of herbicides also act by interfering with this enzyme. We have cloned and characterized the genes encoding ALS from several higher plants. The ALS genes isolated from herbicide sensitive and herbicide resistant plants have been compared, and several mutations which confer the herbicide resistant phenotype have been identified. 

For example, glyphosate inhibits the enzyme, EPSP (5-enolpyruvylshikimate 3-phosphate) synthase, that catalyzes a step in the synthesis of the aromatic amino acids. Similarly, both the imidazolinones and sulfonylureas inhibit acetolactate synthase (ALS), the enzyme that catalyzes the first step in the formation of branched-chain amino acids (11). Triazine herbicides act by binding to a specific protein in the thylakoid membranes of the chloroplasts, preventing the flow of electrons and inhibiting photosynthesis (12). 

This strategy was used to engineer resistance against glyphosate, and imidazoli-nones and sulfonylureas that inhibit acetolactate synthase (ALS), a key enzyme in the biosynthesis of branched chain amino acids. ALS resistant crops have primarily been generated through selection for an herbicide insensitive ALS allele from natural or mutagenized cell or plant populations [3]. 

In AChE-based biosensors acetylthiocholine is commonly used as a substrate. The thiocholine produced during the catalytic reaction can be monitored using spectromet-ric, amperometric [44] (Fig. 2.2) or potentiometric methods. The enzyme activity is indirectly proportional to the pesticide concentration. La Rosa et al. [45] used 4-ami-nophenyl acetate as the enzyme substrate for a cholinesterase sensor for pesticide determination. This system allowed the determination of esterase activities via oxidation of the enzymatic product 4-aminophenol rather than the typical thiocholine. Sulfonylureas are reversible inhibitors of acetolactate synthase (ALS). By taking advantage of this inhibition mechanism ALS has been entrapped in photo cured polymer of polyvinyl alcohol bearing styrylpyridinium groups (PVA-SbQ) to prepare an amperometric biosensor for... 

There are several commercially available sulfonylurea herbicides that contain a 2-pyrimidine group <2006H(68)561>. These compounds, which function by inhibition of acetolactate synthase (ALS), an enzyme involved in the early stage of branched-chain amino acid synthesis, include sulfometuron-methyl 1095, primisulfuron-methyl 1096, chlorimuron-ethyl 1097, bensulfuron-methyl 1098, ethoxysulfuron 1099, nicosulfuron 1100, and pyrazosulfuron-ethyl 1101. Related nonsulfonylureas include the sulfide pyrftalid 1102 and the ether pyriminobac-methyl 1103. 

Inhibition of acetolactate synthase (ALS) [Acetohydroxy acid synthase (AHAS)] Sulfonylureas Imidazolinones T riazolopyrimidines Pyrimidinylthiobenzoates 2... 

Sprague, C.L., E.W. Stoller, and L.M. Wax (1997c). Response of an acetolactate synthase (ALS)-resistant biotype of Amaranthus rudis to selected ALS-inhibiting and alternative herbicides. Weed Res., 37 93-101. 

The mode of their action consists in the acetolactate synthase (ALS) inhibition and, as a result, in the inhibition of the branched-chain amino acid biosynthesis from acetolactate (90MI1, 92MI7, 98MI4). Results of QSAR studies were published (03MI3). Another approach was used to analyze a model of inhibition of photosystem II (96MI1). 

One of the most effective classes of herbicide are the sulfonylureas. They are applied in quantities of no more than a few grams per hectare. The action of sulfonylureas is based on inhibition of acetolactate synthase (ALS) [9]. The fluorine-containing derivative Primsulfuron methyl has selectivity in the cultivation of maize [10]. 

Inhibits ALS activity by 50%. A good correlation exists between the herbicidal activity of sulfonylureas and their ability to inhibit acetolactate synthase (2). This in vitro assay using the target enzyme along with the three-dimensioFTI structure of the enzyme should aid in the generation of a substantial data base that can be used to design potent inhibitors. 

Lolium biotypes exist which have resistance to the sulfonylurea herbicides chlorsulfuron and metsulfuron methyl (4). The biotype used in the studies presented here is resistant to both these sulfonylurea herbicides. Sulfonylurea herbicides inhibit the chloroplastic enzyme acetolactate synthase (ALS), also known as acetohydroxyacid synthase (AHAS) (16). Inhibition of this enzyme results in disruption of the synthesis of the branched-chain amino acids valine and isoleucine (161. The imidazolinone herbicides also inhibit ALS Q2). In some species auxins can protect against chlorsulfuron inhibition (S. Frear, USDA North Dakota, personal communication) the mechanistic basis for this protection is not known. We have measured the ALS activity in the resistant and susceptible Lolium and have also checked for any induction of ALS activity following treatment with the sulfonylurea herbicide chlorsulfuron. 

The sulfonylurea herbicides are a new family of chemical compounds, some of which are selectively toxic to weeds but not to crops. The selectivity of the sulfonylureas results from their metabolism to non-toxic compounds by particular crops, but not by weeds. In addition to efficient weed control, the sulfonylurea herbicides provide environmentally desirable properties such as field use rates as low as two grams/hectare and very low toxicity to mammals. The high specificity of the herbicides for their molecular target contributes to both of these properties. In addition, the low toxicity to mammals results from their lack of the target enzyme for the herbicides. Sulfonylureas inhibit the enzyme acetolactate synthase (ALS), also known as acetohydroxyacid synthase (AHAS), which catalyzes the first common step in the biosynthesis of the branched chain amino acids leucine, isoleucine and valine. In mammals these are three of the essential amino acids which must be obtained through dietary intake because the biosynthetic pathway for the branched chain amino acids is not present. The prototype structure of a sulfonylurea herbicide is shown in Figure 1. 

A great number of herbicides that work through the inhibition of acetolactate synthase (ALS) have been commercialized. They belong to four chemical groups sulfonylureas (23), triazolopyrimidines (2), imidazolinones (5), and pyrimidinyloxybenzoic analogues (3). (The number of active ingredients in parentheses is taken from The Pesticide Manual.) Also in this case, potent herbicides were developed (e.g., chlorsulfuron) before the site of action was found. 

These are chemical compounds that inhibit acetohydroxy acid synthase, also known as acetolactate synthase (ALS). These compounds inhibit the production of the... 

Application of Global Sequence Similarity to Find an Inhibitor of Acetolactate Synthase. Acetolactate synthase (ALS) Is the site of action of sulfonylurea, Imldazollnone, and trlazolo pyrimidine herbicides (10-14). Their mode of Inhibition and binding sites on ALS were ambiguous, because (1) these herbicides bear no obvious similarity In their chemical structures to those of ALS substrates (pyruvate and acetolactate), cofactors (thiamine pyrophosphate, FAD, and Mg ) and effectors (valine, Isoleuclne, and leucine) and (2) they Inhibit ALS In a mode too complex to be analyzed. 

Inhibitors of Acetolactate Synthase (ALS/AHAS) The enzyme acetolactate synthase (ALS) plays in plants an essential role in branched-chain amino acid biosynthesis. In the pathway leading to valine and leucine, ALS catalyzes the formation of 2-acetolactate from two pyruvate molecules, and in the pathway to isoleucine the formation of 2-acetohydroxybutyrate from 2-ketobutyrate and pyruvate. Due to this double function the enzyme is also called with a more general term aceto-hydroxyacid synthase. ALS is inhibited by several groups of herbicides, mainly the sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinylthiobenzoates(PTBs) and sulfonylaminocarbonyltriazolinone (SCTs) (see Chapter 2.1, M. E. Thompson). 

The same is true in the case of imazaquin and chlorimuron-ethyl. It is well known that these two herbicides have the same mode of action (, ), namely blocking the biosynthesis of branched chain amino acids by inhibition of acetolactate synthase (ALS) correspondingly, the response patterns are very similar. 

Imazaquin, an example of the imidazolinone class of herbicides, is particularly selective to soybeans. These compounds have a similar mode of action to that of the sulfonylurea herbicides, which will be discussed later. They block branched chain amino acid biosynthesis by inhibition of the enzyme acetolactate synthase (ALS). 

Acetolactate synthase (ALS) is the enzyme target site of the sulfonyl-ureas. In common with the imidazolinone aryl carboxylates, these herbicides inhibit valine and isoleucine biosynthesis. The imidazolinones are exemplified by Assert (8), which consists of a mixture of m- and p-isomers. The selectivity of ALS inhibitors invariably can be accounted for by differential metabolism or uptake or related phenomena rather than by any significant inherent difference in the properties of the ALS enzymes of crop and weed species. Assert is another example of a herbicide activated in plants by deesterification to the phytotoxic acid, and susceptible species such as Avena fatua (wild oat) are unable to metabolize the molecule further. Facile ring-methyl hydroxylation to the m- and p-benzyl alcohols, however, followed by glycosylation confers tolerance to maize and wheat." ... 

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