Herbicide-binding sites revealed in the structure of plant acetohydroxyacid synthase

Abstract

The sulfonylureas and imidazolinones are potent commercial herbicide families. They are among the most popular choices for farmers worldwide, because they are nontoxic to animals and highly selective. These herbicides inhibit branched-chain amino acid biosynthesis in plants by targeting acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This report describes the 3D structure of Arabidopsis thaliana AHAS in complex with five sulfonylureas (to 2.5 Å resolution) and with the imidazolinone, imazaquin (IQ; 2.8 Å). Neither class of molecule has a structure that mimics the substrates for the enzyme, but both inhibit by blocking a channel through which access to the active site is gained. The sulfonylureas approach within 5 Å of the catalytic center, which is the C2 atom of the cofactor thiamin diphosphate, whereas IQ is at least 7 Å from this atom. Ten of the amino acid residues that bind the sulfonylureas also bind IQ. Six additional residues interact only with the sulfonylureas, whereas there are two residues that bind IQ but not the sulfonylureas. Thus, the two classes of inhibitor occupy partially overlapping sites but adopt different modes of binding. The increasing emergence of resistant weeds due to the appearance of mutations that interfere with the inhibition of AHAS is now a worldwide problem. The structures described here provide a rational molecular basis for understanding these mutations, thus allowing more sophisticated AHAS inhibitors to be developed. There is no previously described structure for any plant protein in complex with a commercial herbicide.inhibition ͉ sulfonylurea ͉ x-ray crystallography ͉ imidazolinone ͉ thiamin diphosphate T he sulfonylurea and imidazolinone herbicides are an essential part of the multibillion-dollar weed-control market. There are now Ͼ30 herbicides from these families registered for worldwide use. A major advantage of these compounds is that they are nontoxic to animals, highly selective, and very potent, thereby allowing low application rates. These herbicides act by inhibiting acetohydroxyacid synthase [AHAS; also known as acetolactate synthase; EC 2.2.1.6 (1)], the first common enzyme in the biosynthetic pathway of the branched-chain amino acids. The reaction carried out by this enzyme is the synthesis of either (S)-2-acetolactate from two molecules of pyruvate or (S)-2-aceto-2-hydroxybutyrate from a molecule each of pyruvate and 2-ketobutyrate.AHAS belongs to a superfamily of thiamin diphosphate (ThDP)-dependent enzymes that are capable of catalyzing a variety of reactions, including both the oxidative and nonoxidative decarboxylation of 2-ketoacids. This cofactor is bound by a divalent metal ion such as Mg 2ϩ , which coordinates to the diphosphate group of ThDP and to two highly conserved residues (2) in these proteins. AHAS also binds a molecule of FAD, although this cofactor does not participate in the principal reactions. To date, most AHAS enzymes that have been characterized have both a catalytic subunit (Ϸ65 kDa) and a smaller regulatory subunit, which varies in s...

Bioensaios para diagnóstico da resistência aos herbicidas imidazolinonas em arroz

“…Entretanto, avaliando a mesma característica, Kuk et al (2003) discriminaram os biótipos S e R ao herbicida imazosulfuron da planta Bioensaios para diagnóstico da resistência aos herbicidas ... A diferente localização das mutações no gene ALS responsáveis pela resistência aos herbicidas desses cultivares pode ter causado distintos níveis de resistência, devido às mudanças da conformação da enzima ALS e à importância específica do aminoácido mutado em relação à ligação de cofatores (McCourt et al, 2006). Em sementes de Setaria viridis embebidas em diversas doses de trifluralina, foi possível diagnosticar a resistência em apenas cinco dias após a embebição nos tratamentos (Beckie et al, 1990) (Tal et al, 2000), ALS (Beckie et al, 2000) e EPSPs (Escorial et al, 2001;Perez & Kogan, 2003).…”

Growth Inhibition of Pathogenic Bacteria by Sulfonylurea Herbicides

“…To determine the prevalence of AHAS isozymes across a sample of clinically relevant bacteria, we performed a phylogenetic analysis on the AHAS catalytic subunit, which contains the SH binding site (2,16). As seen previously (13,22,23), type I AHAS was exclusively present in Enterobacteriaceae (Fig.…”

“…Bacterial isozymes of AHAS are composed of a catalytic and a regulatory subunit that are encoded by adjacent genes. Catalytic subunits of AHAS were identified by searching the genomes of select organisms using the Enzyme Commission number 2.2.1.6 (2,16). In most genomes, multiple genes have been annotated as AHAS catalytic subunits.…”

A Novel Rice Cytochrome P450 Gene, CYP72A31, Confers Tolerance to Acetolactate Synthase-Inhibiting Herbicides in Rice and Arabidopsis

“…Structural data describing the ALS protein in complex with ALS-inhibiting herbicides have been reported (McCourt et al, 2006). It has also been shown that it is difficult to predict substrate preferences from primary sequences in CYP72A proteins (Nelson and WerckReichhart, 2011).…”