Acetolactate synthase isozyme II from Salmonella typhimurium

Design, synthesis and SAR study of novel sulfonylureas containing an alkenyl moiety

A series of sulfonylurea compounds was designed and synthesized via introducing an alkenyl moiety into the aryl-5 position and most title compounds exhibited enhanced antifungal activities and limited herbicidal activities compared with chlorsulfuron. Then, a CoMSIA calculation for antifungal activities was carried out to establish a 3D-QSAR model in which a cross-validated q(2) of 0.585 and a correlation coefficient r(2) of 0.989 were obtained. The derived model revealed that hydrophobic and electrostatic fields were the two most important factors for antifungal activity. Structure optimization was performed according to the CoMSIA model and compound 9z was found to be as potent as chlorothalonil in vitro against C. cornigerum, the pathogen of the wheat sharp eyespot disease. In order to study the fungicidal mechanism, 9z was successfully docked into yeast AHAS using a flexible molecular docking method and the resulting binding pattern was similar to that of chlorimuron-ethyl, indicating that the antifungal activity of compounds 9 was probably due to the inhibition of fungal AHAS.

Detailed structure-function correlations of Bacillus subtilis acetolactate synthase

Isobutanol is deemed to be a next-generation biofuel and a renewable platform chemical.1 Non-natural biosynthetic pathways for isobutanol production have been implemented in cell-based and in vitro systems with Bacillus subtilis acetolactate synthase (AlsS) as key biocatalyst.2-6 AlsS catalyzes the condensation of two pyruvate molecules to acetolactate with thiamine diphosphate and Mg(2+) as cofactors. AlsS also catalyzes the conversion of 2-ketoisovalerate into isobutyraldehyde, the immediate precursor of isobutanol. Our phylogenetic analysis suggests that the ALS enzyme family forms a distinct subgroup of ThDP-dependent enzymes. To unravel catalytically relevant structure-function relationships, we solved the AlsS crystal structure at 2.3 Å in the presence of ThDP, Mg(2+) and in a transition state with a 2-lactyl moiety bound to ThDP. We supplemented our structural data by point mutations in the active site to identify catalytically important residues.

Acetohydroxyacid synthase and its role in the biosynthetic pathway for branched-chain amino acids

The branched-chain amino acids are synthesized by plants, fungi and microorganisms, but not by animals. Therefore, the enzymes of this pathway are potential target sites for the development of antifungal agents, antimicrobials and herbicides. Most research has focused upon the first enzyme in this biosynthetic pathway, acetohydroxyacid synthase (AHAS) largely because it is the target site for many commercial herbicides. In this review we provide a brief overview of the important properties of each enzyme within the pathway and a detailed summary of the most recent AHAS research, against the perspective of work that has been carried out over the past 50 years.

Isoleucine starvation caused by sulfometuron methyl in Salmonella typhimurium measured by translational frameshifting

The authors have developed a tool for the study of inhibitor-induced amino acid starvation in bacteria which exploits the phenomenon of translational frameshifting. The inhibition of acetohydroxyacid synthase II by the herbicide sulfometuron methyl (SMM) has complex effects on branched-chain amino acid biosynthesis. Experiments were done with Salmonella typhimurium containing a plasmid with an isoleucine codon in a 'shifty' region, prone to translational frameshifting. SMM did not cause translational frameshifting in minimal medium under conditions that inhibit growth. A 20-fold higher concentration of SMM was required to cause starvation for isoleucine, e.g. in the presence of valine. This starvation was reflected in translational frameshifting correlated with inhibition of growth. These observations support the authors' previous suggestions based on other techniques. The method used here could be generalized for the study of complex metabolic effects related to amino acids.

Thiamin-dependent enzymes as catalysts in chemoenzymatic syntheses

Enzymes are increasingly being used to perform regio- and enantioselective reactions in chemoenzymatic syntheses. To utilize enzymes for unphysiological reactions and to yield novel products, a broad substrate spectrum is desirable. Thiamin diphosphate (ThDP)-dependent enzymes vary in their substrate tolerance from rather strict substrate specificity (phosphoketolases, glyoxylate carboligase) to more permissive enzymes (transketolase, dihydroxyacetone synthase, pyruvate decarboxylase) and therefore differ in their potential to be used as biocatalysts. We give an overview of the known substrate spectra of ThDP-dependent enzymes and present examples of multi-enzyme or chemoenzymatic approaches which involve ThDP-dependent enzymes as biocatalysts to obtain pharmaceutical compounds as ephedrine and glycosidase inhibitors, sex pheromones as exo-brevicomin, 13C-labeled metabolites, and other intermediates as 1-deoxyxylulose 5-phosphate, a precursor of vitamins and isoprenoids.

Biosynthesis of 2-aceto-2-hydroxy acids: acetolactate synthases and acetohydroxyacid synthases

Two groups of enzymes are classified as acetolactate synthase (EC 4. 1.3.18). This review deals chiefly with the FAD-dependent, biosynthetic enzymes which readily catalyze the formation of acetohydroxybutyrate from pyruvate and 2-oxobutyrate, as well as of acetolactate from two molecules of pyruvate (the ALS/AHAS group). These enzymes are generally susceptible to inhibition by one or more of the branched-chain amino acids which are ultimate products of the acetohydroxyacids, as well as by several classes of herbicides (sulfonylureas, imidazolinones and others). Some ALS/AHASs also catalyze the (non-physiological) oxidative decarboxylation of pyruvate, leading to peracetic acid; the possible relationship of this process to oxygen toxicity is considered. The bacterial ALS/AHAS which have been well characterized consist of catalytic subunits (around 60 kDa) and smaller regulatory subunits in an alpha2beta2 structure. In the case of Escherichia coli isozyme III, assembly and dissociation of the holoenzyme has been studied. The quaternary structure of the eukaryotic enzymes is less clear and in plants and yeast only catalytic polypeptides (homologous to those of bacteria) have been clearly identified. The presence of regulatory polypeptides in these organisms cannot be ruled out, however, and genes which encode putative ALS/AHAS regulatory subunits have been identified in some cases. A consensus sequence can be constructed from the 21 sequences which have been shown experimentally to represent ALS/AHAS catalytic polypeptides. Many other sequences fit this consensus, but some genes identified as putative 'acetolactate synthase genes' are almost certainly not ALS/AHAS. The solution of the crystal structures of several thiamin diphosphate (ThDP)-dependent enzymes which are homologous to ALS/AHAS, together with the availability of many amino acid sequences for the latter enzymes, has made it possible for two laboratories to propose similar, reasonable models for a dimer of catalytic subunits of an ALS/AHAS. A number of characteristics of these enzymes can now be better understood on the basis of such models: the nature of the herbicide binding site, the structural role of FAD and the binding of ThDP-Mg2+. The models are also guides for experimental testing of ideas concerning structure-function relationships in these enzymes, e.g. the nature of the substrate recognition site. Among the important remaining questions is how the enzyme suppresses alternative reactions of the intrinsically reactive hydroxyethylThDP enamine formed by the decarboxylation of the first substrate molecule and specifically promotes its condensation with 2-oxobutyrate or pyruvate.

Expression, purification and characterization of Arabidopsis thaliana acetohydroxyacid synthase

Acetohydroxyacid synthase (EC 4.1.3.18) is the enzyme that catalyses the first step in the synthesis of the branched-chain amino acids valine, leucine and isoleucine. The AHAS gene from Arabidopsis thaliana with part of the chloroplast transit sequence removed was cloned into the bacterial expression vector pT7-7 and expressed in the Escherichia coli strain BL21(DE3). The expressed enzyme was purified by an extensive procedure involving (NH4)2SO4 fractionation followed by hydrophobic and anion-exchange chromatography. The purified enzyme appears as a single band on SDS/PAGE with a molecular mass of about 61 kDa. On gel filtration the enzyme is a dimer, migrating as a single peak with molecular masses of 109 and 113 kDa in the absence and presence of FAD respectively. Ion spray MS analysis yielded a mass of 63864 Da. The enzyme has optimum activity in the pH range 6.5-8.5 and exhibits absolute dependence on the three cofactors FAD, Mg2+ and thiamine diphosphate for activity. It displays negatively co-operative kinetics with respect to pyruvate concentration. A model was derived to explain the non-hyperbolic substrate-saturation curve, involving interaction between the active sites of the dimer. The Km for the first active site was found to be 8.01 +/- 0.66 mM; the Km for the second active site could not be accurately determined but was estimated to be approx. 100 mM. The enzyme is insensitive to valine, leucine and isoleucine but is strongly inhibited by the sulphonylurea herbicide, chlorsulphuron, and the imidazolinone herbicide, imazapyr. Inhibition by both herbicides exhibits slow-binding kinetics, whereas chlorsulphuron also shows tight-binding inhibition.

Silent genes in bacteria: the previously designated 'cryptic' ilvHI locus of 'Salmonella typhimurium LT2' is active in natural isolates

Gene ilvG in Escherichia coli K-12 and ilvI in 'Salmonella typhimurium LT2' (S. enterica serotype Typhimurium, strain LT2) are inactive due to frameshift or nonsense mutations, respectively. These inactive genes have been suggested to be part of 'cryptic' genetic systems which are defined as being of long-term regulatory and evolutionary significance. We have shown that the nonsense mutation in ilvI is present only in derivatives of the laboratory strain 'S. typhimurium LT2'. All natural isolates of Salmonella examined have an arginine codon at the corresponding location of their ilvI sequences. Further, two randomly selected natural isolates of serotype Typhimurium are shown to each have an active ALS III isozyme. Our findings strongly suggest that the only Salmonella strains which lack a functional ilvHI locus are LT2 isolates. We suggest that the mutations leading to inactivation of both ilvI in 'S. typhimurium LT2' and ilvG in E. coli K-12 are more likely to have been acquired during laboratory storage and/or cultivation, rather than representing cryptic systems of gene regulation.