Soluble overexpression in Escherichia coli, and purification and characterization of wild-type recombinant tobacco acetolactate synthase

Heterologous Expression and Functional Characterization of Catalytic Subunit of Rice Acetohydroxyacid Synthase

BACKGROUND

Acetohydroxyacid Synthase (AHAS) is the first enzyme in the biosynthesis pathway of the branched chain amino acids. AHAS is the common target site of five herbicide chemical groups: sulfonylurea, imidazolinone, triazolopyrimidine, pyrimidinyl-thiobenzoates, and sulfonyl-aminocarbonyl-triazolinone.

OBJECTIVE

The purification of protein enabled us to study the physical and biochemical properties of the enzyme. In addition in vitro activity of this enzyme was tested in the presence of four different sulfonylureaherbicides and the feedback regulation of enzyme was analyzed in the presence of branched amino acids.

METHODS

The gene encoding catalytic subunit of rice AHAS (cOsAHAS) without part of the chloroplast transit sequence was cloned into the bacterial expression vector pET41a and heterologously expressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST).The soluble protein was purified using affinity chromatography. The measurement of GSTOsAHAS activity was performed under optimized conditions at present of branched-chain amino acids and sulfonylurea herbicides independently.

RESULTS

The optimum pH and temperature for GST-cOsAHAS activity was 8.0 and 37 °C, respectively. The specific activity and Km value of this enzyme toward pyruvate were 0.08 U/mg and 30 mM, respectively.GST-cOsAHAS was inhibited by herbicides tribenuron, sulfosulfuron, nicosulfuron and bensulfuron while the enzyme was insensitivieto end products.

CONCLUSION

These results suggest that the recombinant form of GST-cOsAHAS is functionally active and carries the binding site for sulfynylurea herbicides. Furthermore, GST-cOsAHAS was insensitive to feedback inhibition by endproducts which indicates the existence of a regulator subunit in rice AHAS as previously has been described in other plant AHASs.

Structural and functional evaluation of three well-conserved serine residues in tobacco acetohydroxyacid synthase

The first step in the common pathway for the biosynthesis of branched-chain amino acids (BCAAs) is catalyzed by acetohydroxyacid synthase (AHAS). The roles of three well-conserved serine residues (S167, S506, and S539) in tobacco AHAS were determined using site-directed mutagenesis. The mutations S167F and S506F were found to be inactive and abolished the binding affinity for cofactor FAD. The Far-UV CD spectrum of the inactive mutants was similar to that of wild-type enzyme, indicating no major conformational changes in the secondary structure. However, the active mutants, S167R, S506A, S506R, S539A, S539F and S539R, showed lower specific activities. Further, a homology model of tobacco AHAS was generated based on the crystal structure of yeast AHAS. In the model, the S167 and S506 residues were identified near the FAD binding site, while the S539 residue was found to near the ThDP binding site. The S539 mutants, S539A and S539R, showed strong resistance to three classes of herbicides, NC-311 (a sulfonylurea), Cadre (an imidazolinone), and TP (a triazolopyrimidine). In contrast, the active S167 and S506 mutants did not show any significant resistance to the herbicides, with the exception of S506R, which showed strong resistance to all herbicides. Thus, our results suggest that the S167 and S506 residues are essential for catalytic activity by playing a role in the FAD binding site. The S539 residue was found to be near the ThDP with an essential role in the catalytic activity and specific mutants of this residue (S539A and S539R) showed strong herbicide resistance as well.

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.

Cysteine protease falcipain 1 in Plasmodium falciparum is biochemically distinct from its isozymes

Falcipains form a class of papain-like cysteine proteases found in Plasmodium falciparum. This group of proteases has been suggested to be promising targets for anti-malarial chemotherapy. Despite being the first falcipain to be identified, the physiological role(s) of falcipain 1 (fp1) remains a mystery. Its suggested functions include haemoglobin degradation, erythrocytic invasion and oocyst production. In this study, the procurement of the gene coding for fp1 and its soluble expression in a heterologous host, Escherichia coli, have enabled further enzyme characterization. The recombinant fp1 protease was found to be unlike falcipain 2 (fp2A) in being more active at neutral pH than at acidic pH against the Z-LR-AMC fluorogenic substrate, suggesting a probable localization in the cytosol and not in the food vacuole. Interestingly, a common cysteine specific inhibitor, E64, did not inhibit fp1 activity, indicating dissimilar biochemical characteristics of fp1 from the other falcipains. This may be explained by computational analysis of the primary structures of the falcipain isozymes, as well as that of papain. The analysis revealed that Tyr61 (papain numbering), which is correspondingly absent in fp1, might be an important residue involved in E64 substrate binding.

Roles of three well-conserved arginine residues in mediating the catalytic activity of tobacco acetohydroxy acid synthase

Acetohydroxy acid synthase (AHAS, EC 2.2.1.6; also known as acetolactate synthase, ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine in plants and microorganisms. AHAS is the target of several classes of herbicides. In the present study, the role of three well-conserved arginine residues (R141, R372, and R376) in tobacco AHAS was determined by site-directed mutagenesis. The mutated enzymes, referred to as R141A, R141F, and R376F, were inactive and unable to bind to the cofactor, FAD. The inactive mutants had the same secondary structure as that of the wild type. The mutants R141K, R372F, and R376K exhibited much lower specific activities than the wild type, and moderate resistance to herbicides such as Londax, Cadre, and/or TP. The mutation R141K showed a strong reduction in activation efficiency by ThDP, while the mutations R372K and R376K showed a strong reductions in activation efficiency by FAD in comparison to the wild type enzyme. Taking into account the data presented here and the homology model constructed previously [Le et al. (2004) Biochem. Biophys. Res. Commun. 317, 930-938], it is suggested that the three amino acid residues studied (R141, R372, and R376) are located essentially at the enzyme active site, and, furthermore, that residues R372 and R376 are possibly responsible for the binding of the enzyme to FAD.

Two consecutive aspartic acid residues conferring herbicide resistance in tobacco acetohydroxy acid synthase

Acetohydroxy acid synthase (AHAS) catalyzes the first common step in the biosynthesis pathway of the branch chain amino acids in plants and microorganisms. A great deal of interest has been focused on AHAS since it was identified as the target of several classes of potent herbicides. In an effort to produce a mutant usable in the development of an herbicide-resistant transgenic plant, two consecutive aspartic acid residues, which are very likely positioned next to the enzyme-bound herbicide sulfonylurea as the homologous residues in AHAS from yeast, were selected for this study. Four single-point mutants and two double mutants were constructed, and designated D374A, D374E, D375A, D375E, D374A/D375A, and D374E/D375E. All mutants were active, but the D374A mutant exhibited substrate inhibition at high concentrations. The D374E mutant also evidenced a profound reduction with regard to catalytic efficiency. The mutation of D375A increased the K(m) value for pyruvate nearly 10-fold. In contrast, the D375E mutant reduced this value by more than 3-fold. The double mutants exhibited synergistic reduction in catalytic efficiencies. All mutants constructed in this study proved to be strongly resistant to the herbicide sulfonylurea Londax. The double mutants and the mutants with the D375 residue were also strongly cross-resistant to the herbicide triazolopyrimidine TP. However, only the D374A mutant proved to be strongly resistant to imidazolinone Cadre. The data presented here indicate that the two residues, D374 and D375, are located at a common binding site for the herbicides sulfonylurea and triazolopyrimidine. D375E may be a valuable mutant for the development of herbicide-resistant transgenic plants.

Effects of deletions at the C-terminus of tobacco acetohydroxyacid synthase on the enzyme activity and cofactor binding

AHAS (acetohydroxyacid synthase) catalyses the first committed step in the biosynthesis of branched-chain amino acids, such as valine, leucine and isoleucine. Owing to the unique presence of these biosynthetic pathways in plants and micro-organisms, AHAS has been widely investigated as an attractive target of several classes of herbicides. Recently, the crystal structure of the catalytic subunit of yeast AHAS has been resolved at 2.8 A (1 A=0.1 nm), showing that the active site is located at the dimer interface and is near the herbicide-binding site. In this structure, the existence of two disordered regions, a 'mobile loop' and a C-terminal 'lid', is worth notice. Although these regions contain the residues that are known to be important in substrate specificity and in herbicide resistance, they are poorly folded into any distinct secondary structure and are not within contact distance of the cofactors. In the present study, we have tried to demonstrate the role of these regions of tobacco AHAS by constructing variants with serial deletions, based on the structure of yeast AHAS. In contrast with the wild-type AHAS, the truncated mutant which removes the C-terminal lid, Delta630, and the internal deletion mutant without the mobile loop, Delta567-582, impaired the binding affinity for ThDP (thiamine diphosphate), and showed different elution profiles representing a monomeric form in gel-filtration chromatography. Our results suggest that these regions are involved in the binding/stabilization of the active dimer and ThDP binding.

Amino acid residues conferring herbicide resistance in tobacco acetohydroxy acid synthase

The enzyme AHAS (acetohydroxy acid synthase), which is involved in the biosynthesis of valine, leucine and isoleucine, is the target of several classes of herbicides. A model of tobacco AHAS was generated based on the X-ray structure of yeast AHAS. Well conserved residues at the herbicide-binding site were identified, and the roles of three of these residues (Phe-205, Val-570 and Phe-577) were determined by site-directed mutagenesis. The Phe-205 mutants F205A, F205H, F205W and F205Y showed markedly decreased levels of catalytic efficiency, and cross-resistance to two or three classes of herbicides, i.e. Londax (a sulphonylurea herbicide), Cadre (an imidazolinone herbicide) and TP (a triazolopyrimidine derivative). None of the mutations caused significant changes in the secondary or tertiary structure of the enzyme. Four mutants of Phe-577, i.e. F577D, F577E, F577K and F577R, showed unaltered V(max) values, but substantially decreased catalytic efficiency. However, these mutants were highly resistant to two or three of the tested herbicides. The three mutants F577D, F577E and F577R had a similar secondary structure to that of wild-type AHAS. Conservative mutations of Phe-577, i.e. F577W and F577Y, did not affect the kinetic properties of the enzyme or its inhibition by herbicides. The mutation Val-570 to Asn abolished the binding affinity of the enzyme for FAD as well as its activity, and also caused a change in the tertiary structure of AHAS. However, the mutant V570Q was active, but resistant to two classes of herbicides, i.e. Londax and TP. The conservative mutant V570I was substantially reduced in catalytic efficiency and moderately resistant to the three herbicides. The results of this study suggest that residues Phe-205, Val-570 and Phe-577 in tobacco AHAS are located at or near the binding site that is common for the three classes of herbicides. In addition, Phe-205 and Val-570 are probably located at the herbicide-binding site that may overlap partially with the active site. Selected mutants of Phe-577 are expected to be utilized to construct herbicide-resistant transgenic plants.

Soluble expression of a functionally active Plasmodium falciparum falcipain-2 fused to maltose-binding protein in Escherichia coli

Falcipain-2 (fp2) is a hemoglobinase required for supplying peptides and amino acids for the proliferation of Plasmodium falciparum in blood. The prospect of circumventing its activity thereby serves as a potential strategy for mining drugs for anti-malarial therapy. However, to date, efforts to express soluble and active fp2 in Escherichia coli have been futile. To overcome this problem, fp2 was expressed under an array of conditions including the exploitation of multiple gene constructs in eukaryotic and prokaryotic hosts. A series of experiments led to the finding that the placement of maltose-binding protein (MBP) before the fp2 mature domain was best in availing the soluble expression of the protease. The results also indicate that the prodomain impaired the bacterial expression of the protease and the amino acid residues at the N-terminal segment of mature fp2 can have a significant effect on the folding and solubility of the enzyme. The overexpressed MBP-fp2 fusion protein was purified and shown to be functionally active, providing a very useful alternative to the use of resolubilized enzyme for future study of structure and function of fp2.

The active site and mechanism of action of recombinant acetohydroxy acid synthase from tobacco

Acetohydroxy acid synthase (AHAS) is one of several enzymes that require thiamine diphosphate and a divalent cation as essential cofactors. Recently, the three-dimensional structure of the enzyme from yeast has been determined [Pang et al., J. Mol. Biol. 317 (2002) 249-262]. While this structure sheds light on the binding of the cofactors and the reaction mechanism, the interactions between the substrates and the enzyme remain unclear. We have studied the pH dependence of kinetic parameters in order to obtain information about the chemical mechanism in the active site. Data are consistent with a mechanism in which substrate selectively catalyzed to the enzyme with an unprotonated base having a pK of 6.48, and a protonated group having a pK of 8.25 for catalysis. The temperature dependence of kinetic parameters was pH-dependent, and the enthalpies of ionization, DeltaH(ion), calculated from the slope of pK(1) and pK(2) are both pH-independent. The solvent perturbation of kinetic parameters was pH-dependent, and the pK(1) from the acidic side and the pK(2) from the basic side were shifted down 0.4 pH units and shifted up 0.6 units as water was replaced by 15% ethanol, respectively. The data are discussed in terms of the acid-base chemical mechanism.

Roles of conserved methionine residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine. ALS is the target of several classes of herbicides, including the sulfonylureas, the imidazolinones, and the triazolopyrimidines. The conserved methionine residues of ALS from plants were identified by multiple sequence alignment using ClustalW. The alignment of 17 ALS sequences from plants revealed 149 identical residues, seven of which were methionine residues. The roles of three well-conserved methionine residues (M350, M512, and M569) in tobacco ALS were determined using site-directed mutagenesis. The mutation of M350V, M512V, and M569V inactivated the enzyme and abolished the binding affinity for cofactor FAD. Nevertheless, the secondary structure of each of the mutants determined by CD spectrum was not affected significantly by the mutation. Both M350C and M569C mutants were strongly resistant to three classes of herbicides, Londax (a sulfonylurea), Cadre (an imidazolinone), and TP (a triazolopyrimidine), while M512C mutant did not show a significant resistance to the herbicides. The mutant M350C was more sensitive to pH change, while the mutant M569C showed a profile for pH dependence activity similar to that of wild type. These results suggest that M512 residue is likely located at or near the active site, and that M350 and M569 residues are probably located at the overlapping region between the active site and a common herbicide binding site.

Roles of lysine 219 and 255 residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine. The ALS is the target of several classes of herbicides, including the sulfonylureas, the imidazolinones, and the triazolopyrimidines. The roles of three well-conserved lysine residues (K219, K255, K299) in tobacco ALS were determined using site-directed mutagenesis. The mutation of K219Q inactivated the enzyme and abolished the binding affinity for cofactor FAD. However, the secondary structure of the enzyme was not changed significantly by the mutation. Both mutants, K255F and K255Q, showed strong resistance to three classes of herbicides Londax (a sulfonylurea), Cadre (an imidazolinone), and TP (a triazolopyrimidine). In addition, there was no difference in the secondary structures of wALS and K255F. On the other hand, the mutation of K299Q did not show any significant effect on the kinetic properties or any sensitivity to the herbicides. These results suggest that Lys219 is located at the active site and is likely involved in the binding of FAD, and that Lys255 is located at a binding site common for the three herbicides in tobacco ALS.

Roles of histidine residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine in plants and microorganisms. ALS is the target of several structurally diverse classes of herbicides, including sulfonylureas, imidazolinones, and triazolopyrimidines. The roles of three well-conserved histidine residues (H351, H392, and H487) in tobacco ALS were determined using site-directed mutagenesis. Both H487F and H487L mutations abolished the enzymatic activity as well as the binding affinity for the cofactor FAD. Nevertheless, the mutation of H487F did not affect the secondary structure of the ALS. The K(m) values of H351M, H351Q, and H351F are approximately 18-, 60-, and fivefold higher than that of the wild-type ALS, respectively. Moreover, the K(c) value of H351Q for FAD is about 137-fold higher than that of wALS. Mutants H351M and H351Q showed very strong resistance to Londax (a sulfonylurea) and Cadre (an imidazolinone), whereas mutant H351F was weakly resistant to them. However, the secondary structures of mutants H351M and H351Q appeared to be different from that of wALS. The mutation of H392M did not have any significant effect on the kinetic parameters nor the resistance to ALS-inhibiting herbicides. These results suggest that the His487 residue is located at the active site of the enzyme and is likely involved in the binding of cofactor FAD in tobacco ALS. Mutational analyses of the His351 residue imply that the active site of the ALS is probably close to its binding site of the herbicides, Londax and Cadre.

Amino acid residues conferring herbicide tolerance in tobacco acetolactate synthase

Acetolactate synthase (ALS) is the common enzyme in the biosynthetic pathways leading to valine, leucine, and isoleucine in plants and microorganisms. ALS is the target site of several classes of structurally unrelated herbicides including sulfonylureas, imidazolinones, and triazolopyrimidines. To identify the residues conferring herbicide tolerance in tobacco ALS, site-directed mutagenesis for three residues, Ala121, Pro187 and Ser652, was performed. Mutant A121T showed strong resistance to Londax (a sulfonylurea) and Cadre (an imidazolinone), while mutant S652T was resistant only to Cadre. The S652N mutation abolished the binding affinity of FAD, and inactivated the enzyme. Double mutation of Ala121 and Ser652 with Thr yielded a mutant highly tolerant to Londax, Cadre, and TP (a triazolopyrimidine sulfonamide), but has enzymatic properties similar to those of wild-type. Substitution of Pro187 with Ser resulted in the enzyme highly susceptible to oxidation and fragmentation. These results suggest that two residues Ala121 and Ser652 are potent residues conferring herbicide resistance in tobacco ALS, and that double mutation of Ala121 and Ser652 by Thr can confer stronger tolerance to Londax, Cadre, and TP.

Structural and functional role of cysteinyl residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) is the common enzyme in the biosynthesis of valine, leucine, and isoleucine. The role of four cysteinyl residues in tobacco ALS was determined using site-directed mutagenesis and cysteine-specific cleavage. The C411A mutation abolished the enzymatic activity, as well as the binding affinity for the cofactor FAD. The activation constant of C411S for FAD is approximately 50-fold higher than that of wALS. The C607S mutation did not significantly affect the kinetic parameters. The IC(50) values of C411S and C607S for ALS-inhibiting herbicides are not much different from those of wALS. Two mutants, C163S and C309S, are labile and readily degraded to peptide fragments. The treatment of wALS with 2-nitro-5-thiocyanobenzoic acid, specific for cleavage of the N-terminal side of cysteine, yielded three peptides of 37.0, 22. 0, and 7.0 kDa. This fragmentation pattern is consistent with that deduced from the amino acid sequence of tobacco ALS, assuming the disulfide bond between Cys163 and Cys309. These results suggest that Cys411 is involved in the binding of FAD and that the intrachain disulfide bond between Cys163 and Cys309 plays a key role in maintaining the correct conformation of tobacco ALS.

Role of tryptophanyl residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine. ALS is the target of three classes of herbicides, the sulfonylureas, the imidazolinones, and the triazolopyrimidines. Five mutants (W266F, W439F, W490F, W503F, and W573F) of the ALS gene from Nicotiana tabacum were constructed and expressed in Escherichia coli, and the enzymes were purified. The W490F mutation abolished the binding affinity for cofactor FAD and inactivated the enzyme. The replacement of Trp573 by Phe yielded a mutant ALS resistant to the three classes of herbicides. The other three mutations, W266F, W439F, and W503F, did not significantly affect the enzymatic properties and the sensitivity to the herbicides. These results indicate that the Trp490 residue is essential for the binding of FAD and that Trp573 is located at the herbicide binding site. The data also suggest that the three classes of herbicides bind ALS competitively.

Synthesis of the quinoline-linked triazolopyrimidine analogues and their interactions with the recombinant tobacco acetolactate synthase

Acetolactate synthase (ALS) is the first common enzyme in the biosynthesis of L-leucine, L-isoleucine, and L-valine. Triazolopyrimidine sulfonamide (TP) is a mixed-type inhibitor of ALS with respect to both pyruvate and thiamine pyrophosphate. In this study, we synthesized new substituted quinoline-linked TP analogues and several TP analogues which contained either unsubstituted aminoquinolines or amino isoquinolines. In addition, we examined the interactions of both the wild-type and the sulfonylurea-resistant recombinant tobacco ALS enzymes in a highly pure and active form with the quinoline-linked TP analogues, respectively. The wild-type tobacco ALS was extremely sensitive to inhibition by the quinoline-linked TP analogues. In contrast, the mutant tobacco ALS was insensitive to both the quinoline-linked triazolopyrimidine and the sulfonylurea herbicides. The results indicate that the ability of the quinoline-linked TP analogues to inhibit ALS is highly sensitive to substitution at the ortho position (C-7) and to the position of the ring nitrogen around the sulfonamide functionality (C-8).

Mutagenesis of Escherichia coli acetohydroxyacid synthase isoenzyme II and characterization of three herbicide-insensitive forms

Sulphonylurea and imidazolinone herbicides act by inhibiting acetohydroxyacid synthase (AHAS; EC 4.1.3.18), the enzyme that catalyses the first step in the biosynthesis of branched-chain amino acids. AHAS requires as cofactors thiamin diphosphate, a bivalent metal ion and, usually, FAD. Escherichia coli contains three isoenzymes and this study concerns isoenzyme II, the most herbicide-sensitive of the E. coli forms. A plasmid containing the large and small subunit genes of AHAS II was mutagenized using hydroxylamine and clones resistant to the sulphonylurea chlorimuron ethyl were selected. Three mutants were isolated; A26V, V99M and A108V. A26V has been described previously whereas the equivalent mutation of A108V has been reported in a herbicide-insensitive variant of yeast AHAS. The V99M mutation has not been discovered previously in AHAS from any source. The mutants were each over-expressed in E. coli, and the enzymes were purified to homogeneity. Some differences from wild type in the kinetic properties (kcat, Km and cofactor affinities) were observed, most notably a 28-fold decrease in the affinity for thiamin diphosphate of V99M. None of the mutants shows marked changes from the wild type in sensitivity to three imidazolinones, with the largest increase in the apparent inhibition constant being a factor of approximately 5. The A26V mutant is weakly resistant (6- to 20-fold) to six sulphonylureas, whereas stronger resistance is seen in V99M (20- to 250-fold) and A108V (35- to 420-fold). Resistance as a result of these mutations is consistent with a molecular model of the herbicide-binding site, which predicts that mutation of G249 might also confer herbicide insensitivity. Three G249 mutants were constructed, expressed and purified but all are inactive, apparently because they cannot bind FAD.

Herbicide-resistant forms of Arabidopsis thaliana acetohydroxyacid synthase: characterization of the catalytic properties and sensitivity to inhibitors of four defined mutants

Acetohydroxyacid synthase (AHAS) catalyses the first step in the synthesis of the branched-chain amino acids and is the target of several classes of herbicides. Four mutants (A122V, W574S, W574L and S653N) of the AHAS gene from Arabidopsis thaliana were constructed, expressed in Escherichia coli, and the enzymes were purified. Each mutant form and wild-type was characterized with respect to its catalytic properties and sensitivity to nine herbicides. Each enzyme had a pH optimum near 7.5. The specific activity varied from 13% (A122V) to 131% (W574L) of the wild-type and the Km for pyruvate of the mutants was similar to the wild-type, except for W574L where it was five-fold higher. The activation by cofactors (FAD, Mg2+ and thiamine diphosphate) was examined. A122V showed reduced affinity for all three cofactors, whereas S653N bound FAD more strongly than wild-type AHAS. Six sulphonylurea herbicides inhibited A122V to a similar degree as the wild-type but S653N showed a somewhat greater reduction in sensitivity to these compounds. In contrast, the W574 mutants were insensitive to these sulphonylureas, with increases in the Kiapp (apparent inhibition constant) of several hundred fold. All four mutants were resistant to three imidazolinone herbicides with decreases in sensitivity ranging from 100-fold to more than 1000-fold.

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.