Acetohydroxy acid synthase

Herbicides also inhibit 5- (9/-pymvylshikiniate synthase, a susceptible en2yme in the pathway to the aromatic amino acids, phenylalanine, tyrosine and tryptophan, and to the phenylpropanes. Acetolactate synthase, or acetohydroxy acid synthase, a key en2yme in the synthesis of the branched-chain amino acids isoleucine and valine, is also sensitive to some herbicides. Glyphosate (26), the sulfonylureas (136), and the imida2oles (137) all inhibit specific en2ymes in amino acid synthesis pathways. 

Singh BK, Shaner DL (1992) Carbon flow through branched-chain amino acid biosynthetic pathway lessons from acetohydroxy acid synthase inhibitors. In Singh BK, Flores HE, Shannon JC (eds) Biosynthesis and molecular regulation of amino acids in plants. American Society of Plant Physiologists, Maryland, p 354... 

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

Formation of a-ketols from a-oxo acids also starts with step b of Fig. 14-3 but is followed by condensation with another carbonyl compound in step c, in reverse. An example is decarboxylation of pyruvate and condensation of the resulting active acetaldehyde with a second pyruvate molecule to give R-a-acetolactate, a reaction catalyzed by acetohydroxy acid synthase (acetolactate synthase).128 Acetolactate is the precursor to valine and leucine. A similar ketol condensation, which is catalyzed by the same synthase, is... 

Acetohydroxy acid synthase (acetolactate synthase) 1-Deoxy-D-xylulose 5-phosphate synthase Transketolase ... 

The first step in valine biosynthesis is a condensation between pyruvate and active acetaldehyde (probably hy-droxyethyl thiamine pyrophosphate) to yield a-acetolactate. The enzyme acetohydroxy acid synthase usually has a requirement for FAD, which, in contrast to most flavopro-teins, is rather loosely bound to the protein. The very same enzyme transfers the acetaldehyde group to a-ketobutyrate to yield a-aceto-a-hydroxybutyrate, an isoleucine precursor. Unlike pyruvate, the a-ketobutyrate is not a key intermediate of the central metabolic routes rather it is produced for a highly specific purpose by the action of a deaminase on L-threonine as shown in figure 21.10. 

Three other classes of compounds, although quite different from each other, are all inhibitors of acetohydroxy acid synthase (an enzyme required for branched-chain amino acid biosynthesis (see fig. 21.10). These three classes are sulfonylureas, imidazolinones, and triazolpyrimidines, which are the active ingredients in, respectively, Oust, Sceptor, and a third commercial herbicide still under development (fig. 21.11). 

Presumably the availability of valine inhibited an enzyme, such as acetohydroxy acid synthase, in the early stages of valine, isoleucine and leucine biosynthesis. 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each half-reaction is an a cleavage which leads to a thiamin- bound enamine (center. Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step h followed by fl in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an tetramer and one with an (aP)2 quaternary structure. The isolation of a-hydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase in the biosynthesis of the plant hormone indole-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25). Formation of a-ketols from a-oxo acids also starts with step h of Fig. 14-3 but is followed by condensation with another carbonyl compound in step c, in reverse. An example is decarboxylation of pyruvate and condensation of the resulting active acetaldehyde with a second pyruvate molecule to give l -a-acetolactate, a reaction catalyzed by acetohydroxy acid synthase (acetolactate synthase). Acetolactate is the precursor to valine and leucine. A similar ketol condensation, which is catalyzed by the same S5mthase, is... 

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

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

Scott, L. W. Guddat, R. G. Duggleby, Herbicide-binding sites revealed in the structure of plant acetohydroxy-acid synthase, Proc. Natl. Acad. Sci. U.S.A., 2006, 103, 569-573. 

Valine overproduction inf. coli K-12 using random mutant strains is unknown. However, systems metaboUc engineering approaches allowed titers of 7.3 g 1 by several methods [27]. Acetohydroxy acid synthase (AHAS), encoded by ilvl, was mutated to remove feedback inhibition. Competing pathways toward leucine. 

Acetolactate synthase, ALS, also referred to as acetohydroxy acid synthase, AHAS, is the first common enzyme in the biosynthetic pathway to the branched-chain amino acids valine, leucine and isoleucine (Fig. 1). 

Gollop, N., Damri, B., Barak, Z., and Chipman, D.M. (1989) Kinetics and mechanism of acetohydroxy acid synthase isozyme III from Escherichia coli. Biochemistry, 28, 6310-6317. 

Compounds 296-299 inhibit acetohydroxy acid synthase (AHAS), formerly known as acetolactate synthase. Its activity is not present in animals, but it has been found in all plants where measurements have been attempted. Acetohydroxy acid synthase catalyses the first step in production of branched amino acids (leucine, valine and isoleucine) (Scheme 73), which are obviously needed for the protein synthesis and cell growth. The compounds 296-299 seem to bind within the substrate-access channel of the enzyme, thus blocking a-ketocarboxylate access to the active site. While these herbicides are undoubtedly highly successful, resistance developed due to mutations within AHAS is becoming a serious problem [274, 275]. 

The study of inhibitors of photosynthetic carbon metabolism has, by contrast, been remarkably unsuccessful. Attempts to design herbicidal inhibitors on rational grounds on the basis of specific enzyme inhibition have not afforded herbicidal compounds that are effective on whole plants. It is possible that in many instances the chloroplast envelope proves to be an insuperable barrier. Nevertheless, recent work identifying specific enzymes such as 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, acetyl-CoA carboxylase, and acetohydroxy acid synthase has demonstrated the effectiveness of enzyme inhibitors, although none were developed on rational grounds. 

Engel S, Vyazmensky M, Berkovich D, Barak Z, Chipman DM. Substrate range of acetohydroxy acid synthase I from Escherichia coli in the stereoselective synthesis of a-hydroxy ketones. RidtecAwo/. Bioeng. 2004 88 825-831. 

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