A-Acetohydroxy acids

Fig, 17.13 The formation of a-acetohydroxy acids and their non-enzymic oxidative decarboxylation to diacetyl and pentane dione. 

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. 

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... 

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. 

Conversion of the acetohydroxy acids to the a,/3-dihydroxyacid precursors of valine and isoleucine is catalyzed by acetohydroxy acid isomeroreductase. The a,/3-dihydroxy acids are both converted to the a-keto acid precursors of valine and isoleucine by a dihydroxy acid dehydrase. Finally, the two amino acids are formed in trans-... 

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). 

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. 

Proust de Martin F, Dumas R, Field MJ. A hybrid-potential free-energy study of the isomerization step of the acetohydroxy acid isomeroreductase reaction. J Am Chem Soc 2000 122 7688-7697. 

Ahpiperidine-2,6-dicarboxylate dehydrogenase (Q) N-succinyl-2-amino-6-ketopimelate synthase succinyl diaminopimelate aminotransferase succinyl diaminopimelate desuccinylase diaminopimelate epimerase diaminopimelate decarboxylase (Q threonine dehydratase (serine dehydratase) acetolactate synthase acetohydroxy acid isomeroreductase dihydroxy acid dehydratase valine aminotransferase a-isopropylmalate synthase isopropylmalate isomerase -isopropylmalate dehydrogenase leucine aminotransferase... 

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. 

Of the pyruvic acid formed during glycolysis, a proportion is used for biosynthetic reactions (see Fig. 17.8). Pyruvate is converted to a-acetolactate by the enzyme acetohydroxy acid synthetase. The substrates for this reaction are pyruvate and hydroxyethyl thiamine pyrophosphate. In a similar reaction involving hydroxyethyl TPP and a-oxobutyrate, a-acetohydroxybutyrate is formed (Fig. 17.13). Both acetohydroxy acids are excreted by yeast and are non-enzymically converted, in the medium, to vicinal diketones. 

Because these compounds have extremely low taste thresholds and impart a strong toffee, honey or butter-like aroma to beer, considerable attention is paid to controlling their levels. The practices used encourage the breakdown of acetohydroxy acids and the subsequent reduction of diketones to inocuous diols (Chapter 19). 

There is some evidence [57] that towards the end of fermentation, a-acetolactate forms a complex with as yet unknown substances. This complex is more stable than the free acetohydroxy acid and may present problems when attempting to use short (rapid) conditioning processes. 

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]. 

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. 

By using colistine for the enrichment procedure, many auxotrophic mutants defective in the biosynthetic pathway of valine and isoleucine have been isolated. From an isoleucine-requiring mutant, defective in threonine desaminase, a prototrophic revertant has been isolated. The threonine desaminase of this revertant differs from the wild type enzyme in that its affinity for isoleucine is diminished. This revertant excretes isoleucine. Another revertant of an isoleucine-deficient mutant was obtained which formed the enzyme acetohydroxy add synthetase constitutively. During heterotrophic growth with fructose or lactate as substrates, valine, isoleucine and leucine were excreted into the culture medium. Approximately 0.6 g of amino acids were produced per liter suspension when lactate was supplied as a substrate under autotrophic conditions the excretion was negligible (Reh, 1970 Fig. 12). 

Although pyruvate and 2-oxobutyrate are substrates of acetohydroxyacid synthase, measurements of the activity of this enzyme have been almost exclusively based on the production of acetolactate from pyruvate. This reaction product is readily decarboxylated under acidic conditions and the acetoin produced can be measured spectrophotometrically. However, ace-toin can be formed during reactions which need not be related to amino acid biosynthesis. Therefore it is unclear whether the enzyme activity characterized by Saytanarayana and Radhakrishnan (1963) can be completely ascribed to acetohydroxyacid synthase. Only a portion of the acetolactate forming activity measured in pea extracts was considered to represent the activity of this enzyme (Davies, 1964). However, the enzyme(s) isolated from barley was shown to facilitate formation of acetohydroxy derivatives of 2-oxobutyrate and pyruvate (Miflin, 1971). Mg or Mn " " as well as the substrate, hydroxyethylthiamine-pyrophosphate, was required for maximum enzyme activity. The fact that the acetolactate forming activity of the barley... 

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