Amino acid synthesis chorismate

Figure 4. Enzymes of Rhizobium (a) and Lemna (b) proposed as sites of glyphosate inhibition of aromatic amino acid synthesis. Abbreviations CM, chorismate mutase PDH, prephenate dehydrogenase and PD, prephenate dehydratase.

Chorismate is an intermediate in the biosynthesis of the aromatic amino acids tryptophan, phenylalanine, and tyrosine. Mammals do not synthesize these amino acids bom chorismate. Instead, they obtain the essential aromatic amino acids tryptophan and phenylalanine from the diet, and they can synthesize tyrosine from phenylalanine. Glyphosate is an effective herbicide because it prevents synthesis of aromatic amino acids in plants. But the compound has no effect on mammals because they have no active pathway for de novo aromatic amino acid synthesis. 

The synthesis of tryptophan in microorganisms is affected at several levels by end-product inhibition. Thus, end-product feedback inhibition partly regulates the synthesis of chorismic acid which is the final product of the common aromatic pathway and serves as a substrate for the first reaction in the tryptophan-synthesizing branch pathway (see Fig. 2). Regulation of the common aromatic pathway was recently reviewed by Doy [72]. The first enzyme of the common aromatic pathway, 3-deoxy-D-flrah/>jo-heptulosonate 7-phosphate synthetase (DAHPS), has been reported to exist as at least three isoenzymes, each specifically susceptible to inhibition by one of the aromatic amino acid end products (tyrosine, phenylalanine, and tryptophan), in E. coli (see reference [3]). It should be noted that many reports have indicated that in E. coli the DAHPS (trp), the isoenzyme whose synthesis is repressed specifically by tryptophan, was not sensitive to end-product inhibition by tryptophan. Recently, however, tryptophan inhibition of DAHPS (trp) activity has been demonstrated in E. coli [3,73,74]. The E. coli pattern, therefore, represents an example of enzyme multiplicity inhibition based on the inhibition specificity of isoenzymes. It is interesting to note the report by Wallace and Pittard [75] that even in the presence of an excess of all three aromatic amino acids enough chorismate is synthesized to provide for the synthesis of the aromatic vitamins whose individual pathways branch from this last common aromatic intermediate. In S. typhimurium, thus far, only two DAHPS isoenzymes, DAHPS (tyr) and DAHPS (phe) have been identified as sensitive to tyrosine and phenylalanine, respectively [76]. 

Of the enzymes associated with aromatic amino acid biosynthesis, there is good evidence that unique isozymes of DAHP (Rubin and Jensen, 1985), chorismate synthetase (d Amato et al 1984), and anthranilate synthase (Brotherton et ai, 1986) are differentially localized within chloroplasts and in the cytoplasm. The regulatory properties of the plastid isozymes are consistent with their involvement in amino acid synthesis. Many of the remaining pathway enzymes have also been detected in plastids, including all those required for the synthesis of EPSP [(1) to (6)] (Mousdale and Coggins, 1985). These results, combined with those obtained during measurements of the biosynthetic capabilities of isolated chloroplasts (Bickel and Schultz, 1979 Buchholz and Schultz, 1980 Schulze-Siebert et ai, 1984), leave little doubt that these organelles are a primary site of aromatic amino acid biosynthesis. 

Inhibition of Chorismate Synthase Shikimic and quinic acids are used by microorganisms, fungi, and superior plants for the synthesis of essential aromatic amino acids from acyclic sugars. Fluorinated analogues of substrates and reaction intermediates have been synthesized in order to inhibit enzymes involved in... 

HGURE 22-16 Biosynthesis of chorismate, an intermediate in the synthesis of aromatic amino acids in bacteria and plants. 

The exbB mutants (47) are derepressed in enterobactin synthesis and produce this siderophore in iron-containing media. The means whereby iron represses enterobactin synthesis is still obscure. Several years ago it was noted that growth of E. coli on low iron media led to changes in various fRNAs (77). In E. coli K-12 aromatic amino-acid synthesizing enzymes are also derepressed in low iron media, possibly because of the diversion of the chorismate pool to enterobactin (78). 

The biosynthetic pathway from SA into L-Phe [69, 70] is shown in Fig. 8.15. The synthesis of chorismate (CHA), the common intermediate in the biosynthesis of the aromatic amino acids, requires an extra equivalent of PEP, which limits the yield of L-Phe from glucose to 0.30 mol mol-1 if PEP is not conserved [91]. The further transformation of CHA into phenylpyruvic acid (PPY) suffers from inhibition by L-Phe and is also subject to transcriptional control [69, 92]. The final step is a reductive amination of PPY into L-Phe with consumption of l-G1u. 

The quinone ring is derived from isochorismic acid, formed by isomerization of chorismic acid, an intermediate in the shikirnic acid pathway for synthesis of the aromatic amino acids. The first intermediate unique to menaquinone formation is o-succinyl benzoate, which is formed by a thiamin pyrophosphate-dependent condensation between 2-oxoglutarate and chorismic acid. The reaction catalyzed by o-succinylbenzoate synthetase is a complex one, involving initially the formation of the succinic semialdehyde-thiamin diphosphate complex by decarboxylation of 2-oxoglutarate, then addition of the succinyl moiety to isochorismate, followed by removal of the pyruvoyl side chain and the hydroxyl group of isochorismate. 

Chorismate synthase (CS) catalyzes the formation of chorismate, the last step in the shikimate pathway. Chorismate is a branch-point metabolite used for the synthesis of aromatic amino acids, p-aminobenzoic acid, folate, and other cyclic metabolites such as ubiquinone. The shikimate pathway is found only in plants, fungi, and bacteria, making the enzymes of the pathway potential targets for herbicides, antifungals, and antibiotics. 

Besides showing the unbranched pathway from erythrose-4-phosphate and phosphoenolpyruvate to shikimic acid. Figure 2L13 also shows the sequence of reactions from shikimic acid to chorismate, the first major branch point in the synthesis of the aromatic amino acids and histidine. The sixth reaction of the shikimic acid pathway is inhibited specifically by glyphosate (see here), which is the active ingredient in the broad spectrum herbicide known as Roundup. 

Figure 21.12 provides an overview of the biosynthesis of aromatic amino acids and histidine. All of the carbons in phenylalanine and tyrosine are derived from erythrose-4-phosphate and phosphoenolpyruvate. A key intermediate in synthesis of virtually all aromatic compounds (including p-aminobenzoic acid) in plant and bacterial cells is shikimic acid. Shikimic acid gives rise to chorismate... 

Chorismic acid is a metabolic intermediate that is the branch point in the synthesis of coenzyme Q and the aromatic amino acids, phenylalanine, tyrosine, and tryptophan (Figure 21.12). 

When a P. aeruginosa mutant (PALS 128) was grown under iron rich conditions, the specific activity of the SA-forming enzymes was below the limits of detection [79]. Liu et al. [88], suggest that entC gene expression may be limited at the translational level as well, even when the operon is induced under iron deficiency. This may be understandable because chorismic acid is an essential metabolite for Phe, Trp, Tyr, folate and ubiquinone synthesis. In B. subtilis it was shown that the accumulation of 2,3-DHBA(Glycine) was influenced by the levels of aromatic amino acids and anthranilic acid. Anthranilic acid inhibited the synthesis of DHBA from chorismic acid [117]. It seemed that the reduction in phenolic acid accumulation caused by aromatic amino acids is a consequence of enzyme repression [121]. The synthesis of 2,3-DHBA in B. subtilis is also reduced by other phenolic acids, such as m-substimted benzoic acids. Inhibition of accumulation of phenolic acid by other phenolic acids, would indicate a fairly specific effect on phenolic acid synthesis, but not on the accumulation of coproporphyrin that also accumulates in iron-deficient cultures oiB. subtilis [121]. 

Fig. 6. Regulation of the synthesis of aromatic amino acids in plants. Each of the sequential arrows represents the enzyme catalyzed steps detected in Figs. 2, 3 and 4. Bold lines indicate inhibition of chorismate mutase by phenylalanine and tyrosine in addition to anthranilate synthase inhibition by tryptophan. The dashed arrow symbolizes the ability of tryptophan to both activate chorismate mutase and antagonize inhibition of this enzyme by either phenylalanine or tyrosine.

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