Anthranilate synthase, tryptophan synthesis

L-Tryptophan is formed through a biosynthetic pathway consisting of five enzymatically controlled steps (152). In plants, tryptophan is required for protein synthesis and also provides precursors for secondary metabolites, such as indole alkaloids. The formation of anthranilate and the following four steps in the tryptophan biosynthesis are invariant in all organisms studied to date (153) (Fig. 9). Anthranilate synthase (AS, EC 4.1.3.27) catalyzes the conversion of chorismate to anthranilate, the first step in this pathway. 

The pivotal position occupied by chorismic acid in the shikimic acid pathway has been established in several higher plants as well as microorganisms (Fig. 2) (Edwards and Jackman, 1965 Cotton and Gibson, 1968 Schmit and Zalkin, 1969 Gilchrist et al., 1972). By action of chorismate mutase [Fig. 3 (8)], chorismate is converted to prephenate which is subsequently metabolized by two independent pathways [Fig. 3 (9 and 11)] to form phenylalanine and tyrosine. Alternatively, chorismate serves as a substrate for anthranilate synthase, the first enzyme in the pathway branch leading to the synthesis of tryptophan [Fig. 4 (13)]. 

Studies with isolated enzymes in vitro reveal feedback inhibition of chorismate mutase by phenylalanine and tyrosine. Tryptophan apparently controls its own synthesis by feedback inhibition of anthranilate synthase and furthermore exerts control in the partitioning of chorismate between the two competing routes of chorismate metabolism by its ability to both activate chorismate mutase and relieve the inhibition imposed on this step by phenylalanine and tyrosine. In addition, carbon flux throuch chorismate to prephenate is also sensitive to fluctuations in chorismate concentration due to the allosteric substrate activation of chorismate mutase by chorismate. 

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.

The first step in the formation of tryptophan involves conversion of chorismate (9) to anthranilate (11) (Fig. 7.4). Although the reaction is not well understood, it is catalyzed by the enzyme anthranilate synthase and utilizes L-gluta-mine. By means of specifically labeled chorismic acid, it was determined that the protonation involved in the formation of anthranilic acid had occurred from the re face (Figure 4) (Floss, 1986). Anthranilic acid (11) also serves as an intermediate for the synthesis of a number of secondary compounds and occurs free and as various derivatives in many plants and other organisms (Dewick, 1989). 

Tryptophan pool size is regulated by feedback inhibition of anthranilate synthase by tryptophan. Other mechanisms, as yet unidentified, may regulate tryptophan and indoleacetic acid synthesis. 

Chorismate serves as the branch point metabolite for the synthesis of tryptophan or phenylalanine and tyrosine. The branch of the pathway leading to tryptophan is considered to be tightly regulated by feedback inhibition of the first committed reaction, catalyzed by anthranilate synthase (8). This is supported by the observation that cells containing tryptophan-resistant anthrani-... 

Phosphorylation of 3-hydroxyl group of shikimate by shikimate kinase (EC 2.7.1.71) with ATP as a cosubstrate initiates the biosynthesis pathway of anthranilic acid [2], This step also presents the first step of the shikimate pathway, which is a metabolic route used by bacteria, fungi, and plants for the biosynthesis of many aromatic products such as lignins, alkaloids, flavonoids, benzoic acid, and plant hormones, in addition to the aromatic amino acids (phenylalaiune, tyrosine, and tryptophan). The sequential EPSP synthesis is catalyzed by EPSP synthase (EC 2.5.1.19) through the addition of phosphoenolpyruvate to 3-phospho-shikimate followed elimination of phosphate. EPSP synthase belongs to the family of transferases, specifically to those transferring aryl... 

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