Tryptophan synthase, aromatic amino acid biosynthesis

Aromatic Amino Acid Biosynthesis. The shikimate pathway is the biosynthetic route to the aromatic amino acids tryptophan, tyrosine and phenylalanine as well as a large number of secondary metabolites such as flavonoids, anthocyanins, auxins and alkaloids. One enzyme in this pathway is 5-enolpyruvyl shikimate-3-phosphate synthase (EPSP synthase) (Figure 2.9). 

Also in C. roseus two forms are present (150, R. Bongaerts et al, unpublished results). One is strongly regulated inhibition by phenylalanine and t)u-osine and induction by tryptophan. The other is not influenced by these aromatic amino acids. The apparent molecular mass, determined by gel filtration, of the regulated form CM-1 is 44 kDa. The activity measured for CM is about 100-fold higher than for anthranilate synthase, catalyzing the first conunitted step in tryptophan biosynthesis (151). 

Biosynthesis Like other aromatic amino acids, e.g., Phe and Tyr, Trp is formed on the shikimic acid pathway. There is a branching point at chorismic acid one branch leads to Phe and Tyr, the other to Trp choris-mic acid - anthranilic acid (anthranilic acid synthase, EC 4.1.3.27)- A-(5 -0-phosphoribosyl)-anthranilic acid (anthranilic acid phosphoribosyl transferase, EC 2.4.2.18)- 1 -o-carboxyphenylamino-1 -deoxyribu-lose 5-phosphate [A-(5 -phosphoribosyl)anthranilic acid isomerase]- indole-3-glycerol phosphate (in-dole-3-glycerol phosphate synthase, EC 4.1.1.48) - indole (tryptophan synthase, EC 4.2.1,20)+serine - Trp. Many biologically active indole compounds are derived from Trp, e. g., 5-hydroxytryptophan, 5-hydroxy-tryptamine ( serotonin), and melatonin as well as many indole alkaloids. 

Fig. 9. Sequential pattern of allosteric control over biosynthesis of aromatic amino acids in the plastid compartment. In the presence of excess aromatic amino acids, L-tyrosine (TYR) inhibits arogenate dehydrogenase, L-phenylalanine (PHE) inhibits arogenate dehydratase and L-tryptophan (TRP) inhibits anthranilate synthase. The three aromatic amino acids exert allosteric inhibition (-) or activation (+) effects upon chorismate mutase-1 as symbolized. However, activation dominates over inhibition. The outcome of these events is to trap L-arogenate (AGN) between the various foci of control in the pathway. As shown symbolically, -arogenate (AGN) then acts to feedback inhibit DAHP synthase-Mn.

Fig. 1.1. Biosynthesis and regeneration of tetrahydrobiopterin including possible metabolic defects and catabolism of phenylalanine. l.l=phenylalanine-4-hydroxylase (PAH) 1.2/1.6 = GTP cyclohydrolase I (GTPCH), 1.3 = 6-pyruvoyl-tetra-hydropterin synthase (PTPS), 1.4 = dihydropteridine reductase (DHPR), 1.5 = pterin-4a-carbinolamine dehydratase (PCD), 1.7 = sepiapterin reductase SR, carbonyl reductase (CR), aldose reductase (AR), dihydrofolate reductase (DHFR), aromatic amino acid decarboxylase (AADC), tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), nitric oxide synthase (NOS). Pathological metabolites used as specific markers in the differential diagnosis are marked in squares. n.e.=non-enzymatic...

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... 

Chorismate Synthase. - Chorismate synthase catalyses the conversion of 5-enolpyruvylshikimate-3-phosphate to chorismate. It is the seventh and last enzyme of the shikimate pathway. Chorismate constitutes a major building block for the biosynthesis of an array of aromatic compounds, including the amino acids phenylalanine, tryptophan and tyrosine. Although this reaction does not involve a change in redox states, the enzyme requires reduced FMN for activity, and binds oxidized flavin only very weakly which results in its isolation as the flavin-free apo-enzyme. Macheroux and co-workers have used spectrophotometry, fluorimetry and EPR and ENDOR to investigate binding of the oxidized, reduced and radical forms of FMN to chorismate synthase in the presence of (6R)-6-fluoro-5-enolpyruvylshikimate-3-phosphate(a substrate ana-... 

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