Arogenate dehydratase

Figure 3-5. Biosynthesis of salicylic acid. The enzymes involved in this pathway are (a) chorismate mutase (E.C. 5.4.99.5), (b) prephenate aminotransferase (E.C. 2.6.1.78 and E.C. 2.6.1.79), (c) arogenate dehydratase (E.C. 4.2.1.91), (d) phenylalanine ammonia lyase (E.C. 4.3.1.5), (e) presumed P-oxidation by a yet to be identified enzyme, (f) benzoic acid 2-hydroxylase, (g) isochorismate synthase (E. C. 5.4.4.2), and (h) a putative plant pyruvate lyase.

Figure 6 Proposed biosynthetic pathways from chorismate (37), prephenate (38), and arogenate (41) to Phe (1), Tyr (2), and Trp (43) in plants and microorganisms. ADT, arogenate dehydratase AS, anthranilate synthase CM, chorismate mutase HPPAT, p-hydroxyphenylpyruvate aminotransferase PDH, prephenate dehydrogenase PPAAT, prephenate aminotransferase PPYAT, phenylpyruvate aminotransferase.

Bacterial Prephenate Dehydratases and Plant Arogenate Dehydratases ... 

Arogenic acid is converted into phenylalanine by the action of arogenate dehydratase. Although plants normally do not appear to make phenylpyruvic acid (28) and p-hydroxy-phenylpyruvic acid (29), there is some evidence that these compounds can serve as precursors for phenylalanine and tyrosine, respectively (Jensen, 1986 Widholm, 1974). 

In higher plants prephenate dehydratase, which converts prephenate to phenylpyruvate, has never been demonstrated. Since we have recently demonstrated the presence of arogenate dehydratase, which converts -arogenate to L-phenylalanine, in several higher plants (this paper), the arogenate route to phenylalanine may be characteristic of higher plants. 

Fig. 4. HPLC assay for L-phenylalanine produced as the result of the reaction catalyzed by arogenate dehydratase. The reaction mixture contained 1 ir L-arogenate, 0.5 nM L-tyrosine and enzyme purified about 3-fold by precipitation with NHi+SOi at 40% of saturation. Reaction time was 30 min at 33 C.

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.

Until recently, much less information was available concerning the biochemical mechanisms associated with the synthesis of phenylalanine in plants. No definitive reports of prephenate dehydratase (14) have appeared in the literature. Evidence for the existence of arogenate dehydratase (18) in higher plants was first reported by Jensen (1986a). Enzymes from Nieotiana silvestris, spinach, tobacco (Jung et al, 1986), and Sorghum bicolor (Siehl and Conn, 1988) have subsequently been identified and partially characterized. These are typically inhibited by phenylalanine and stimulated by tyrosine. 

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