Arogenic acid

Fig. 4-23. Separation of arogenic acid on a latex anion exchanger. — Chromatographic conditions see Fig. 4-22.

L-Pratyrosine [L-arogenic acid, (5)-cis-a-amino-l-carboxy-4-hydroxy-2,5-cyclohexadiene-1 -propanoic acid]. 

In higher plants, phenylalanine seems to be formed in an alternative manner by formation of prephenate (26) and conversion of this intermediate into arogenic acid (32). Chorismate mutase occurs as two isozymes which have been purified to homogeneity from mung bean and sorghum. All chorismate mutase isozymes show allosteric activation by chorismate (Poulsen and Verpoorte, 1991). One chorismate mutase isozyme is inhibited by phenylalanine or tyrosine and activated by tryprophan, whereas the second is not affected by any of the aromatic amino acids. 

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). 

Furthermore, anion-exchange chromatography is suited for the separation of arogenic acid, an intermediate in the biosynthesis of phenylalanine and tyrosine. 

Arogenic acid is not stable at acidic pH. Thus, it cannot be analyzed by cation-exchange chromatography. However, on a nanobead-agglomerated anion exchanger with alkaline eluents, the separation of this compound from the amino acids phenylalanine and tyrosine is accomplished without any problem. 

Gallic acid is present in tea leaf and is a known reactant during the complex enzymatic and organochemical reactions that occur when tea components are oxidized.51 The gallic and quinic acids originate via the shikimate/arogenate pathway. The key enzymes in shikimic acid biosyn-... 

The shikimate/arogenate pathway leads to the formation of three aromatic amino acids L-phenylalanine, L-tyrosine, and L-tryptophane. This amino acids are precursors of certain homones (auxins) and of several secondary compounds, including phenolics [6,7]. One shikimate/arogenate is thought to be located in chloroplasts in which the aromatic amino acids are produced mainly for protein biosynthesis, whereas the second is probably membrane associated in the cytosol, in which L-phenylalanine is also produced for the formation of the phenylpropanoids [7]. Once L-phenylalanine has been synthesized, the pathway called phenylalanine/hydroxycinnamate begins, this being defined as "general phenylpropanoid metabolism" [7]. 

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.

The presumed primary determinant of which pathway is used for Phe (1) biosynthesis in planta is the substrate specificity of the dehydratase(s) for prephenate (38) or arogenate (41) conversion into phenylpyruvic acid (39) or Phe (1), respectively (Figure 6). Initially, vascular plants were assumed to have dehydratases similar to those in microorganisms, and these were therefore thought to employ a PDT (EC 4.2.1.51) to afford phenylpyruvate... 

L-Phenylalanine and L-tyrosine are formed from chorismic acid (D 8). Two pathways exist for the biosynthesis of L-tyrosine, the 4-hydroxyphenylpyruvate and the L-pretyrosine (arogenate) route (Fig. 266). Both pathways occur in microorganisms and plants. Higher animals are unable to synthesize L-phenyl-alanine and L-tyrosine de novo, but hydroxylate L-phenylalanine to L-tyrosine. Certain insects, however, contain colonies of bacteria in the fat body synthesizing L-phenylalanine and L-tyrosine, which may be used by their hosts. 

Figure 12.4 Simplified diagram of the general aromatic amino acid biosynthesis pathway. (Tryptophan biosynthesis proceeds from chorismate in five steps in all organisms. Phenylalanine is synthesized from arogenate or phenylpyruvate (Cyanobacteria, Saccharomyces cerevisiae, E. coli, C. glutamicum), whereas tyrosine is synthesized from arogenate or 4-hydroxy phenylpyruvate Saccharomyces cerevisiae, E. coli). In Pseudomonas aeuroginosa two alternative pathways coexist.)...

Bacteria, fungi, and plants share a common pathway for the biosynthesis of aromatic amino acids with shikimic acid as a common intermediate and therefore named after it—the shikimate pathway. Availability of shikimic acid has proven to provide growth requirements to tryptophan, tyrosine, and phenylalanine triple auxotrophic bacterial strains. Chorismate is also the last common precursor in the aromatic amino acid biosynthetic pathway, but the pathway is not named after it, as it failed to provide growth requirements to the triple auxotrophs. The aromatic biosynthetic pathway starts with two molecules of phosphoenol pyruvate and one molecule of erythrose 4-phosphate and reach the common precursor, chorismate through shikimate. From chorismate, the pathway branches to form phenylalanine and tyrosine in one and tryptophan in another. Tryptophan biosynthesis proceeds from chorismate in five steps in all organisms. Phenylalanine and tyrosine can be produced by either or both of the two biosynthetic routes. So phenylalanine can be synthesized from arogenate or phenylpyruvate whereas tyrosine can be synthesized from arogenate or 4-hydroxy phenylpyruvate. 

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