Arogenate pathway

Biosynthesis of Tea Flavonoids. The pathways for the de novo biosynthesis of flavonoids in both soft and woody plants (Pigs. 3 and 4) have been generally elucidated and reviewed in detail (32,51). The regulation and control of these pathways in tea and the nature of the enzymes involved in synthesis in tea have not been studied exhaustively. The key enzymes thought to be involved in the biosynthesis of tea flavonoids are 5-dehydroshikimate reductase (52), phenylalanine ammonia lyase (53), and those associated with the shikimate/arogenate pathway (52). At least 13 enzymes catalyze the formation of plant flavonoids (Table 4). 

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-... 

There are diffent pathways by which all phenolic compounds are synthesized [6,7]. The shikimate/arogenate pathway leads, through phenylalanine, to the majority of plant phenolics, and therefore we shall centre the present revision on the detailed description of this pathway. The acetate/malonate pathway leads to some plant quinones but also to various side-chain-elongated phenylpropanoids (e.g. the group of flavonoids). Finally, the acetate/mevalonate pathway leads by dehydrogenation reactions to some aromatic terpenoids. 

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]. 

For example, the arogenate pathway-to-tyrosine and the phenylpyruvate pathway-to-phenylalanine is an exceedingly common biochemical arrangement in prokaryotes. 

Figure 1. Hypothetical mechanism for shuttling of intermediates of the common aromatic pathway between plastidic and cytosolic compartments. Enzymes denoted with an asterisk (DAHP synthase-Co, chorismate mutase-2, and cytosolic anthranilate synthase) have been demonstrated to be isozymes located in the cytosol. DAHP molecules from the cytosol are shown to be shuttled into the plastid compartment in exchange for EPSP molecules synthesized within the plastid. Abbreviations C3, phosphoenolpyruvate C4, erythrose 4-P DAHP, 3-deoxy-D-arabino-heptulosonate 7-phosphate EPSP, 5-enolpyruvylshikimate 3-phosphate CHA, chorismate ANT, anthranilate TRP, L-tryptophan PPA, prephenate AGN, L-arogenate TYR, L-tyrosine and PHE, L-phenylalanine.

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... 

To fully establish whether arogenate (41), phenylpyruvate (39), or both were pathway intermediates to Phe (1) in vascular plants, it was essential to unambiguously identify all enzymatic processes (and encoding genes) needed for conversion of prephenate (38) into Phe (1) both in vivo and in vitro. Moreover, it was also essential to obtain rigorous enzymatic kinetic data using highly purified recombinant enzymes in vitro for all potential substrates, in order to compare and contrast relative efficacies/feedback inhibition properties and so forth. 

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.

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. 

Organic synthesis has made significant contributions to the study of the shikimate pathway, including the total synthesis of arogenate (Fig. 4) by Danishefsky s group. This synthesis relies on the Diels-Alder strategy used earlier in Danishefsky s synthesis of prephenic acid. ... 

DAHP synthase-Co was insensitive to allosteric effects of aromatic-pathway compounds, although caffeic acid was inhibitory. On the other hand, DAHP synthase-Mn was found to be sensitive to feedback inhibition by L-arogenate in both mung bean and in N. silvestris. In mung bean, where the most detailed studies have been done thus far, a number of other pathway intermediates produced allosteric effects. 

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