Shikimate/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]. 

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. ... 

L-Tyrosine biosynthesis starts with the condensation of phosphoenolpyruvate (PEP) and erythrose-4-phosphate (E4P), the intermediates of the glycolytic pathway and pentose phosphate pathway, respectively, which is catalyzed by 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS aroE/aroG/aroH). The resultant 3-deoxy-D-arabino-heptulosonate (DAHP) is converted into chorismate through the shikimate pathway with seven reactions. In plants, prephenate (PPA) is converted into L-arogenate by transamination whereas in E. coli, PPA is converted to p-hydroxyphenylpyruvate (HPP) by prephenate dehydrogenase, which is a bifunctional enzyme that behaves as chorismate mutase/prephenate... 

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