Arogenate route, phenylalanine biosynthesis

Figure 2. Alternative enzymatic routing for L-phenylalanine biosynthesis. Dehydration followed by transamination defines the phenylpyruvate route, whereas the reverse order of reactions defines the arogenate route. Abbreviations GLU, L-glutamate aKG, 2-ketoglutarate.

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. 

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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