The shikimate pathway as an entry to aromatic secondary metabolism.

The shikimate pathway is often referred to as the common aromatic biosynthetic pathway, even though nature does not synthesize a11 aromatic compounds by this route. This metabolic sequence converts the primary metabolites PEP and erythrose-4-P to chorismate, the last common precursor for the three aromatic amino acids Phe, Tyr, and Trp and for p-amino and p-hydroxy benzoate (Fig. 1). The shikimate pathway is found in bacteria, fungi, and plants. In monogastric animals, Phe and Trp are essential amino acids that have to come with the diet and Tyr is directly derived from Phe. Since bacteria use in excess of 90% of their metabolic energy for protein biosynthesis, for most prokaryotes, the three aromatic amino acids represent nearly the entire output of aromatic biosynthesis, and regulatory mechanisms for shikimate pathway activity are triggered by the intracellular concentrations of Phe, Tyr, and Trp. This is not so in higher plants, in which the aromatic amino acids are the precursors for a large variety of secondary metabolites with aromatic ring structures that often make up a substantial amount of the total dry weight of a plant. Among the many aromatic secondary metabolites are flavonoids, many phytoalexins, indole acetate, alkaloids such as morphine, UV light protectants, and, most important, lignin.