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. THE PATHWAYAlthough the enzyme-catalyzed reactions of the shikimate pathway seem to be identical for prokaryotes and eukaryotes, plants have branches off of the main pathway that so far have not been demonstrated in bacteria or fungi.In this update I will focus on the main pathway.First, I will briefly review the basic regulatory features of the shikimate pathway in bacteria and then describe what is known about these features in higher plants.Here the differences between prokaryotes and eukaryotes are rather striking.I will close with some projections into future research in this exciting field.The main trunk of the shikimate pathway consists of reactions catalyzed by seven enzymes.