Tyrosine and Phenylalanine Biosynthesis: Relationship between Alternative Pathways, Regulation and Subcellular Location

Abstract

The metabolic pathway responsible for biosynthesis of aromatic amino acids and for vitamin-like derivatives such as folic acid and ubiquinones is a major enzyme network in nature.¹ In higher plants this pathway plays an even larger role since it is the source of precursors for numerous phenylpropanoid compounds, lignins, auxins, tannins, cyano-genic glycosides and an enormous variety of other secondary metabolites.² Such secondary metabolites may originate from the amino acid end products or from intermediates in the pathway (Fig. 1). The aromatic pathway interfaces with carbohydrate metabolism at the reaction catalyzed by 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, the condensation of erythrose-4-phosphate and PEP to form the 7-carbon sugar, DAHP. Evidence is accumulating to show that separate biochemical pathways of aromatic biosynthesis may exist in the spatially separated microenvironments of the plastid and cytosolic compartments.³ Separate enzyme systems of carbohydrate metabolism exist in the cytosol and in plastids⁴ that would be able to generate erythrose-4-phosphate and PEP for entry into aromatic biosynthesis. Secondary metabolites appear to be synthesized by enzymes located primarily in the cytosol.