The Shikimate Pathway: Early Steps in the Biosynthesis of Aromatic Compounds.

The shikimate pathway was discovered as the biosynthetic route to the aromatic amino acids phenylalanine, tyrosine, and tryptophan through the classic studies of Bernhard Davis and David Sprinson and their collaborators. This pathway has been found only in microorganisms and plants. Phenylalanine and tryptophan are essential components of animal diets, and animals synthesize tyrosine in a single step from phenylalanine. Thus, with respect to plant specificity, the shikimate pathway is a bit more widespread than nitrogen fixation or photosynthesis but less ubiquitous than, for example, nitrogen assi milat ion . Bacteria spend >90% of their total metabolic energy on protein biosynthesis. Consequently, the bacterial shikimate pathway serves almost exclusively to synthesize the aromatic amino acids (Herrmann, 1983; Pittard, 1987). In contrast, higher plants use these amino acids not only as protein building blocks but also, and in even greater quantities, as precursors for a large number of secondary metabolites, among them plant pigments, compounds to defend against insects and other herbivores (see Dixon and Paiva, 1995, this issue), UV light protectants, and, most importantly, lignin (Bentley, 1990; Singh et al., 1991; see Whetten and Sederoff, 1995, this issue). Under normal growth conditions, 20% of the carbon fixed by plants flows through the shikimate pathway (Haslam, 1993). Globally, this amounts to ~7 x 1015 kg each year, most of it used for the synthesis of the various secondary metabolites. And the variation in shikimate pathway-derived secondary metabolites is very extensive among plant species. The secondary metabolite makeup of a plant could be used for species classification. Different plants not only synthesize different aromatic secondary metabolites but also synthesize varying amounts of them at specific times and in specific subcellular compartments. One would expect that regulation of the differential biosynthesis of sometimes very complex molecular structures might involve regulation of the supply of the precursors influencing the rate-limiting step for carbon flow through the shikimate pathway. Recent data on transgenic potatoes give some indication that this is indeed the case (Jones et al., 1995). This review gives a short overview of the shikimate pathway and briefly introduces the seven enzymes that catalyze the sequential steps of the pathway. This is followed by a discussion of some enzymes of quinate metabolism, which use shikimate pathway intermediates as substrates, thus forming branches off the main trunk. I end by discussing some regulatory features of severa1 of the enzymes.