Shikimate pathway

Shikimate pathway - The Plant Cell, Vol. 7,907-919,July...

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The Plant Cell, Vol. 7, 907-919, July 1995 O American Society of Plant Physiologists The Shikimate Pathway: Early Steps in the Biosynthesis of Aromatic Compounds Klaus M. Herrmann Department of Biochemistry, Purdue University, West Lafayette, Indiana 47907 INTRODUCTION 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 phenylala- nine. Thus, with respect to plant specificity, the shikimate pathway is a bit more widespread than nitrogen fixation or pho- tosynthesis but less ubiquitous than, for example, nitrogen assi milat ion . Bacteria spend >90% of their total metabolic energy on pro- tein 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 her- bivores (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 metabo- lite makeup of a plant could be used for species classification. Different plants not only synthesize different aromatic sec- ondary metabolites but also synthesize varying amounts of them at specific times and in specific subcellular compart- ments. One would expect that regulation of the differential biosynthesis of sometimes very complex molecular structures might involve regulation of the supply of the precursors influenc- ing the rate-limiting step for carbon flow through the shikimate pathway. Recent data on transgenic potatoes give some indi- cation that this is indeed the case (Jones et al., 1995). This review gives a short overview of the shikimate path- way and briefly introduces the seven enzymes that catalyze the sequential steps of the pathway. This is followed by a dis- cussion 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 regula- tory features of severa1 of the enzymes.
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Shikimate pathway - The Plant Cell, Vol. 7,907-919,July...

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