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Abstract
The shikimate pathway links metabolism of carbohydrates to biosynthesis of aromatic
compounds. In a sequence of seven metabolic steps, phosphoenolpyruvate and erythrose
4-phosphate are converted to chorismate, the precursor of the aromatic amino acids
and many aromatic secondary metabolites. All pathway intermediates can also be considered
branch point compounds that may serve as substrates for other metabolic pathways.
The shikimate pathway is found only in microorganisms and plants, never in animals.
All enzymes of this pathway have been obtained in pure form from prokaryotic and eukaryotic
sources and their respective DNAs have been characterized from several organisms.
The cDNAs of higher plants encode proteins with amino terminal signal sequences for
plastid import, suggesting that plastids are the exclusive locale for chorismate biosynthesis.
In microorganisms, the shikimate pathway is regulated by feedback inhibition and by
repression of the first enzyme. In higher plants, no physiological feedback inhibitor
has been identified, suggesting that pathway regulation may occur exclusively at the
genetic level. This difference between microorganisms and plants is reflected in the
unusually large variation in the primary structures of the respective first enzymes.
Several of the pathway enzymes occur in isoenzymic forms whose expression varies with
changing environmental conditions and, within the plant, from organ to organ. The
penultimate enzyme of the pathway is the sole target for the herbicide glyphosate.
Glyphosate-tolerant transgenic plants are at the core of novel weed control systems
for several crop plants.