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phenylpyruvic acid

Figure 8.1 The shikimic acid pathway from erythrose-4-phosphate and phosphoenol-pyruvic acid to phenylpyruvic acid referred to as phenyl-C3 compounds. Many plant substances have this phenyl-C3 structure. The details of the shikimic acid pathway are shown in Figure 8.1.

In the first step, a molecule of phosphoenol pyruvic acid adds on to a carboxyl group on the enzyme. The resulting ester loses phosphate to give an enolpyruvate ester, which is in a reactive form to add to ery-throse-4-phosphate. The product is 3-deoxy-D-arabinoheptulosonic acid 7-phosphate, a name mercifully shortened to DAHP. The enzyme catalyzing this condensation is DAHP synthase. Again loss of phosphate and cyclization, through the nucleophilic addition of the enol group to the a-keto-group derived from pyruvic acid, gives the six-membered ring that will become a phenyl group. This first cyclized product is dehydro-quinic acid, and the enzyme reponsible for it is dehydroquinate synthase. By dehydration of a p-hydroxy-ketone in the usual way, followed by reduction of the ketone (actually through a Schiifs base formed with an NH2 group on the enzyme), shikimic acid is reached, the key compound in the series. This is phosphorylated (with adenosine triphosphate and shikimate kinase) before addition of the second molecule of phosphoe-nol pyruvic acid. Loss of another phosphate gives an enol ether combining shikimic acid phosphate and pyruvic acid. Loss of phosphate for the fourth time gives a second double bond, and another key intermediate called chorismic acid. This compound undergoes a Claisen rearrangement to prephenic acid, followed by a concerted loss of C02 and H20 to give the third double bond and the first aromatic phenyl-C3 compound, phenylpyruvic acid (Figure 8.1).

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