Chorismic acid properties

Chemical properties appropriate to a compound found at a branch point of metabolism are displayed by chorismic acid. Simply warming the compound in acidic aqueous solution yields a mixture of prephen-ate and para-hydroxybenzoate (corresponding to reactions h and l of Fig. 25-1). Note that the latter reaction is a simple elimination of the enolate anion of pyruvate. As indicated in Fig. 25-1, these reactions correspond to only two of several metabolic reactions of the chorismate ion. In E. coli the formation of phe-nylpyruvate (steps h and i, Fig. 25-1) is catalyzed by a single protein molecule with two distinctly different enzymatic activities chorismate mutase and prephenate dehydratase.34-36 However, in some organisms the enzymes are separate.37 Both of the reactions catalyzed by these enzymes also occur spontaneously upon warming chorismic acid in acidic solution. The chorismate mutase reaction, which is unique in its mechanism,373 is discussed in Box 9-E. Stereochemical studies indicate that the formation of phenylpyruvate in Fig. 25-1, step z, occurs via a... 

Of the enzymes associated with aromatic amino acid biosynthesis, there is good evidence that unique isozymes of DAHP (Rubin and Jensen, 1985), chorismate synthetase (d Amato et al 1984), and anthranilate synthase (Brotherton et ai, 1986) are differentially localized within chloroplasts and in the cytoplasm. The regulatory properties of the plastid isozymes are consistent with their involvement in amino acid synthesis. Many of the remaining pathway enzymes have also been detected in plastids, including all those required for the synthesis of EPSP [(1) to (6)] (Mousdale and Coggins, 1985). These results, combined with those obtained during measurements of the biosynthetic capabilities of isolated chloroplasts (Bickel and Schultz, 1979 Buchholz and Schultz, 1980 Schulze-Siebert et ai, 1984), leave little doubt that these organelles are a primary site of aromatic amino acid biosynthesis. 

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