The Shikimate Pathway

The shikimate pathway results in the biosynthesis of chorismate, which can subsequently serve as a recursor for the biosynthesis of the aromatic amino acids tryptophan, phenylalanine and tyrosine. The biochemistry of

Figure 3-3. The pentose phosphate pathway. The enzymes involved in this pathway are: (a) glucose-6-phosphate dehydrogenase (E.C. 1.1.1.49), (b) gluconate-6-phospate lactonase (E.C. 3.1.1.31), (c) gluconate-6-phosphate reductase (E.C. 1.1.1.44), (d) ribose-5-phosphate isomerase (E.C. 5.3.1.6), (e) ribulose5-phosphate 3-epimerase (E.C. 5.1.3.1.), f) transketolase (E.C. 2.2.1.1), (g) transaldolase (E.C. 2.2.1.2), (h) transketolase, and (i) hexose-phosphate isomerase (E.C. 5.3.1.9).

the shikimate pathway has been extensively reviewed by Weaver and Herrmann (1997) and Hermann and Weaver (1999).

The shikimate pathway is common to both plants and microorganisms (Figure 3-3). Shikimate is synthesized from the substrates phosphoenolpyruvate (3.9) and erythrose 4-phosphate (3.17). These two precursors are derived from glycolysis and the pentose phosphate pathway, respectively, and are condensed to 3-deoxy-D-ara&ino-heptulosonate 7-phosphate (DAHP; 3.18) by the enzyme DAHP synthase. The subsequent steps result in the formation of 3-dehydro-quinate (3.19) by the enzyme 3-dehydroquinate synthase, 3-dehydroshikimate

(3.20) by the enzyme 3-dehydroquinate dehydratase, and finally shikimate

(3.21) by the enzyme shikimate dehydrogenase.

Shikimate is further converted to shikimate 3-phosphate (3.22) by shikimate kinase, and subsequently to 5-enolpyruvylshikimate 3-phosphate (EPSP; 3.23) by 5-enolpyruvylshikimate 3-phosphate synthase. EPSP is then converted to chorismate (3.24) by chorismate synthase.

Chorismate is at a branch point for the biosynthesis of aromatic amino acids: tryptophan on the one hand, and phenylalanine (3.27) and tyrosine (3.28) on the other hand. While this is strictly speaking no longer part of the shikimate pathway, the biosynthesis of phenylalanine and tyrosine is included in Figure 3-3, because they are the precursors of the important class of phenolic compounds, the phenylpropanoids, as well as several other classes of phenolic compounds. This requires the conversion of chorismate to prephenate (3.25), catalyzed by chorismate mutase, and arogenate (3.26), catalyzed by prephenate aminotransferase. The enzyme arogenate dehydratase converts arogenate to phenylalanine (3.27), whereas the enzyme arogenate dehydrogenase generates tyrosine (3.28).

Figure 3-3. The shikimate pathway. The enzymes involved in this pathway are: (a) DAHP synthase (E.C. 2.5.1.54), (b) 3-dehydroquinate synthase (E.C. 4.2.3.4), (c) 3-dehydroquinate dehydratase (E.C. 4.2.1.10), (d) shikimate dehydrogenase (E.C. 1.1.1.25), (e) shikimate kinase (E.C 2.7.1.71), (f) 5-enolpyruvylshikimate 3-phosphate synthase (E.C. 2.5.1.19), (g) chorismate synthase (E.C. 4.2.3.5), (h) chorismate mutase (E.C. 5.4.99.5), (i) prephenate aminotransferase (E.C. 2.6.1.78 and E.C. 2.6.1.79), (j) arogenate dehydratase (E.C. 4.2.1.91), and (k) arogenate dehydrogenase (E.C. 1.3.1.43, E.C. 1.3.1.78, E.C. 1.3.1.79).

OH OH

OH OH

HOI

HOItii

HON"

HOItii

HON"

HOIH

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