Poster: Secondary Metabolism
Abs #
301: p-Coumaroyl 5-O-Shikimate: an Intermediate in the General Phenylpropanoid Pathway
Although most steps in the phenylpropanoid pathway have been known for decades, the substrate for the “3-hydroxylation” step had remained unknown. New evidence suggests that p-coumaroyl 5-O-shikimate is indeed this missing intermediate.
CYP98A enzymes in Arabidopsis thaliana and Ocimum basilicum (basil) are required for the formation of 3-hydroxylated phenylpropanoid products. These enzymes catalyze at high efficiencies the 3´-hydroxylation of the p-coumaroyl moiety of p-coumaroyl 5-O-shikimate. Free p-coumaric acid and p-coumaroyl-CoA are not substrates for these enzymes. Although p-coumaroyl quinate serves as a substrate for the Arabidopsis CYP98A (at lower efficiency than p-coumaroyl 5-O-shikimate), it is a much poorer substrate for the basil enzyme.
Furthermore, the enzyme that produces p-coumaroyl 5-O-shikimate has been identified in basil, tobacco and Arabidopsis. Protein extracts from all three plants produce p-coumaroyl 5-O-shikimate from p-coumaroyl-CoA and shikimic acid. The tobacco and Arabidopsis enzymes (HCTs, hydroxycinnamoyl-CoA transferases) prefer shikimic acid and p-coumaroyl-CoA as substrates, although quinic acid and caffeoyl-CoA are also reasonable (though less efficient) substrates. In contrast, the basil acyltransferase (CST, p-coumaroyl-CoA 5-O-shikimate transferase) demonstrates high substrate specificity: only shikimic acid serves as acyl acceptor and only p-coumaroyl-CoA as acyl donor. CST appears to be the control point for production of eugenol vs. chavicol in basil. cDNAs for these HCTs and CST have been identified.
Genes for CYP98As and HCTs are also expressed in tissues of other plants where high levels of phenylpropanoid pathway-derived compounds are produced, suggesting a universal role for these enzymes and this intermediate.
