Phenylalanine-derived Metabolites Phenylpropanoids

Origin of Some Central Trivial Names. Some names were chosen according to the discovery of the corresponding compounds such as cinnamic acid [Cinnamomum verum Presl., cinnamon tree (Lauraceae)], coumaric acids/ coumarin [Dipterix odorata (Aubl.) Wu-ld. sub nom. Coumarouna odorata Aubl. (Fabaceae)], caf-feic acid [Coffea arabica L., coffee tree (Rubiaceae)], and ferulic add (Ferula spp., Apiaceae). Quinic acid is named according to its discovery as a constituent of cinchona bark [Cinchona L. spp. (Rubiaceae)] this compound was named according to the German Chinarinde (cinchona bark) - corrupted from quina (Spanish 

Eckart Eich, Solanaceae and Convolvulaceae Secondary Metabolites Springer-Verlag Berlin Heidelberg 2008 

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Figure 1. Phenylalanine ammonia-lyase (PAL) involvement in the biosynthesis of phenylpropanoid-derived secondary metabolites in plants and Ba-sidiomycetes.

Compounds derived from phenylalanine and/or tyrosine are among the most common of all secondary metabolites in plants, bacteria, and fungi. These include phenylpropanoid compounds as well as many C -Ci compounds (Fig. 8.1) (Gross, 1981). Relatively simple Ce-Cs compounds, such as cinnamic (1) and p-coumaric acids (2), are modified to produce more complex derivatives (Fig. 8.2) (Conn, 1981, 1986). The term phenylpropanoid is sometimes used to refer to any compound bearing a 3-carbon chain attached to 6-carbon aromatic ring (C6-C3 compounds). Most phenylpropanoids are formed from cinnamic or p-coumaric acids. 

Occurrence in the Solanaceae. p-Coumaric acid, caffeic acid, methyl caffeate, and methyl ferulate as well as certain of their 2,3-dihydro derivatives have been identified as constituents of the leaves of Cestrum parqui L Herit. with good phytotoxic activity against different species (D Abrosca et al. 2004). Family-specific phenylpropanoid acids like tropic acid or 2-hydroxytropic acid as acyl moieties of tropane alkaloids are synthesized via phenylalanine -> phenylpyruvic acid (l )-3-phenyllactic acid (Fig. 3.14 Table 3.1 (T5-T7-B)]. Tropic acid may occur as a metabolite of, e.g., hyoscyamine, but the free acid is not synthesized as such (for details see Sect. 3.4). 

Biosynthesis of Plant Phenoiics. Phenolic compounds in plant foods are secondary metabolites which are derived from phenylalanine, and in some plants from tyrosine via enzymatic deamination assisted by ammonia lyase (Figure 1). Phenylpropanoids, the first products of deamination of phenylalanine and/or tyrosine consist of a phenyl ring (C ) and a 3 carbon side chain (C,). These Q-C, compounds may subsequently undergo hydroxylation in the phenyl ring and possibly subsequent methylation. This would lead to the formation of a large number of products which include cinnamic acid, p-coumaric acid, caffeic acid, ferulic acid and sinapic acid (5). 

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