Fragrance, phenylpropanoid-derived

Figure 4.2 Structures of some phenylpropanoid-derived fragrance compounds.

Some of the pathways of animal and bacterial metabolism of aromatic amino acids also are used in plants. However, quantitatively more important are the reactions of the phenylpropanoid pathway,173-1743 which is initiated by phenylalanine ammonia-lyase (Eq. 14-45).175 As is shown at the top of Fig. 25-8, the initial product from phenylalanine is trails-cinnam-ate. After hydroxylation to 4-hydroxycinnamate (p-coumarate) and conversion to a coenzyme A ester,1753 the resulting p-coumaryl-CoA is converted into mono-, di-, and trihydroxy derivatives including anthocyanins (Box 21-E) and other flavonoid compounds.176 The dihydroxy and trihydroxy methylated products are the starting materials for formation of lignins and for a large series of other plant products, many of which impart characteristic fragrances. Some of these are illustrated in Fig. 25-8. 

The smell and taste of plants rely on aroma and fragrance compounds, many of which (besides fhe terpenoids) are derived from phenylpropanoid metabolism. In food and cosmetic industry, such fragrance and aroma compounds play an important economical role. Simple phenolic fragrance compounds are, e.g., eugenol, isoeugenol or (methyl)chavicol (Fig. 4.2), the biosynthesis of which has been clarified recently more complex compounds are phenolic esters. Evolutionary aspects of the bios)mthesis of flavours and scents have been reviewed by Gang (2005). 

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