Phenylpropanoid acetate pathway

It has been noted that the chemical diversity of plant phenolics is as vast as the plant diversity itself. Most plant phenolics are derived directly from the shikimic acid (simple benzoic acids), shikimate (phenylpropanoid) pathway, or a combination of shikimate and acetate (phenylpropanoid-acetate) pathways. Products of each of these pathways undergo additional structural elaborations that result in a vast array of plant phenolics such as simple benzoic acid and ciimamic acid derivatives, monolig-nols, lignans and lignin, phenylpropenes, coumarins, stilbenes, flavonoids, anthocyanidins, and isollavonoids. 

There are diffent pathways by which all phenolic compounds are synthesized [6,7]. The shikimate/arogenate pathway leads, through phenylalanine, to the majority of plant phenolics, and therefore we shall centre the present revision on the detailed description of this pathway. The acetate/malonate pathway leads to some plant quinones but also to various side-chain-elongated phenylpropanoids (e.g. the group of flavonoids). Finally, the acetate/mevalonate pathway leads by dehydrogenation reactions to some aromatic terpenoids. 

Initial investigations into the biosynthesis of the curcuminoids and gingerols were performed many years ago (Denniff and Whiting, 1976 Macleod and Whiting, 1979 Denniff et al., 1980). These initial radiotracer feeding studies suggested that these compounds were derived from intermediates in the phenylpropanoid pathways that were condensed with other molecules, derived in turn from the acetate and short-and medium-chain fatty acid pathways. 

Guterman, L, Masci, T., Chen, X., Negre, R, Pichersky, E., Dudareva, N., Weiss, D. and Vainstein, A. (2006) Generation of phenylpropanoid pathway-derived volatiles in transgenic plants rose alcohol acetyltransferase produces phenylethyl acetate and benzyl acetate in petunia flowers. Plant Mol. Biol, 60, 555-63. 

The biosynthetic pathway (Figure 10.5) of polyphenols, including phenolic acids is well known. Phenylalanine formed in plants via the shikimate pathway is a common precursor for most of the phenolic compounds. Forming hydroxycinnamic acids from phenylalanine requires hydroxylation and methylation steps. The formation of hydroxybenzoic acids is simple and they can directly be formed from the corresponding hydroxycinnamic acids with the loss of acetate or with an alternate path stemming from an intermediate in the phenylpropanoid pathway [69,77,78]. 

Three different metabolic pathways are known to be involved in the synthesis of different classes of phenolic compounds, namely, (1) (Ce — C3) phenylpropanoid derivatives produced by the shikimate/chorismate pathway (2) side chain elongated phenylpropanoids, flavonoids (Ce - C3 - Cg), and few quinones synthesized by the acetate/malOTiate or polyketide pathway and (3) the aromatic terpenoids synthesized throu the acetate/mevalonate pathway. 

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