The general phenylpropanoid pathway (Figure 3-4), as the name implies, generates a substrate common to a number of phenylpropanoid compounds, including flavonoids, monolignols, hydroxycinnamic acids, sinapoyl esters, coumarins and stilbenes. The general phenylpropanoid pathway starts with phenylalanine (3.27) generated via the shikimate pathway (see Section 6). Deamination of phenylalanine is catalyzed by the enzyme phenylalanine ammonia lyase (PAL) and results in cinnamic acid (3.29). Cinnamic acid is subsequently hydroxylated by cinnamic acid 4-hydroxylase (C4H) to give p-coumaric acid (3.30). In graminaceous species, such as maize, this compound can also result from the deamination of tyrosine (3.28). In vitro assays with recombinant enzyme demonstrated that the catalytic activity towards tyrosine resides in the same enzyme, in other words, in grasses PAL has activity against both phenylalanine and tyrosine (Roesler et al., 1997). p-Coumaric acid (3.30) is converted to p-coumaroyl Coenzyme A (3.31) by the enzyme 4-coumaric acid:CoA ligase (4CL). The general phenylpropanoid pathway ends with p-coumaroyl Coenzyme A (3.31), as further reactions lead to the biosynthesis of specific classes of compounds.
Older text books and articles typically describe the general phenylpropanoid pathway as the pathway leading to the full set of hydroxycinnamic acids (p-coumaric acid (3.30), caffeic acid (3.32), ferulic acid (3.33), 5-hydroxyferulic acid (3.34) and sinapic acid (3.35), as well as their corresponding CoA-esters (3.31, 3.36-3.39). This is indicated by the grey structures in Figure 3-4. Recent advances in the cloning and characterization of genes encoding enzymes involved in phenylpropanoid metabolism have made it possible to determine substrate specificity and catalytic activity of a number of enzymes. This provided evidence
Figure 3-4. The general phenylpropanoid pathway. The enzymes involved in this pathway are: (a) phenylalanine ammonia lyase (PAL; E.C. 22.214.171.124), (b) cinnamic acid 4-hydroxylase (C4H; E.C. 126.96.36.199), and f) 4-coumaric acid:CoA ligase (4CL; E.C. 188.8.131.52). (a) depicts tyrosine ammonia lyase activity in PAL of graminaceous species. The grey structures in the box represent an older version of the phenylpropanoid pathway in which the ring substitution reactions were thought to occur at the level of the hydroxycinnamic acids and/or hydroxycinnamoyl esters. The enzymes involved in these conversions are (c) coumarate 3-hydroxylase (C3H; E.C. 184.108.40.206), (d) caffeate O-methyltransferase (COMT; EC 220.127.116.11), (e) ferulate 5-hydroxylase (F5H; EC 1.14.13), and (g) caffeoyl-CoA O-methyltransferase (CCoA-OMT; EC 18.104.22.168). These enzymes are discussed in more detail in Section 10.
against the hydroxylation and methylation reactions occurring at the level of the hydroxycinnamic acids. Rather, these reactions appear to occur at the level of the hydroxycinnamoyl esters, aldehydes, and alcohols (reviewed by Humphreys and Chapple, 2002). The hydroxycinnamic acids are thought to be generated from cinnamaldehydes, as will be discussed in Section 12. As a consequence, the current model of the general phenylpropanoid pathway is more streamlined, as shown in black in Figure 3-4. There is some evidence, however, that in certain plant species, specifically poplar (Meyermans et al., 2000), variations of the general phenylpropanoid pathway exist that are different from the general model depicted here.
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