Phenylpropanoids

Phenylpropanoids are biologically synthesized from phenylalanine as described above. Among them, cinnamic acid is synthesized directly from phenylalanine by phenylalanine ammonia-liase (PAL), and p-hydroxycinnamic acid p-coumaric acid) is synthesized from cinnamic acid by cinnamic acid 4-hydroxylase (C4H, an enzyme in the cytochrome P-450 family).The phenylpropanoid metabolic pathway is important for plants to synthesize lignin, and some phenylpropanoids are seen at junctions of cell wall polysaccharides such as hemicellulose and pectin. 

Synthetic Green Polymers from Renewable Monomers 

The condensation (homo- or co-)polymerization of p-hydroxycinnamic acid (p-coumaric add) is achieved by several research groups as described below. 

Coumarin and its derivatives, which are found in many plants, also show photodimerization by irradiation of UV light with longer wavelength (X = 300 350 nm) than a cinnamoyl group. Trenor et al wrote a detailed 

Wondraczek et to obtain cross-linkable poly(Lys) and polysaccharide sulfates, respectively. 

Cinnamic Acid, p-Coumaric acid, and Related Compounds BiosynAesis Physiological Roles 

Some Important Phenylpropanoids Biological Activity Lignin 

Composition of Lignin Origin of the Monomeric Units Polymerization of Lignin Lignans Biosynthesis Biological Activity Neolignans 

Distribution of Hydroxybenzoic Acids and Their Derivatives Biological Activity Allelopathy References 

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. 

These are CgC3 compounds, made up of a benzene ring with a three-carbon side chain. The most important are the hydroxycinnamic acids caffeic acid, p-coumaric acid, ferulic acid and sinapic acid. They can be derived from different stages of the shikimic acid pathway. These acids are of much benefit therapeutically and are non-toxic. They may also occur as glycosides. 

Modification of the side chain of these acids produces alcohols such as coniferyl alcohol, which act as precursors to the formation of lignins (high-molecular-weight polymers that give strength and structure to stems of herbs and tree trunks). By modification of their C3 side chains or changes in substitution patterns of their aromatic nucleus, hydroxycinnamic acids are able to form a host of secondary 

Caffeic acid is an inhibitor of the enzymes DOPA-decarboxylase and 5-lipoxygenase. It is an analgesic and anti-inflammatory, and promotes intestinal motility (Adzet and Camarasa 1988). It is of widespread occurrence and is found in green and roasted coffee beans. 

Cynarin (1.5 dicaffeoyl-D-quinic acid), the major active principle of globe artichoke, Cynara scolymus (Asteraceae), is formed from the bonding of two phenolic acids, caffeic and quinic acids. Cynarin is a proven hepatoprotective and hypocholesterolaemia agent. 

Curcumin is the yellow pigment from the turmeric rhizome Curcuma longa (Zingiberaceae). Curcumin and its derivatives are diarylheptanoids. They have significant anti-inflammatory, hypotensive and hepatoprotective properties (Ammon and Wahl 1990). 

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The Diabrotica spp. com rootworm beetles are specifically attracted to a variety of plant-produced phenylpropanoids, eg, ( )-cinnamaldehyde [14371-10-9] for the southern com rootworm D. undecimpunctata howardr, ( )-cinnamyl alcohol [4407-36-7] for the northern com rootworm D. barberi and indole [120-72-9] for the western com rootworm, D. virgifera virgifera. Especially powerflil lures for these rootworm beetles are 2-(4-methoxyphenyl)ethanol for the northern com rootworm and 4-methoxycinnamaldehyde [71277-11-7] (177) for the western com bootworm. 

According to a widely accepted concept, lignin [8068-00-6] may be defined as an amorphous, polyphenoHc material arising from enzymatic dehydrogenative polymerization of three phenylpropanoid monomers, namely, coniferyl alcohol [485-35-5] (2), sinapyl alcohol [537-35-7] (3), and /)-coumaryl alcohol (1). 

The earliest references to cinnamic acid, cinnamaldehyde, and cinnamyl alcohol are associated with thek isolation and identification as odor-producing constituents in a variety of botanical extracts. It is now generally accepted that the aromatic amino acid L-phenylalanine [63-91-2] a primary end product of the Shikimic Acid Pathway, is the precursor for the biosynthesis of these phenylpropanoids in higher plants (1,2). 

Tenerife and La Palma, revealed the existence of luteolin and an array of simple phenolic derivatives as well as three known phytosterols, B-amyrin, sitosterol, and stigmasterol. The phenols identified comprised a set of phenylpropanoids myristicin [566] (see Fig. 6.16 for structures 566-573), methyleugenol [567], todadiol [568], todatriol [569], crocatone [570], elemicin [571], apiole [572], and the coumarin scopoletin [573]. The occurrence of these compounds is recorded in Table 6.5. The differences between the two profiles were taken by Gonzalez and his co-workers... 

Antioxidant potential of intermediates in phenylpropanoid metabolism in higher FEBS Letters, 368, 188-92. 

Root flavonoids that may act as signals for the initiation and development of endomycorrhizal and ectomycorrhizal symbio.ses have been identified (see Chap. 7). Metabolites of the phenylpropanoid pathways apparently act as signaling molecules in endo- and ectomycorrhizal interactions (14). The role of flavonoids is still controversial, but a variety of flavanones, flavones, and isoflavones... 

Russell, W. R. Forrester, A. R. Chesson, A. Burkitt, M. J. Oxidative coupling during lignin polymerization is determined by unpaired electron delocalization within parent phenylpropanoid radicals. Arch. Biochem. Biophys. 1996, 332, 357-366. 

Humphreys, J. M. Hemm, M. R. Chappie, C. Ferulate 5-hydroxylase fromArahidopsis is a multifunctional cytochrome P450-dependent monooxygenase catalyzing parallel hydroxylations in phenylpropanoid metabolism. Proc. Natl. Acad Sci. USA 1999, 96, 10045-10050. 

Achenbach H, Hemrich H. Alkaloids, flavonoids and phenylpropanoids of the West African plant Oxymitra velutina. Phytochemistry 1991 30 1265-1267. 

Walker, K., Fujisaki, S., Long, R. and Croteau, R. (2002) Molecular cloning and heterologous expression of the C-13 phenylpropanoid side chain-CoA acyltransferase that functions in Taxol biosynthesis. Proceedings of the National Academy of Sciences of the United States of America, 99, 12715-12720. 

The use of nitrogen fertilization results in higher content of N-containing compounds, including free amino acids, and also increases in terpene content in wood plants, whilst starch, total carbohydrates, phenylpropanoids and total carbon-based phytochemicals decreased (Koricheva et al., 1998). Higher levels of nitrogen favoured its uptake and increased the nitrate content of the crop, which is critical for salad vegetables and baby foods. 

Aliotta, G., Cafiero G., Fiorentino, A. and Strumia, S. (1993). Inhibition of radish germination and root growth by coumarin and phenylpropanoids. Journal of Chemical Ecology 19 175-183. 

In addition, it has been discovered that there are naturally occurring enzymes that facilitate Diels-Alder type reactions within certain metabolic pathways and that enzymes are also instrumental in forming polyketides, isoprenoids, phenylpropanoids, and alkaloids (de Araujo et al., 2006). Agresti et al. (2005) identified ribozymes from RNA oligo libraries that catalyzed multiple-turnover Diels-Alder cycloaddition reactions. 

Hydroxy cinnamic acids are included in the phenylpropanoid group (C6-C3). They are formed with an aromatic ring and a three-carbon chain. There are four basic structures the coumaric acids, caffeic acids, ferulic acids, and sinapic acids. In nature, they are usually associated with other compounds such as chlorogenic acid, which is the link between caffeic acid and quinic acid. 

Plant metabolism can be separated into primary pathways that are found in all cells and deal with manipulating a uniform group of basic compounds, and secondary pathways that occur in specialized cells and produce a wide variety of unique compounds. The primary pathways deal with the metabolism of carbohydrates, lipids, proteins, and nucleic acids and act through the many-step reactions of glycolysis, the tricarboxylic acid cycle, the pentose phosphate shunt, and lipid, protein, and nucleic acid biosynthesis. In contrast, the secondary metabolites (e.g., terpenes, alkaloids, phenylpropanoids, lignin, flavonoids, coumarins, and related compounds) are produced by the shikimic, malonic, and mevalonic acid pathways, and the methylerythritol phosphate pathway (Fig. 3.1). This chapter concentrates on the synthesis and metabolism of phenolic compounds and on how the activities of these pathways and the compounds produced affect product quality. 

Replacement of the hydroxyl group on the phenyl ring with a carboxyl group forms a molecule of benzoic acid. Addition of a hydroxyl at the 2-position on a benzoic acid molecule forms 2-hydroxybenzoic acid or salicylic acid. The slightly more complex phenylpropanoid skeleton contains a linear three-carbon chain (the propanoic group) added to the benzene ring (the phenyl group). Addition of ammonia to carbon 2 of this three-carbon side chain yields the amino acid phenylalanine (Fig. 3.3). Phenylalanine... 

Figure 3.3. General phenylpropanoid pathway. Each arrow represents one enzymatic reaction.

Precursors of phenylpropanoids are synthesized from two basic pathways the shikimic acid pathway and the malonic pathway (see Fig. 3.1). The shikimic acid pathway produces most plant phenolics, whereas the malonic pathway, which is an important source of phenolics in fungi and bacteria, is less significant in higher plants. The shikimate pathway converts simple carbohydrate precursors into the amino acids phenylalanine and tyrosine. The synthesis of an intermediate in this pathway, shikimic acid, is blocked by the broad-spectrum herbicide glyphosate (i.e., Roundup). Because animals do not possess this synthetic pathway, they have no way to synthesize the three aromatic amino acids (i.e., phenylalanine, tyrosine, and tryptophan), which are therefore essential nutrients in animal diets. 

Many secondary phenolic compounds are derived from the amino acids phenylalanine and tyrosine and therefore contain an aromatic ring and a three-carbon side chain (see Fig. 3.3). Phenylalanine is the primary substrate for phenylpropanoid synthesis in most higher vascular plants, with tyrosine being used to a lesser extent in some plants. Because of their common structure, compounds derived from these amino acids are collectively called phenylpropanoids. 

Simple phenolic compounds include (1) the phenylpropanoids, trans-cinnamic acid, p-coumaric acid and their derivatives (2) the phenylpropanoid lactones called coumarins (Fig. 3.4) and (3) benzoic acid derivatives in which two carbons have been cleaved from the three carbon side chain (Fig. 3.2). More complex molecules are elaborated by additions to these basic carbon skeletons. For example, the addition of quinic acid to caffeic acid produces chlorogenic acid, which accumulates in cut lettuce and contributes to tissue browning (Fig. 3.5). 

The key reaction that links primary and secondary metabolism is provided by the enzyme phenylalanine ammonia lyase (PAL) which catalyzes the deamination of l-phenylalanine to form iran.v-cinnamic acid with the release of NH3 (see Fig. 3.3). Tyrosine is similarly deaminated by tyrosine ammonia lyase (TAL) to produce 4-hydroxycinnamic acid and NH3. The released NH3 is probably fixed by the glutamine synthetase reaction. These deaminations initiate the main phenylpropanoid pathway. 

Flavonoids are the largest class of phenylpropanoids in plants. The basic flavonoid structure is two aromatic rings (one from phenylalanine and the other from the condensation of three malonic acids) linked by three carbons (Fig. 3.6). Chalcone is converted to naringenin by the enzyme chalcone isomerase, which is a key enzyme in flavonoid synthesis. This enzyme, like PAL and chalcone synthase (CHS), is under precise control and is inducible by both internal and external signals. Naringenin is the... 

Heldt HW and Heldt F. 2005. Phenylpropanoids comprise a multitude of plant secondary metabolites and cell wall components. In Plant Biochemistry, 3rd ed. San Diego, CA Elsevier Academic Press, pp. 435 154. 

Ring B and the central three-carbon bridge forming the C ring (see Fig. 5.1) originate from the amino acid phenylalanine, itself a product of the shikimate pathway, a plastid-based process which generates aromatic amino acids from simple carbohydrate building blocks. Phenylalanine, and to a lesser extent tyrosine, are then fed into flavonoid biosynthesis via phenylpropanoid (C6-C3) metabolism (see Fig. 5.1). 

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