Biosynthetic pathways phenylpropanoids

Figure 8.5 Phenylpropanoid biosynthetic pathway. IFS 2-hydroxyisoflavanone synthase F2H flavanone-2-hydroxylase F3 5 H flavnonoid-3 ,5 -hydroxylase...

Based on the chemical structures of the isolated norlignans, several hypothetical biosynthetic pathways were proposed [16-18], All these proposed pathways involved coupling of two phenylpropanoid monomer units followed by a loss of one carbon atom to give rise to norlignans. Although this mechanism seemed plausible, another mechanism that involved the addition of two carbon atoms to flavonoid compounds (C6-C3-C6) to give norlignans (Ce-Cs-Ce) could not be mled out. 

In a recent study (54), we showed increased activities of two enzymes of the general phenylpropanoid pathway, PAL and 4-coumarate CoA lig-ase, as well as one enzyme of the specific pathway of lignin biosynthesis, cinnamy 1-alcohol dehydrogenase (CAD), in resistant plants at the time of the hypersensitive host cell death. On the other hand, decreased activities were observed at the same time with susceptible host plants (54). Furthermore, we showed that the well known increase in peroxidase activities, which is strong in resistant and only weak in susceptible plants (55-58), is at least partly due to the increased activity of the lignin biosynthetic pathway (54,59). 

Fig. 1. Simplified diagram of the phenylpropanoid and flavonoid biosynthetic pathways. Enzymes that catalyze the reactions are placed on the left-hand side, and transcription factors on the right-hand side of the arrows. Both transcription factors for which their control over the enzymatic steps has been genetically proven, as well as transcription factors that have been shown to interact with promoters of the structural genes, are shown. PAL Phenylalanine ammonia lyase C4H cinnamate 4-hydroxylase 4CL 4-coumaroyl-coenzyme A ligase CHS chalcone synthase CHI chalcone-flavanone isomerase F3H flavanone 3(3-hydroxylase DFR dihydroflavonol 4-reductase AS anthocyanin synthase UFGT UDP glucose-flavonol glucosyl transferase RT anthocyanin rhamnosyl transferase...

Phenylpropanoids have an aromatic ring with a three-carbon substituent. Caffeic acid (308) and eugenol (309) are known examples of this class of compounds. Phenylpropanoids are formed via the shikimic acid biosynthetic pathway via phenylalanine or tyrosine with cinnamic acid as an important intermediate. Phenylpropanoids are a diverse group of secondary plant compounds and include the flavonoids (plant-derived dyes), lignin, coumarins, and many small phenolic molecules. They are known to act as feeding deterrents, contributing bitter or astringent properties to plants such as lemons and tea. 

Aroma-active molecules of natural origin are mainly formed via well-known biosynthetic pathways.17,27-29 The major class is the terpenoids followed by phenylpropanoid compounds (see Chapters 1.15, 1.16, and 1.24). Enzymatic and biosynthetic transformation and cleavage of fatty acid is another important source of aroma-active compounds (see Chapter 8.07). Transformation of amino acids and carbohydrates by fermentation is also... 

Fig. 11.1 Simplified diagram of the flavonoid biosynthetic pathway, starting with the general phenylpropanoid metabolism and leading to the main types of flavonoids. Only a few examples are illustrated of the large variety of flavonoids that arise through modification at different positions (not indicated or shown as R). Enzymes catalysing some key reactions are indicated by the following abbreviations PAL, phenylalanine ammonia-lyase CHS, chalcone synthase CHI, chalcone isomerase DFR, dihydroflavonol reductase F3H, flavanone 3-hydroxylase F3 5 H, flavonoid 3 5 -...

Natural products presumably biosynthesized through a [4 + 2] cycloaddition frequently occur in the literature. Several reviews on natural Diels-Alder-type cycloadducts covered more than 300 cycloadducts, including polyketides, terpenoids, phenylpropanoids, alkaloids, and natural products formed through mixed biosynthetic pathways. Representative examples of natural [4 + 2] adducts are shown in Figure 1. These include intramolecular adducts pinnatoxin (5) and nargenicin (6), a simple intermolecular adduct... 

All these functions that have the phenylpropanoids, have conduced to the genetic engineer to modify the biosynthetic pathways. In that way, it has induced illness resistance, or foraged plants digestibility increase [10]. 

Besides the phenylpropanoid pathway, which includes trani-cinnamic acid (CA) as a putative intermediate, SA can also be formed along a completely different biosynthetic pathway, via isochorismate, which is directly derived from chorismate [15,16] (Fig. 1). The latter pathway is found in microorganisms and its occurrence in plants cannot yet be excluded, though in the few plants studied more extensively so far, SA seems to be derived from the phenylpropanoid pathway. 

Fig. 1. Biosynthetic pathway and the enzymes involved in the biosynthesis of salicylic acid and 2,3-dihydroxybenzoic acid in plants, via the phenylpropanoid pathway and in microorganisms, via the chorismate/isochorismate pathway.

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]. 

The biosynthetic pathway for isoflavonoids in soybean and the relationship of the isoflavonoids to several other classes of phenylpropanoids is presented in Fig. 8.2. Production of /i-coumaryl-CoA from phenylalanine requires phenylalanine ammonia lyase to convert phenylalanine to cinnamate, cinnamic acid hydroxylase to convert cinnamate to /7-coumarate, and coumaraterCoA ligase to convert jt -coumarate to -coumaroyl-CoA. Lignins may be produced from j3-coumaroyl-CoA or from />-coumarate. Chalcone synthase catalyzes the condensation of three molecules of malonyl CoA with p-coumaroyl-CoA to form 4, 2 , 4 , 6 -tetrahydroxychalcone, which is subsequently isomerized in a reaction catalyzed by chalcone isomerase to naringenin, the precursor to genistein, flavones, flavonols, condensed tannins, anthocyanins, and others. 

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