Of phenylpropanoids

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... 

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. 

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... 

BOREVITZ, J.O., XIA, Y, BLOUNT, J DIXON, R.A., LAMB, C., Activation tagging identifies a conserved MYB regulator of phenylpropanoid biosynthesis, Plant Cell,... 

Validation of the role of femloyl-CoA in the synthesis of the vanillin precursor will be detection of the appropriate intermediates and/or enzyme activities in placental extracts that could account for the production of the predicted levels of capsaicinoids. The presence of low levels of monolignol intermediates could be explained by lignin biosynthesis. An alternate route from phenylalanine to vanillin has been considered by some investigators Orlova et al. [68] demonstrated the role of the benzenoid pathway in petunia flowers for the biosynthesis of phenylpropanoid/benzenoid volatiles. 

Hall R, Holden M, Yeoman M (1987) The accumulation of phenylpropanoid and capsaicinoid compounds in cell cultures and whole fruit of the chili pepper, Capsicum frutescens MiU. Plant Cell Tissue Organ Cult 8 163-176... 

The functions of phenylpropanoid derivatives are as diverse as their structural variations. Phenylpropanoids serve as phytoalexins, UV protectants, insect repellents, flower pigments, and signal molecules for plant-microbe interactions. They also function as polymeric constituents of support and surface structures such as lignins and suberins [1]. Therefore, biosynthesis of phenylpropanoids has received much interest in relation to these functions. In addition, the biosynthesis of these compounds has been intensively studied because they are often chiral, and naturally occurring samples of these compounds are usually optically active. Elucidation of these enantioselective mechanisms may contribute to the development of novel biomimetic systems for enantioselective organic synthesis. 

Using this system, (Z)-hinokiresinol isolated from cultured cells of A. officinalis was determined to be the optically pure (75 )-isomer, while ( )-hinokiresinol isolated from cultured cells of C. japonica had 83.3% e.e. in favor of the (7S)-enantiomer (Table 12.1). The enzymatically formed (Z)-hinokiresinol obtained following incubation of p-coumaryl p-coumarate with a mixture of equal amounts of recZHRSa and recZHRSf) was found to be the optically pure (75)-isomer, which is identical to that isolated from A. officinalis cells (Table 12.1). A similar result was obtained with the crude plant protein from A. officinalis cultured cells, where the formed (Z)-hinokiresinol was almost optically pure, 97.2% e.e. in favor of the (75)-isomer (Table 12.1). In sharp contrast, when each subunit protein, recZHRSa or recZHRSP, was individually incubated with p-coumaryl p-coumarate, ( )-hinokiresinol was formed (Table 12.1). The enantiomeric compositions of ( )-hinokiresinol thus formed were 20.6% e.e. (with recZHRSa) and 9.0% e.e. (with recZHRSP) in favor of the (7S)-enantiomer (Table 12.1). Taken together, these results clearly indicate that the subunit composition of ZHRS controls not only cis/trans selectivity but also enantioselectivity in hinokiresinol formation (Fig. 12.3). This provides a novel example of enantiomeric control in the biosynthesis of natural products. Although the mechanism for the cis/trans selective and enantioselective reaction remains to be elucidated, for example by x-ray crystallography, the enantioselective mechanism totally differs from the enantioselectivity in biosynthesis of lignans, another class of phenylpropanoid compounds closely related to norlignans in terms of structure and biosynthesis. 

Hahlbrock K, Scheel D (1989) Physiology and molecular biology of phenylpropanoid metabolism. Annu Rev Plant Physiol Plant Mol Biol 40 347-369... 

A four-year study of field-grown commercial carrot roots revealed that recently harvested, unprocessed carrot roots contained 24 ppm falcarinol and 65 ppm falcarindiol (8). 6-Methoxymellein (6-MM) had not been identified by Yates al (8) at that time, and was not measured in that study. Reexamination of data revealed that 6-MM was absent from most samples, but present in a few at concentrations of 2 to 8 ppm. Myristicin, 1 ppm, was detected in only one sample. Wulf et 1978, reported that myristicin was present in supermarket carrots. Other studies have shown that certain brands of supermarket carrots contain myristicin while others do not (Yates, unpub.). The presence of myristicin in some samples from the supermarket and its absence in unprocessed carrots analyzed as soon after harvest as possible suggests that myristicin formation is induced during some stage of processing. Since light is known to be an elicitor of a plant system that results in the synthesis of phenylpropanoid compounds, a study of the effect of light on harvested carrot roots was undertaken. 

Surprisingly, little attention has been paid to (i) the mechanism of phenylpropanoid transport from the cytoplasm into the cell wall and (ii) subsequent chemical and biochemical modifications within the cell wall... 

As regards the second topic, namely that of phenylpropanoid reactions within plant cell walls, a more comprehensive discussion is possible and also timely, due to the recent increase in interest in this area. For the purpose of this review, the phenylpropanoids present in plant cell walls are first classified according to structural complexity (monomers, dimers, polymers, etc.), following which their main reactions are discussed. 

The second group of phenylpropanoids, which is the main emphasis of this chapter, consists of those components which are integrated into the cell wall framework. This group can be subdivided into three categories monomers, such as hydroxycinnamic acids, dimers, such as didehydrofer-ulic and 4,4 -dihydroxytruxillic acids, and polymers, such as lignins and suberins. It is important to emphasize, at this juncture, that the dimers (4,5) and polymers (8,9) discussed in this chapter are considered to be formed within the cell walls from their corresponding monomers. 

There are at least five types of phenylpropanoid related reactions which appear to occur in plant cell walls. Two are UV-mediated photochemical reactions, and hence may be restricted only to the first few layers of cells under the plant surface due to poor penetrability of the light (3). The other reactions appear to be enzymatically mediated, and result in the formation of dimers or polymers from the corresponding monomeric units. 

Vascular plant cell walls contain a wide variety of phenylpropanoids, such as monomers, dimers and polymers. Of these, the polymers (i.e., lignins and suberins) are the most abundant. According to our current knowledge, all cell-wall phenylpropanoids are derived from monomers synthesized in the cytoplasm. Following their excretion into the plant cell wall, these monomers can then be either photochemically or biochemically modified within the cell wall. 

In higher plants aromatic amino acids are required not only for protein synthesis, but as precursors for hormones, and a vast diversity of phenylpropanoid or other secondary metabolites. Thus, the availability of aromatic amino acids in a number of the spatially separate compartments of the plant-cell microenvironment is essential. 

Figure 1. Phenylalanine ammonia-lyase (PAL) involvement in the biosynthesis of phenylpropanoid-derived secondary metabolites in plants and Ba-sidiomycetes.

Legrand, M., B. Fritig, and L. Hirth. Metabolism of phenylpropanoids in the leaf veins of healthy hypersensitive tobacco or tobacco infected by tobacco mosaic virus. C R Acad Sci Ser D 1974 279 1043. 

Petersen, M., Strack, D., and Matern, U., Biosynthesis of phenylpropanoids and related compounds, in Biochemistry of Plant Secondary Metabolism, Wink, M., Ed., Sheffield Academic Press, Sheffield, 1999, 151. 

Franke, R. et al.. The Arabidopsis REFS gene encodes the 3-hydroxylase of phenylpropanoid metabolism. Plant J., 30, 33, 2002. 

The observations that flavonols are not involved in the fertilization process in certain species, and that this function can be completed using other compounds, suggest that flavonols only affect fertility indirectly. There are various examples of cross-talk between branch pathways of phenylpropanoid metabolism, or the shikimate pathway. The absence of flavonols in maize and Petunia could affect the accumulation of other compounds that are more specifically required for male fertility. Thus, differences between species in terms of flavonoid... 

Awale, S. et al., Facile and regioselective synthesis of phenylpropanoid-substituted flavan-3-ols, Org. Lett., 4, 1707, 2002. 

---