Phenylpropanoid pathways metabolism

Costa MA, Collins RE, Anterola AM, Cochrane FC, Davin LB. 2003. An in silico assessment of gene function and organization of the phenylpropanoid pathway metabolic networks in Arabidopsis thaliana and limitations thereof. Phytochemistry 64 1097-112... 

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. 

Dixon RA, Achnine L, Kota P, Liu CJ, Reddy MSS, Wang LJ (2002) The phenylpropanoid pathway and plant defence - a genomics perspective. Mol Plant Pathol 3 371-390 Dixon RA, Paiva NL (1995) Stress-induced phenylpropanoid metabolism. Plant Cell 7 1085-1097... 

Juwadi PR et ai, Genomics reveals traces of fungal phenylpropanoid-flavonoid metabolic pathway in the filamentous fungus Aspergillus oryzae, J Microbiol 43 475-486, 2005. 

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

Rasmussen, S. and Dixon, R.A., Transgene-mediated and elicitor-induced perturbation of metabolic channeling at the entry point into the phenylpropanoid pathway. Plant Cell, 11, 1537, 1999. 

Yao, K.N., de Luca, V., and Brisson, N., Creation of a metabolic sink for tryptophan alters the phenylpropanoid pathway and the susceptibility of potato to Phytophthora infestans. Plant Cell, 1, 1787, 1995. 

Matsuda, F., Morino, K., Ano, R., Kuzawa, M., Wakasa, K., Miyagawa, H. (2005). Metabolic flux analysis of the phenylpropanoid pathway in elicitor-treated potato tuber tissue. Plant Cell Physiol., 46,454 66. 

Some of the pathways of animal and bacterial metabolism of aromatic amino acids also are used in plants. However, quantitatively more important are the reactions of the phenylpropanoid pathway,173-1743 which is initiated by phenylalanine ammonia-lyase (Eq. 14-45).175 As is shown at the top of Fig. 25-8, the initial product from phenylalanine is trails-cinnam-ate. After hydroxylation to 4-hydroxycinnamate (p-coumarate) and conversion to a coenzyme A ester,1753 the resulting p-coumaryl-CoA is converted into mono-, di-, and trihydroxy derivatives including anthocyanins (Box 21-E) and other flavonoid compounds.176 The dihydroxy and trihydroxy methylated products are the starting materials for formation of lignins and for a large series of other plant products, many of which impart characteristic fragrances. Some of these are illustrated in Fig. 25-8. 

The general phenylpropanoid pathway begins with the deamination of L-phenylalanine to cinnamic acid catalyzed by phenylalanine ammonia lyase (PAL), Fig. (1), the branch-point enzyme between primary (shikimate pathway) and secondary (phenylpropanoid) metabolism [5-7]. Due to the position of PAL at the entry point of phenylpropanoid metabolism, this enzyme has the potential to play a regulatory role in phenolic-compound production. The importance of this is illustrated by the high degree of regulation both during development as well as in response to environmental stimuli. 

Studies have shown that phenylpropanoid metabolism can be stimulated by ozone. The activity of PAL increased in soybean [91], Scots pine (Pinus sylvestris L.) [92], and parsley (Petroselinum crispum L.) [93] soon after treatment with 150-200 nmol O3 mol 1. Rapid increases in transcript levels for PAL in response to ozone have been observed in parsley [93], Arabidopsis thaliana L. Heynhold [94] and tobacco (Nicoticma tabacum L.) [95]. Transcript levels for 4-coumarate CoA ligase (4CL), the last enzyme in the general phenylpropanoid pathway, increased commensurately with PAL transcripts in ozone-treated parsley seedlings [93]. Phenolic compunds reported to accumulate in leaf tissue in response to ozone include hydroxycinnamic acids, salicylic acid, stilbenes, flavonoids, furanocoumarins, acetophenones, and proanthocyanidins [85, 92, 93, 96, 97]. 

From the 1970s to the 1990s, there was a rapid and substantial progress in the research on the phenylpropanoid pathway, focusing mainly on a broad understanding of the metabolic pathway [Hahlbrock and Grisebach, 1975 Ebel and Hahlbrock, 1982 Heller and Forkmann, 1988]. However, in more recent... 

Phenylpropanoid Pathway, Subcellular Localization, and Metabolic Channeling 509... 

Ro D-K, Douglas CJ. 2004. Reconstitution of the entry point of plant phenylpropanoid metabolism in yeast (Saccharomyces cerevisiae) Implication for control of metabolic flux into the phenylpropanoid pathway. J Biol Chem 279 2600-2607. 

One of the major features of phenylpropanoid metabolism is the diversity of end products. The set of enzymic reactions leading from phenylalanine to 4-coumaroyl coenzyme A is common to pathways which lead to these diverse end products and is known as the general phenylpropanoid pathway (Fig. 1). Those biochemical reactions which lead to the synthesis of specialised products are known as branch pathways. 

The examples discussed here show that for two secondary metabolite pathways, metabolic engineering using transcription factors was successful. Modification of the phenylpropanoid/flavonoid pathway made use of the tissue-specific MYB... 

By contrast, the 3D structural analysis of a bona fide CAD has been reported only for the Arabidopsis AtCAD4 and AtCAD5, as two other reports of putative CADs from Saccharomyces cerevisiae and Populus tremuloides (aspen) are not currently considered as CADs proper. Saccharomyces cerevisiae does not produce metabolites in the phenylpropanoid pathway, thereby making it unclear as to why this specific metabolic role was being contemplated. In addition, the putative sinapyl aldehyde dehydrogenase (SAD) from aspen was not established to have the specific function claimed (see Davin etal and Anterola and Lewis " for a full discussion). 

As a result, these authors proposed in 1997 a new definition based on the metabolic origin of these substances. Concerning the plant phenols, and consequently the flavonoids, it may be considered as those compounds originating in the shikimate and phenylpropanoid pathways Notwithstanding, the flavonoids, as a differentiate subgroup inside the phenolic compounds, show a characteristic metabolic intermediate, the naringeninchalcone, from which all the bioflavonoids originate. 

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