Plants, phenolic pathway

Figure 1. Schematic outline of various products and associated enzymes from the shikimate and phenolic pathways in plants (and some microorganisms). Enzymes (1) 3-deoxy-2-oxo-D-arabino-heptulosate-7-phosphate synthase (2) 5-dehydroquinate synthase (3) shikimate dehydrogenase (4) shikimate kinase (5) 5-enol-pyruvylshikimate-3-phosphate synthase (6) chorismate synthase (7) chorismate mutase (8) prephenate dehydrogenase (9) tyrosine aminotransferase (10) prephenate dehydratase (11) phenylalanine aminotransferase (12) anthranilate synthase (13) tryptophan synthase (14) phenylalanine ammonia-lyase (15) tyrosine ammonia-lyase and (16) polyphenol oxidase. (From ACS Symposium Series No. 181, 1982) (62).

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. 

There are diffent pathways by which all phenolic compounds are synthesized [6,7]. The shikimate/arogenate pathway leads, through phenylalanine, to the majority of plant phenolics, and therefore we shall centre the present revision on the detailed description of this pathway. The acetate/malonate pathway leads to some plant quinones but also to various side-chain-elongated phenylpropanoids (e.g. the group of flavonoids). Finally, the acetate/mevalonate pathway leads by dehydrogenation reactions to some aromatic terpenoids. 

The focus of this book is centered on structure, nomenclature and occurrence of phenolic compounds (Chapter 1), and their chemical properties (Chapter 2). Chapter 3 describes the biosynthetic pathways leading to the major classes of phenolics. This chapter presents an up-to-date overview of the genetic approaches that have been used to elucidate these pathways. Chapter 4 presents an overview of methods for the isolation and identification of plant phenolic compounds. Given that much of the recent... 

In addition to the molecular techniques, technical advances both in chromatographic techniques and in identification tools, particularly the diverse forms of mass spectrometry, has allowed successful challenges to the separation and characterization of compounds of increasing complexity, poor stability, and low abundance [Whiting, 2001]. Information generated utilizing these techniques has resulted in characterization of a plethora of complex secondary metabolites that, in conjunction with the characterization of the enzymatic steps, has permitted the complete or partial elucidation of the flavonoid and the phenolic pathways present in many plants (Figs. 1.35 and 1.36). 

The shikimate pathway links the metabolism of carbohydrates to the biosynthesis of aromatic natural products via aromatic amino acids. This pathway, which is found only in plants and microorganisms, provides a major route to aromatic and phenolic natural products in plants. To date, over 8,000 phenolic natural products are known, which accounts for about 40% of organic carbon circulating in the biosphere. Although the bulk of plant phenolics are components of cell wall stmctures, many phenolic natural products are known to play functional roles that are essential for the survival of plants. 

It has been noted that the chemical diversity of plant phenolics is as vast as the plant diversity itself. Most plant phenolics are derived directly from the shikimic acid (simple benzoic acids), shikimate (phenylpropanoid) pathway, or a combination of shikimate and acetate (phenylpropanoid-acetate) pathways. Products of each of these pathways undergo additional structural elaborations that result in a vast array of plant phenolics such as simple benzoic acid and ciimamic acid derivatives, monolig-nols, lignans and lignin, phenylpropenes, coumarins, stilbenes, flavonoids, anthocyanidins, and isollavonoids. 

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. 

Benzoic and cinnamic acid derivatives and flavonoids are the two most distributed phenolics within plants. Polyphenolic units are biosynthesized via shikimate pathway, resulting in cinnamic acids C -C phenylpropanoid building block that also contributes to other plant phenolics backbones such as those from flavonoids (Q-Ca-Ce), anthocyanidins (C6-C3-C6), and coumarins (C6-C3). Stilbeneoids (C6-C2-C6) and benzoic acid derivatives (Cfi-Ci) such as gallic and ellagic acids are also synthesized through this metabolic pathway (Fig. 1). 

Plant phenols are not only widespread among plants, but also comprise a wide variety of structural diversity. Classification of plant phenols is based on structural similarities, as well as biogenetic pathways from which they originate. Brief notes ate given here about the major classes of natural phenolics and their biological importance. Under each class, some stmctures for corresponding compoimds in Table 1 are given. 

Flavonoids (C-15 compounds) are the largest group of plant phenols. They occur either in the free state or in the form of glycosides. Ring-B of flavonoids is formed from phenylpropane via the shikimate pathway, while rings A and C arise from condensation of acetate units via malonlyl CoA. Flavonoids are classified into several subclasses according to their stractural variations (Fig. 1). 

Naphthalenes are a small group of plant phenols formed from mixed shikimate-mevalonate pathways. ... 

Against this backcloth it is perhaps not surprising to learn, that, despite its distinctive position in the overall patterns of plant phenol metabolism, ambiguity still surrounds the biosynthesis of gallic acid. Several pathways have been proposed. Zenk formulated a conventional pathway (Fig. 7, a) from L-phenylalanine to 3,4,5-trihydroxy-cinnamic acid followed by 6-oxidation to give gallic acid. 

Lattanzio V, Cardinali A, Ruta C, Fortunato IM, Lattanzio VMT, Linsalata V, Cicco N (2009) Relationship of secondary metabolism to growth in oregano (Origanum vulgare L.) shoot cultures under nutritional stress. Environ Exp Bot 65 54-62 Shetty K (2004) Role of proline-linked pentose phosphate pathway in biosynthesis of plant phenolics for functional food and environmental applications a review. Proc Biochem 9 789-803... 

Plant phenolics are cmisidered to have a key role as defense compounds when environmental stresses, such as high light, low temperatures, pathogen infection, herbivores, and nutrient dehciency, can lead to an increased production of free radicals and other oxidative species in plants. Both biotic and abiotic stresses stimulate carbon fluxes from the primary to the secondary metabolic pathways. 

In plants, phenolic metabolites can stimulate cellular protective response coupled to antioxidant function in the presence of biotic and abiotic stress (Briskin 2000). Among abiotic stress, UV light induces phenolic phytochemicals through the phenylpropanoid and flavonoid glycoside pathways as a protective of metabolic response (Logemann et al. 2000). This UV inducible phenolic phytochemical response can help to counter intracellular ROS produced in response to UV. This UV-inducible phenolic response ean be coupled to antioxidant enzyme response (Rao 1996) to attenuate damage from UV radiation. 

The shikimic acid pathway was discovered by Davis in investigations with bacterial auxotrophs. However, it is found not only in microorganisms but also in higher plants. Most of the enzymes of the shikimic acid pathway have been demonstrated in a cell-free system, even in higher plants. The pathway is named after an intermediate, shikimic acid. Its importance lies not only in its furnishing phenols but especially in the provision of the aromatic amino acids, phenylalanine, tyrosine, and tryptophan. 

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