Phenolics shikimate pathway

The shikimate pathway is the major route in the biosynthesis of ubiquinone, menaquinone, phyloquinone, plastoquinone, and various colored naphthoquinones. The early steps of this process are common with the steps involved in the biosynthesis of phenols, flavonoids, and aromatic amino acids. Shikimic acid is formed in several steps from precursors of carbohydrate metabolism. The key intermediate in quinone biosynthesis via the shikimate pathway is the chorismate. In the case of ubiquinones, the chorismate is converted to para-hydoxybenzoate and then, depending on the organism, the process continues with prenylation, decarboxylation, three hydroxy-lations, and three methylation steps. - ... 

The effects of glyphosate on phenolic compound production are two-fold 1) accumulation of phenolic compounds that are derivatives of aromatic amino acids is reduced and 2) pools of phenolic compounds derived from constituents of the shikimate pathway prior to 5-enolpyruvylshikimate-3-phosphate become larger. Assays that do not distinguish between effects on these two groups, such as that for hydroxyphenolics of Singleton and Rossi (18), can lead to equivocal and difficult to interpret results (e.g. 3-5). 

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. 

Phenolic compounds and others derivedfrom the Shikimic pathway (refer to Structures 15-17)... 

The enzyme complex that catalyses steps d to/of Fig. 25-20 has an unusual composition. An a3 trimer of 23.5-kDa subunits is contained within an icosahe-dral shell of 60 16-kDa (3 subunits, similar to the protein coats of the icosahedral viruses (Chapter 7). The (3 subunits catalyze the formation of dimethylribityllu-mazine (steps d, e), while the a3 trimer catalyzes the dismutation reaction of step/, the final step in riboflavin formation.365 A separate bifunctional bacterial ATP-dependent synthetase phosphorylates riboflavin and adds the adenylyl group to form FAD.366 Two separate mammalian enzymes are required.367 Synthesis of deazaflavins of methanogens (Fig. 15-22) follows pathways similar to those of riboflavin. However, the phenolic ring of the deazaflavin originates from the shikimate pathway.368... 

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. 

Many other natural products contain terpenoid elements in their molecules, in combination with carbon skeletons derived from other sources, such as the acetate and shikimate pathways. Many alkaloids, phenolics, and vitamins discussed in other chapters are examples of this. A particularly common terpenoid fragment in such cases is a... 

Flavonoids, products of the shikimate pathway from the phenylalanine and acetate pathways,11 share a diphenylpropane (C6-C3-C6) structure of three phenolic rings (Figure 2.1). Flavonoids are ubiquitous in plant foods—for example, flavonols like... 

Aromatic natural products of polyketide origin are less prevalent in plants compared with microorganisms. The majority of the plant constituents that contain aromatic stmctures are known to arise from the shikimate pathway (see below). Unlike those derived from the shikimate pathway, aromatic products of the polyketide pathway invariably contain a meta oxygenation pattern because of their origin from the cyclization of polyketides. Phenolic compounds such as chrysophanol-anthrone (Bl), and emodin-anthrone (B2), and the anthraquinones, aloe-emodin (B3) and emodin (B4) (Fig. 2), are products of the polyketide pathway and are found to occur in some plants of the genera Cassia (Leguminosae) (21), Rhamnus (Rhamnaceae) (22), and Aloe (Liliaceae) (23). The dimer of emodin-anthrone (B2), namely hypericin, (B5) is a constituent of the antidepressant herbal supplement, St. John s wort (Hypericumperforatum, Hy-pericaceae) (24). 

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. 

Phenolic compounds include a wide range of secondary metabolites that are biosynthesised from carbohydrates through the shikimate pathway [14]. This is the biosynthetic route to the aromatic amino acids, phenylalanine, tyrosine, and tryptophan, and only occurs in microorganisms and plants. In the first step, the glycolytic intermediate phosphoenol pyruvate and the pentose phosphate intermediate erythrose-4-phosphate are condensed to 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP), a step catalysed by DAHP synthase. Intermediates of the shikimate pathway are 3-dehydroquinate, shikimate, and chorismate (Fig. 1). Phenylalanine is biosynthesised from chorismate, and from phenylalanine all the phenylpropanoids. Quinate is produced from 3-dehydroquinate and incorporated into chlorogenic and isochlorogenic acids (caffeoyl quinic acids) by combination with caffeic acid. Gallic acid is produced from shikimate. 

The enzyme DAHP synthase regulates the carbon flow in the shikimate pathway. Different biotic and abiotic stresses, including mechanical wounding and fungal elicitation, induce the accumulation of DAHP synthase mRNA, and, therefore, of phenolic metabolites. 

Plant secondary metabolites are biosynthesized from rather simple building blocks supplied by primary metabolism. Two important metabolic routes in this are the shikimate pathway and the isoprenoid biosynthesis. The shikimate pathway leads to the synthesis of phenolic compounds and the aromatic amino acids phenylalanine, tyrosine and tryptophan. The isoprenoid biosjmthesis is a heavily branched pathway leading to a broad spectrum of compounds (fig. 1). From plants and microorganisms more than 37,000 isoprenoid compounds have been isolated so far [1]. 

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]. 

EPSPS is the sixth enzyme in the shikimate pathway that leads to the biosynthesis of aromatic amino acids, tryptophan, tyrosine, and phenylalanine. These aromatic amino adds along with intermediates of the pathway give rise to important secondary metabolites commonly referred to as phenylpropanoids that include phenolics, lignins, tannins, phytoalexins, etc. [1]. The shikimate pathway is localized in plastids and EPSPS is a key enzyme in regulating the flux through the pathway. 

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). 

Higher plants accumulate a wide variety of phenolic substances, including lignins and flavonoids, which are biogenetically derived from the common precursor cinnamic acid. The enzyme, which catalyzes the formation of cinnamic acid from phenylalanine, phenylalanine ammonia-lyase (PAL,sc 4.3,1,5), is responsible for channeling a considerable amount of carbon from primary metabolism (shikimate pathway) into these secondary products. Be-... 

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). 

Phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) is not considered an enzyme of the shikimate pathway proper, but since in higher plants it channels a large flux of fixed carbon into phenolic compounds, in particular lignin, the shikimate pathway must furnish its substrate, phenylalanine, in sufficient amounts. Literature on PAL has periodically been reviewed (literature in References 88 and 89) and I will concentrate here on a selective and brief discussion of our work on PAL inhibitors in the Bochum laboratory. 

Phenolic compounds or polyphenols constitute one of the most abtmdant and widely distributed groups of substances in the plant kingdom with more than 8,000 phenolic structures currently known. They are products of the secondary metabolism of plants and arise biogenetically from two main primary synthetic pathways the shikimate pathway and the acetate pathway. Both acetic acid and shikimic acid are derived from glucose metabohsm [15] (Scheme 74.1). 

Fig. 3.8. Derivation of some classes of natural products from the final metabolites of the shikimate pathway. In the cases of cinnamic acid and aldehyde derivatives X = H or X = OR. In the case of the cinnamyl alcohol derivatives, reduction occurs only if X = OR. The two phenolic units represent triketide precursors...

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