The shikimic acid pathway

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. 

The shikimic acid pathway begins with phosphoenolpyruvate which is obtained from glycolysis, and D-erythrose-4-phosphate, which comes from the pentose phosphate cycle. The two are linked to form an intermediate with 7 C atoms which cyclizes to 5-dehydroquinic acid. The latter exists in equilibrium with quinic acid. The pathway proceeds via 5-dehydroshikimic acid and shikimic acid to 5-phosphoshikimic acid. An additional phosphoenolpyruvate unit is now attached to the last-mentioned compound. The product of this reaction is converted, in several steps, to chorismic acid. 

With chorismic acid an important junction in the shikimic acid pathway has been reached. For as the Greek name implies (chorizo = split), the synthetic route divides into two branches after this substance. One branch leads via anthranilic acid to tryptophan, and from it to the 

The second branch leads from chorismic acid first to prephenic acid. After this substance the pathway forks again via phenylpyruvate to phenylalanine and via p-hydroxyphenylpyruvate to tyrosine. These two aromatic amino acids are closely related to each other since phenylalanine can be oxidized to tyrosine. However, this last reaction does not seem to be very important in higher plants. On deamination, phenylalanine yields cinnamic acid and tyrosine p-coumaric acid, a derivative of cinnamic acid. 

Summarizing, it can be said that the shikimic acid pathway furnishes 

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E. E. Conn, The Shikimic Acid Pathway, Plenum Press, New York, 1986. 

The earliest references to cinnamic acid, cinnamaldehyde, and cinnamyl alcohol are associated with thek isolation and identification as odor-producing constituents in a variety of botanical extracts. It is now generally accepted that the aromatic amino acid L-phenylalanine [63-91-2] a primary end product of the Shikimic Acid Pathway, is the precursor for the biosynthesis of these phenylpropanoids in higher plants (1,2). 

Phenazines — The phenazines are biosynthesized by the shikimic acid pathway, through the intermediate chorismic acid. The process was studied using different strains of Pseudomonas species, the major producers of phenazines. The best-known phenazine, pyocyanine, seems to be produced from the intermediate phenazine-1-carboxylic acid (PCA). Although intensive biochemical studies were done, not all the details and the intermediates of conversion of chorismic acid to PCA are known. In the first step, PCA is N-methylated by a SAM-dependent methyltransferase. The second step is a hydroxylative decarboxylation catalyzed by a flavoprotein monooxygenase dependent on NADH. PCA is also the precursor of phenazine-1-carboxamide and 1-hydroxyphenazine from Pseudomonas species. - - ... 

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. 

Allelopathic compounds consist of a wide variety of chemical types which arise through either the acetate or the shikimic acid pathway (5 ). These compounds range from very simple gases and aliphatic compounds to complex multi-ringed aromatic compounds. Oniy a few examples are mentioned below. 

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

Alkaloid biosynthesis needs the substrate. Substrates are derivatives of the secondary metabolism building blocks the acetyl coenzyme A (acetyl-CoA), shikimic acid, mevalonic acid and 1-deoxyxylulose 5-phosphate (Figure 21). The synthesis of alkaloids starts from the acetate, shikimate, mevalonate and deoxyxylulose pathways. The acetyl coenzyme A pathway (acetate pathway) is the source of some alkaloids and their precursors (e.g., piperidine alkaloids or anthraniUc acid as aromatized CoA ester (antraniloyl-CoA)). Shikimic acid is a product of the glycolytic and pentose phosphate pathways, a construction facilitated by parts of phosphoenolpyruvate and erythrose 4-phosphate (Figure 21). The shikimic acid pathway is the source of such alkaloids as quinazoline, quinoline and acridine. 

The plant polyphenols—the phenylpropanoids made via the shikimic acid pathway... 

A large number of volatile phenols and related compounds occur in vegetables and fruits, and some of them are potent aroma compounds. The majority of volatile phenols and related compounds in plants are formed mainly through the shikimic acid pathway, and are present in intact plant tissue either as free... 

Claisen rearrangement plays an important part in the biosynthesis of several natural products. For example, the chorismate ion is rearranged to the prephenate ion by the Claisen rearrangement, which is catalysed by the enzyme chorismate mutase. This prephenate ion is a key intermediate in the shikimic acid pathway for the biosynthesis of phenylalanine, tyrosine and many other biologically important natural products. 

Coumarins and isocoumarins appear to be of varied origins. Simple coumarins, such as umbelliferone, are formed by the shikimic acid pathway in which hydroxylation of p-hydroxycinnamic acid occurs. Other coumarins, for example alternariol (690), are derived from a polyketide unit, as are a number of chromanones, chromones, pyranones and isocoumarins (B-78MI22400). The biosynthesis of 5-hydroxy-2-methylchromone has been shown to involve the chromanone (60JCS654). However, isocoumarins are also derived from the mixed acetate-shikimate route, through initial cyclization of the polyketide and subsequent lactonization. 

Isoprenoid structures for carotenoids, phytol, and other terpenes start biosynthetically from acetyl coenzyme A (89) with successive additions giving mevalonate, isopentyl pyrophosphate, geranyl pyrophosphate, farnesyl pyrophosphate (from which squalene and steroids arise), with further build-up to geranyl geranyl pyrophosphate, ultimately to a- and /3-carotenes, lutein, and violaxanthin and related compounds. Aromatic hydrocarbon nuclei are biosynthesized in many instances by the shikimic acid pathway (90). More complex polycyclic aromatic compounds are synthesized by other pathways in which naphthalene dimerization is an important step (91). 

The 3-deoxy D-arabino-heptulosonic acid 7-phosphate (DAHP, 34), intermediate of the shikimic acid pathway (cf. Sect. 2.2.4), has also been prepared... 

Humphreys JL, Lowes DJ, Wesson KA, Whitehead RC (2006) Arene cw-Dihydrodiols -Useful Precursors for the Preparation of Antimetabolites of the Shikimic Acid Pathway Application to the Synthesis of 6,6-Difluoroshikimic Acid and (6S)-6-Fluoroshikimic Acid. Tetrahedron 62 5099... 

In contrast to the rutelines, the melolonthine scarabs generally use terpenoid-and amino acid-derived pheromones (reviewed in Leal, 1999). For example, the female large black chafer, Holotrichia parallela Motschulsky, produces methyl (2.S, 3. Sj - 2 - am ino-3-methy lpcn tanoatc (L-isoleucine methyl ester) as an amino acid-derived sex pheromone (Leal et al., 1992 Leal, 1997). There is no direct evidence that the chafer beetles or any other Coleoptera use the shikimic acid pathway for de novo pheromone biosynthesis, but some scarabs and scolytids (see section 6.6.4.2) may convert amino acids such as tyrosine, phenylalanine, or tryptophan to aromatic pheromone components (Leal, 1997,1999). In another melolonthine species, the female grass grab beetle, Costelytra zealandica (White), the phenol sex pheromone is produced by symbiotic bacteria (Henzell and Lowe, 1970 Hoyt et al. 1971). 

The evidence then is that, for rifamycin and other ansamycins, biosynthesis diverts at a so-far unidentified (but early) compound in the shikimic acid pathway to give 3-amino-5-hydroxybenzoic acid (91) (as its CoA ester). This compound then yields, on the one hand, the mitomycins [e.g. porfiromycin (88)1 and, on the other, the CoA ester of P8/1-OG (92), which then affords diverse metabolites such as rifamycin B (87) and actamycin (86) (cf. ref. 83 for a detailed scheme). 

This key intermediate has given its name to Nature s general route to aromatic compounds and many other related six-membeied ring compounds the shikimic acid pathway. This pathway contains some of the most interesting reactions (from a chemist s point of view) in biology. It starts with an aldol reaction between phosphoenol pyruvate as the nucleophilic enol component and the C4 sugar erythrose 4-phosphate as the electrophilic aldehyde. 

Even the structural material of plants, lignin, comes from the shikimic acid pathway. Lignin—from which wood is made—has a variable structure according to the plant and the position in the plant. A typical splinter is shown here. 

We have arrived at prephenic acid, which as its name suggests is the last compound before aromatic compounds are formed, and we may call this the end of the shikimic acid pathway. The final stages of the formation of phenylalanine and tyrosine start with aromatization. Prephenic acid is unstable and loses water and CO2 to form phenylpyruvic acid. This a-keto-acid can be converted into the amino acid by the usual transamination with pyridoxal. 

An important shikimate metabolite is podophyllotoxin, an antitumour compound—some podophyllotoxin derivatives are used to combat lung cancer. The compound can be split up notion-ally into two shikimate-derived fragments (shown in red and green). Both are quite different and therejjs obviously a lot of chemistry to do after-the shikimic acid pathway is finished. 

Many amino acids can lose ammonia to give an unsaturated acid. The enzymes that catalyse these reactions are known as amino acid ammonia lyases. The one that concerns us at the end of the shikimic acid pathway is phenylalanine ammonia lyase, which catalyses the elimination of ammonia from phenylalanine to give the common metabolite cinnamic acid. 

This chemical reaction might be said to be similar to a reaction in the shikimic acid pathway. Compare the two mechanisms and suggest how the model might be made closer and more interesting. 

The fatty acid pathway or, as we should call it now, the acyl polymalonate pathway, also gives rise to an inexhaustible variety of aromatic and other compounds belonging to the family of the polyketides. You saw in Chapter 50 how the shikimic acid pathway makes aromatic compounds but the compounds below are from the polyketide route. 

It is now well established that the primary metabolic target of glyphosate is an enzyme of the shikimic acid metabolic pathway, enolpyruvyl shikimate-3-phosphate synthase (2.f ). Via this action, glyphosate blocks the synthesis of the end products of this pathway, notably phenylalanine and tryptophan, but also various subsequent products (Figure 1) ( ,i). It has seemed logical to conclude that the herbicidal effect of glyphosate is a direct result of its effect on the shikimic acid pathway. 

Sublethal doses of glyphosate can have some effects on plants in the long term that are difficult to attribute directly to inhibition of the shikimic acid pathway. Shortly after the introduction of glyphosate as a commercial herbicide, its use for control of suckers in commercial raspberry plantations was evaluated with initially promising results (flL). The effects of these treatments in the long term is less well known. Similar... 

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