Shikimate 3-phosphate aromatic amino acid synthesis

The well-known cytoplasmic shikimate pathway is important to aromatic amino acid synthesis. It takes a long metabolic pathway to reach to an aromatic amino acid from D-glucose. Two metabolic intermediates, phosphoenolpymvate from the glycolytic pathway and D-erythrose-4-phosphate from the pentose-phosphate pathway, must combine to form 3-deoxy-7-phospho-D-arabinoheptulosonate before formation of 3-dehydroquinate (Fig. 13.4). Thus, there are still several barriers difficult to overcome in the already existing technologies of shikimate fermentation from D-glucose (Draths et al. 1999). 

Deoxy-araWno-heptulosonic acid 7-phosphate (10) is a metabolic intermediate before shikimic acid in the biosynthetic pathway to aromatic amino-acids in bacteria and plants. While (10) is formed enzymically from erythrose 4-phosphate (11) and phosphoenol pyruvate, a one-step chemical synthesis from (11) and oxalacetate has now been published.36 The synthesis takes place at room temperature and neutral pH... 

The reversible reaction of phosphoenolpyruvate (PEP) with shikimate 3-phosphate is a step in the synthesis of the aromatic amino acids (see Fig. 25-1). The chemical mechanism indicated... 

Aromatic compounds arise in several ways. The major mute utilized by autotrophic organisms for synthesis of the aromatic amino acids, quinones, and tocopherols is the shikimate pathway. As outlined here, it starts with the glycolysis intermediate phosphoenolpyruvate (PEP) and erythrose 4-phosphate, a metabolite from the pentose phosphate pathway. Phenylalanine, tyrosine, and tryptophan are not only used for protein synthesis but are converted into a broad range of hormones, chromophores, alkaloids, and structural materials. In plants phenylalanine is deaminated to cinnamate which yields hundreds of secondary products. In another pathway ribose 5-phosphate is converted to pyrimidine and purine nucleotides and also to flavins, folates, molybdopterin, and many other pterin derivatives. 

In addition to acetyl-CoA, shikimic acid, mevalonic acid, and deoxyxylulose phosphate, other building blocks based on amino acids are frequently employed in natural product synthesis. Peptides, proteins, alkaloids, and many antibiotics are derived from amino acids, and the origins of the most important amino acid components of these are briefly indicated in Figure 2.1. Intermediates from the glycolytic pathway and the Krebs cycle are used in constructing many of them, but the aromatic amino acids phenylalanine, tyrosine,... 

Deoxy-D-araZhrao-hept-2-ulosonic acid-7-phosphate ( DAHP, 122) is the precursor for the synthesis of aromatic amino acids in all microorganisms and plants (shikimic pathway).306,307... 

Inhibits 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS), an enzyme of the aromatic acid and biosynthesis pathway. This prevents synthesis of essential aromatic amino acids needed for protein biosynthesis... 

The biologically active monosaccharide 3-deoxy-D-ura6//io-heptulosonic acid 7-phosphate (8 DAMP) is an important intermediate in the biosynthesis of aromatic amino acids in plants (the shikimate pathway). As shown in Scheme 2, this compound has been produced in a combined chemical and enzymatic synthesis from racemic V-acetylaspartate 3-semialdehyde (4) and DHAP (1). The four-step synthesis proceeds in an overall yield of 13% (37% for the aldolase reaction). The enzymatic step generates the required, enantiomerically pure, syn aldol adduct compound (5). In view of the broad range of substrates tolerated by FDP aldolase, this method may be applicable to the production of analogs of DAMP. 

The 3-deoxy-D-ara6mo-2-heptulosonic acid 7-phosphate (DAHP) synthetase (EC 4.1.2.15) is an enzyme involved in the shikimic pathway of aromatic amino acids biosynthesis in bacteria and plants, where catalyzes the construction of 3-deoxy-D-ara6/ o-2-heptulosonic acid 7-phosphate from phosphoenolpyruvate and D-erythrose 4-phosphate [6]. Although 3-deoxy-D-ara6/H0-2-heptulosonic acid 7-phosphate (DAHP) synthetase has not been widely investigated it has been employed for the DAHP synthesis on preparative scale from D-fructose in multienzyme system [68], This one-pot synthesis was subsequently even more simplified by the results of further studies which indicated that it was more efficient and economical to use the whole cells containing a DAHP synthetase plasmid [69]. 

Besides showing the unbranched pathway from erythrose-4-phosphate and phosphoenolpyruvate to shikimic acid. Figure 2L13 also shows the sequence of reactions from shikimic acid to chorismate, the first major branch point in the synthesis of the aromatic amino acids and histidine. The sixth reaction of the shikimic acid pathway is inhibited specifically by glyphosate (see here), which is the active ingredient in the broad spectrum herbicide known as Roundup. 

Figure 21.12 provides an overview of the biosynthesis of aromatic amino acids and histidine. All of the carbons in phenylalanine and tyrosine are derived from erythrose-4-phosphate and phosphoenolpyruvate. A key intermediate in synthesis of virtually all aromatic compounds (including p-aminobenzoic acid) in plant and bacterial cells is shikimic acid. Shikimic acid gives rise to chorismate... 

Phosphorylation of 3-hydroxyl group of shikimate by shikimate kinase (EC 2.7.1.71) with ATP as a cosubstrate initiates the biosynthesis pathway of anthranilic acid [2], This step also presents the first step of the shikimate pathway, which is a metabolic route used by bacteria, fungi, and plants for the biosynthesis of many aromatic products such as lignins, alkaloids, flavonoids, benzoic acid, and plant hormones, in addition to the aromatic amino acids (phenylalaiune, tyrosine, and tryptophan). The sequential EPSP synthesis is catalyzed by EPSP synthase (EC 2.5.1.19) through the addition of phosphoenolpyruvate to 3-phospho-shikimate followed elimination of phosphate. EPSP synthase belongs to the family of transferases, specifically to those transferring aryl... 

This route, often called the shikimic acid pathway involves the condensation of phosphoenolpyruvate (2) and a 4-carbon sugar erythrose-4-phosphate (1) which is derived from the pentose phosphate pathway. The product of this reaction is converted to shikimic acid (3). Phosphorylation of shikimic acid to yield 5-phosphoshikimic acid (4) is followed by the addition of another molecule of phospho-enol pyruvate (2) which results in the synthesis of prephenic acid (5). Aromatization of the prephenic acid can give rise to phenylpyruvic acid (6) which upon transamination becomes phenylalanine. The carbon skeletons of the other aromatic amino acids, tryptophane and tyrosine are also synthesised via the shikimic acid pathway as is lignin and many of the aromatic secondary products described in Chapter 6. 

A consideration of the biosynthesis of p-aminobenzoic acid (PABA) involves primarily the mechanism of formation of the ring structure of aromatic compounds. Studies with mutants of E. colt indicated that shikimic acid (Fig. 3) was the precursor of the aromatic ring as occurring in PABA and also in tyrosine, tryptophan, phenylalanine, and p-hydroxy-benzoic acid (12). Much is now known about the manner in which kimic acid is formed from intermediates of carbohydrate metabolism (IS, ISa). Certain E. colt mutants also accumulate shikimic acid-5 -phosphate (ISb). Recent work of Weiss and Srinivasan (13c) demonstrated that PABA could be formed from shikimic acid-5 -phosphate and L-glutamine in an enzyme system derived from baker s yeast. Free shikimic acid was utilized very poorly for PABA synthesis but was more active when incubated in the presence of ATP. Glutamine was a specific amino donor and could not be replaced by glutamic acid or asparagine. When uniformly C -labeled shikimic acid was used as substrate for PABA synthesis in the enzyme system, the PABA formed had the same molar specific activity as the initial shikimic acid 5 -phosphate. PABA synthesis was also dependent on small amounts of yeast or liver concentrates but the nature of the cofactor and the mechanism of the over-all reaction are not known. 

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