Amino acid synthesis shikimate

Herbicides that inhibit enzymes important for amino acid synthesis account for 28% of the herbicide market. Just three enzymes are involved the enzyme that adds phosphoenolpyruvate to shikimate-3-phoshate in the pathway leading to aromatic compounds, the enzyme that makes glutamine from glutamate and ammonia, and the first common enzyme in the biosynthesis of the branched-chain amino acids. 

The role of quinic acid in the shikimic acid pathway is not understood. In some plants, compared with shikimate, quinate administered exogenously is more effectively incorporated into the aromatic amino acids. These results have been interpreted to mean that quinate occurs on the pathway of aromatic amino acid synthesis and is not a shunt metabolite as depicted in Fig. 2. Different routes of metabolism for quinate and shikimate are also suggested by other experiments. For example, when p CJglucose and [ C]ery-throse are compared for eflSciency of their conversion into shikimate and... 

The evidence that (- )-shikimic acid plays a central role in aromatic biosynthesis was obtained by Davis with a variety of nutritionally deficient mutants of Escherichia coli. In one group of mutants with a multiple requirement for L-tyrosine, L-phenylalanine, L-tryptophan and p-aminobenzoic acid and a partial requirement for p-hydroxybenzoic acid, (—)-shikimic acid substituted for all the aromatic compounds. The quintuple requirement for aromatic compounds which these mutants displayed arises from the fact that, besides furnishing a metabolic route to the three aromatic a-amino acids, the shikimate pathway also provides in micro-organisms a means of synthesis of other essential metabolites, and in particular, the various isoprenoid quinones involved in electron transport and the folic acid group of co-enzymes. The biosynthesis of both of these groups of compounds is discussed below. In addition the biosynthesis of a range of structurally diverse metabolites, which are derived from intermediates and occasionally end-products of the pathway, is outlined. These metabolites are restricted to certain types of organism and their function, if any, is in the majority of cases obscure. 

The carbon flow from 3-phosphoglycerate, phosphoenolpyruvate, pyruvate and acetyl-CoA. Even if the synthesis of aromatic amino acids by shikimate pathway /28,29,30,31/ and also prenyl-PP synthesis via mevalonate /32,33,34/ has been established in chloroplasts by identification of respective plastidic enzymes, it is still a matter of discussion from where PEP origins to supply DAHP synthesis of the shikimate pathway and from where pyruvate is delivered to supply the plastidic pyruvate dehydrogenase complex (for isolation see Treede and Heise, this Conference). Because phosphoglycerate mutase (PGM) to form 2-PGA from 3-PGA could not be detected in chloroplasts /35/ and acetyl-CoA is preferably synthesized from added acetate by the actetyl-CoA synthetase /36/, particularly in spinach chloroplasts, it was argued that chloroplasts are dependent on import of these substrates from the external site. Evidence for PEP formation from 3-PGA within the chloroplast could be obtained by three different approaches (D. Schulze-Siebert, A. Heintze and G. Schultz, in preparation D. Schulze-Siebert and G. Schultz, in preparation, for plastidic isoenzyme of PGM in Ricinus see /37/ and in Brassica /38/). 

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). 

The organism utilized is a mutant of E. coli blocked in the synthesis of aromatic amino acids before the shikimate step. Cells are first grown in the presence of adenosine, a technique that temporarily derepresses the system of en-... 

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. 

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... 

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). 

Inhibition of Chorismate Synthase Shikimic and quinic acids are used by microorganisms, fungi, and superior plants for the synthesis of essential aromatic amino acids from acyclic sugars. Fluorinated analogues of substrates and reaction intermediates have been synthesized in order to inhibit enzymes involved in... 

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... 

In Scheme 1.2 one possible retrosynthetic analysis of the unnatural enantiomer of shikimic acid, a major biosynthetic precursor of aromatic a-amino acids, is sketched. Because cis dihydroxylations can be performed with high diastereoselectiv-ity and yield, this step might be placed at the end of a synthesis, what leads to a cyclohexadienoic acid derivative as an intermediate. Chemoselective dihydroxylation of this compound should be possible, because the double bond to be oxidized is less strongly deactivated than the double bond directly bound to the (electron-withdrawing) carboxyl group. 

Several other important compounds found in the common aromatic amino acid pathway whose overproduction has been studied are shikimic acid (61) and, to a lesser extent, quinic acid (62) (Scheme 19.41).323 Both 61 and 62 are naturally occurring, highly functionalized carbocyclic rings with asymmetric centers, which can be used as starting material for the synthesis of GS4104 (63), a neuraminidase inhibitor discovered by Gilead Sciences and developed by Roche Pharmaceuticals under the trade name of Tamiflu .324 325 Manipulation of the aromatic amino acid pathway in E. coli has allowed for numerous strains to be assembled that produce both 61 and 62 as well as other intermediates.326 327 As reported by Chandran and co-workers, an E. coli strain has been constructed that synthesized 87 g/L (0.5m) of 61 in 36% (mol/mol) yield with a maximum productivity of 5.2 gL- lr1.328... 

Shikimic acid pathway chemical pathway common in plants, bacteria, and fungi, where aromatic amino acids (e.g., tryptophan, phenylalanine, tyrosine) are synthesized, thereby providing the parent compounds for the synthesis of the phenylpropanoid units in lignins. 

Two main attributes are ascribed to natural shikimic acid the first, of practical nature, is related to its use as a chiral source for asymmetric synthesis, the second, of biochemical prominence, is connected to the key role it exerts in the production of benzenoid rings of natural aromatic amino acids and other important metabolites [45]. The biological relevance of shikimic acid and the challenging nature of its multichiral structure have motivated an active search for the development of viable asymmetric syntheses of this compound and novel structural variants [46],... 

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... 

EPSP synthase catalyzes the synthesis of EPSP by an addition-elimination reaction through the tetrahedral intermediate shown in Fig. 2a. This enzyme is on the shikimate pathway for synthesis of aromatic amino acids and is the target for the important herbicide, glyphosate, which is the active ingredient in Roundup (The Scotts Company EEC, Marysville, OH). Transient-state kinetic studies led to proof of this reaction mechanism by the observation and isolation of the tetrahedral intermediate. Moreover, quantification of the rates of formation and decay of the tetrahedral intermediate established that it was tmly an intermediate species on the pathway between the substrates (S3P and PEP) and products (EPSP and Pi) of the reaction. The chemistry of this reaction is interesting in that the enzyme must first catalyze the formation of the intermediate and then catalyze its breakdown, apparently with different requirements for catalysis. Quantification of the rates of each step of this reaction in the forward and reverse directions has afforded a complete description of the free-energy profile for the reaction and allows... 

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