Shikimic acid pathway amino acids

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. - ... 

Precursor selection An important step in designing a microbial labeling process is the identification of one or more suitable labeled precursors. Generally, secondary metabolites are biosynthesized via primary metabolites from five metabolic sources. These are amino acids, shikimic acid (shikimic acid pathway), acetate and its homologues (polyketide pathway), mevalonic acid (isoprene pathway) and carbohydrates. Selection of a suitable precursor is primarily influenced by the biosynthetic pathway(s) involved, but also depends on the desired position of label in the product and the availability of labeled precursors. 

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. 

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). 

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. 

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). 

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. 

The quinone ring is derived from isochorismic acid, formed by isomerization of chorismic acid, an intermediate in the shikimic acid pathway for synthesis of the aromatic amino acids. The first intermediate unique to menaquinone formation is o-succinyl benzoate, which is formed by a thitunin pyrophosphate-dependent condensation between 2-oxogluttnate emd chorismic acid. The reaction catalyzed by o-succinylbenzoate synthettise is a complex one, involving initially the formation of the succinic semialdehyde-thiamin diphosphate complex by decarboxylation of 2-oxogluttnate, then addition of the succinyl moiety to isochorismate, followed by removed of the pyruvoyl side chain emd the hydroxyl group of isochorismate. 

Several studies of the biosynthesis of chloramphenicol have led to the conclusion that it is formed via the shikimic acid pathway, specifically from chorismic acid. An arylamine synthetase promotes formation of p-amino-L-phenyl alanine (1 ) 50>51. This product is converted to chloramphenicol (15) by oxidation of the amine function to a nitro group, by hy-droxylation of the benzylic methylene group, reduction of the carboxyl... 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

Despite the thousands of secondary metabolites made by microorganisms, they are synthesized from only a few key precursors in pathways that comprise a relatively small number of reactions and which branch off from primary metabolism at a limited number of points. Acetyl-CoA and propionyl-CoA are the most important precursors in secondary metabolism, leading to polyketides, terpenes, steroids, and metabolites derived from fatty acids. Other secondary metabolites are derived from intermediates of the shikimic acid pathway, the tricarboxylic acid cycle, and from amino acids. The regulation of the biosynthesis of secondary metabolites is similar to that of the primary processes, involving induction, feedback regulation, and catabolite repression [6]. 

In 1967 it was shown by Young et al. [84] that 2,3-DHBA in K. pneumoniae and E. coli is produced via the shikimic acid pathway (Fig. 1). Evidence showed that the centra) intermediate chorismic acid, leading to the aromatic amino acid pathways, is also the precursor for 2,3-DHBA. The formation of 2,3-DHBA required NAD, and Mg (Fig. 4)... 

The naturally occurring naphthoquinones such as lawsone and juglone are products of the shikimic acid pathway to aromatic amino-acids but the path which leads to these naphthoquinones branches from the main pathway before the formation of aromatic compounds, probably no later than chorismic acid.It will be most interesting to see whether the biosynthesis of shihunine also follows this route all the other bases of plant origin which arise from products of the shikimic acid pathway derive from aromatic precursors. 

Chorismate mutase (CM) catalyzes the Claisen rearrangement of chorismate to prephenate in the shikimic acid pathway used in the biosynthesis of aromatic amino acids. It represents a reference enzyme to explore the fundamentals of catalysis and has been the subject of extensive experimental and computational research. These have shown both that catalysis proceeds without covalent binding of the substrate to the enzyme, and that the uncatalyzed reaction in water proceeds by the same mechanism. This makes CM a particularly convenient target for QM/MM studies. 

C7H,o05, Mr 174.15. needles, D. 1.6, mp. 178-180°C, [a]g -157° (H2O), pKg4.15 (14.1 °C), soluble in water. S. is a widely distributed component of plants and occurs especially in fruits of the star anise (lllicium anisatum, syn. /. religiosum, Illiciaceae Japanese shi-kimi-no-ki). S. is a key intermediate of the so-called shikimic acid pathway which includes the biosynthesis of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. These, in turn, are precursors of numerous alkaloids, flavonoids, and lignans, as well as 4-amino- and 4-hydroxybenzoic acid, gallic acid, tetrahydrofolic acid, ubiquinones, vitamin K, and nicotinic acid. The synthetic racemate melts at 191-192 °C. 

The shikimic acid pathway is responsible for the biosynthesis of a large number of aromatic compounds, particularly in plants. Most important for many mammals is the fact that plants manufacture the aromatic amino acids Phe (phenylalanine), Tyr (tyrosine), and Trp (tryptophan). These are essential amino acids for humans—we have to have them in our diet as we cannot make them ourselves. 

Fig. 1. Generalized relationship of shikimic acid pathway intermediates, aromatic amino acids, and selected secondary metabolites in higher plants.

Fig. 2. Prechorismate portion of the shikimic acid pathway involved in the synthesis of aromatic amino acid precursors. Nomenclature of enzymes and substrates are described in Table I and cross-referenced to each numbered reaction.

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... 

Regulation of biosynthesis in the multibranched shikimic acid pathway appears to rely heavily on feedback mechanisms, some of which have been demonstrated both in vivo as well as in vitro. It would seem from the composite of plant studies described herein, which by no means are extensive enough to permit inclusive generalization, that adequate control measures have been observed to account for the maintenance of pool sizes at reasonably low levels by the various metabolites. Furthermore, negative evidence for repression of pathway enzymes tends to emphasize the role of enzyme inhibition and activation in the regulation of aromatic amino acid biosynthesis in higher plants which at the present is depicted in Fig. 6. 

Lastly, the coordinate regulation of the shikimic acid pathway in concert with other ancillarly aromatic pathways is a fertile area for future study. It is likely that assessment of the integrated control of aromatic amino acid biosynthesis within the context of secondary metabolites will provide answers to many fundamental questions of the role of the plethora of secondary shikimate products in numerous physiological processess. 

These data and the different intracellular location of two SHORases in Pisum (Ref. 6) suggest the duplication of the first steps of the shikimate pathway. However, it is not yet clear, by the time being, if each Isoenzyme (or isocomplex) is involved in a specific pathway synthesis of aromatic amino-acids, benzoic acids or the putative qui-nate pathway (Ref. 7). 

Most of the enzymes of this pathway have now been isolated and studied (Floss, 1986 Jensen, 1986). The shikimic acid pathway is predominately found in plastids in higher plants isolated spinach chloroplasts can assimilate CO2 or shikimate and produce aromatic amino acids. The controls for this pathway have been reviewed (Jensen, 1986). 

A wide range of aromatic products in the plant kingdom originate from intermediates of the shikimic acid pathway. These include amino acids and ubiquinone among important primary metabolites and also many other compounds (in contrast fo fungi which possess many acetate-derived products), such as lignins, alkaloids, and phenolic acids. Flavonoids (and stilbenes) also arise in part by this route but additionally utilize acetate in the course of their biosynthesis, which will therefore be described in this chapter. 

Extensive studies support the hypothesis that these phenazine precursors are derived from the shikimic acid pathway, as outlined in Scheme 1, with chorismic acid (51) as the most probable branch point intermediate. Shikimic acid (50) is converted to chorismic acid (51) in known transformations that are part of the common aromatic amino acid biosynthetic pathway. The transformation from chorismic acid (51) to the phenazine precursors has been discussed and investigated through intensive biochemical studies so far, no intermediates have been identified and little is known about the genetic origin and details of the phenazine biosynthesis. ... 

The multibranched shikimic acid pathway provides the intermediates for the synthesis of the three amino acids phenylalanine, tyrosine and tryptophan in microorganisms and plants. In plants, these three amino acids are precursors for a variety of secondary metabolites such as alkaloids, coumarins, flavonoids, lignin precursors, indole derivatives and numerous phenolic compounds (Fig. 1). The role of the aromatic amino acids in protein synthesis is well known as is the role of indoleacetic acid in plant development however, the function of the various secondary products is much less clear. Various physiological roles have been proposed including pest resistance, chromagens in flowers and fruits, and precursors for the structural component, lignin. 

The shikimic acid pathway leading to the production of chorismic acid is regulated in the cytosol of the fungal cells. Cytosol or intracellular fluid (cytoplasmic matrix) is a complex mixture of substances dissolved in water. These include ions (such as calcium, sodium, and potassium), macromolecules, and large complexes of enzymes that act together to carry out metabolic pathways. Production of chorismic acid in the cytosol is ultimately utilized in the synthesis of folate, ubiquinone, and amino acids, the most important of which is tryptophan which plays a major role in the biosynthesis of psilocybin. 

Bartlett, P.A., McLaren, K.L., Alberg, D.G., Fdssler, A., Nyfeler, R., Lauhon, C.T. and Grissom, C.B. (1989) Exploration of the Shikimic Acid Pathway (Opportunities for the Study of Enzyme Mechanisms Through the Synthesis of Intomedi-ates and Inhibitors, in L.G. Copping (ed). Prospects for Amino Acid Biosynthesis Inhibitors in Crop Protection and Pharmaceutical Chemistry, Society of Chemical Industry, pp. 155-170. 

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