Prephenic acid, biosynthesis

Pseudoakuammicine is the first racemic base to be discovered in the strychnine-yohimbine series of alkaloids, and the question of its origin naturally arises. The only stage in the extraction of Picralima seeds during which racemization of akuammicine might have occurred involved prolonged percolation with hot methanol however, as already discussed, akuammicine is not racemized under these conditions but suffers a more extensive decomposition. In any event, such a racemization would necessarily involve fission of the 3,7 and 15,16 bonds, followed by a nonspecific resynthesis, which is considered to be a very unlikely possibility. It was therefore suggested that, in the plant, pseudoakuammicine is produced by a nonspecific biosynthesis this would accord with its formation from a tryptophan-phenylalanine precursor, but not from an optically pure prephenic acid derivative (40). 

The mode of biosynthesis of none of these alkaloids is known but, in the case of the iboga group, some guesses have been made (39, 63, 64), all of which start from the amino acids, tryptophan and dihydroxy-phenylalanine, and involve a fission of the latter s aromatic ring. A more sophisticated approach (65), starting from precursors of the aromatic amino acids, namely shikimic and prephenic acids, is apparently not in agreement with recent work on other indole alkaloids (66). The genesis of most indole alkaloids appears to stem from tryptophan and three... 

Fig. 8.15 Biosynthesis of L-Phe. Compounds EPSP, 5-enol-pyruvoylshikimic acid-3-phosphate CHA, chorismic acid PPA, prephenic acid PPY, phenylpyruvic acid S3P, shikimic acid 3-phosphate.

Some of the most interesting applications of organic structural theory to the elucidation of biosynthetic pathways were stimulated by efforts to formulate mechanisms for the biosynthesis of alkaloids. Conversely, consideration of implied biogenetic relations have occasionally helped structural determination. An important aspect of theories concerning alkaloid biosynthesis has been the assumed role of the aromatic amino acids in their formation. Only limited experimental evidence is available in this area. The incorporation of tyrosine- 8-C into morphine has been shown to be in accordance with a theory for its formation from 3,4-dihydroxyphenyl-alanine plus 3,4-dihydroxyphenylacetaldehyde. A stimulating theory of the biosynthesis of indole alkaloids, based on a condensation between trypt-amine and a rearrangement product of prephenic acid, has recently been published. The unique stereochemistry of C15 of these alkaloids had an important part in the formulation of the theory. Experimental proof of this theory would be valuable for several areas of alkaloid chemistry and biosynthesis. 

Experiments with mutants have also revealed the final stages of phenylalanine biosynthesis. Both Davis (190) and Japanese workers (459) obtained mutants excreting a labile substance, subsequently called prephenic acid, which was very readily converted into a second substance, Y, which was in turn converted into phenylalanine. Y was identified as phenyl-pyruvic acid (190). Prephenic acid was isolated (907) and its structure (see diagram 2) demonstrated (c/. 288a). 

The Enigma of Prephenate and/or /Vogenate Dehydrogenases in Plants The Enigma of Prephenate and/or Phenylpyruvate/p-Hydroxyphenylpyruvate Aminotransferases in Plants Regulation of /Vomatic /Voino Acid Biosynthesis... 

Discussed below therefore is what is known currently about Phe (1)/Tyr (2) biosynthesis in planta, as it relates to the conversion of prephenic acid (38) to both aromatic amino acids 1/2 (see Figure 6), and the large scientific and technical gaps that still remain. 

Claisen rearrangement of chorismic acid 1 to prephenic acid 2 (Scheme 1), which is catalyzed by the enzyme chorismate mutase, can be considered as the key step in the biosynthesis of aromatic compounds, that is the so-called shikimic acid pathway. The chair-like transition state geometry 3 was proved by double isotope-labeling experiments [2]. However, in the laboratory this particular reaction can be accelerated not only by enzymes but also by catalytic antibodies [3]. For the generation of such antibodies haptenes such as 4 were used, that is, molecules whose structure is very similar to the transition state of the particular reaction and which are tightly bound by the antibody. 

The three aromatic amino acids that are biosynthesized in the shikimic acid pathway have much in common. The many stereochemical events occurring between the condensation of compounds 288a and 289 derived from carbohydrates to the formation of prephenic acid 296 have been extensively reviewed including a recent review by ourselves (82), and so we have summarized the stereochemistry of the biosynthesis in Scheme 79. Prephenic acid 296 leads to phenylalanine 297 and tyrosine 298. The mem-substituted amino acids 299 are derived from chorismate 295, as is tryptophan 302, as shown. 

This new route to cyclohexadienones has been used in a total synthesis of the disodium salt of prephenic acid (2j, an unstable intermediate in biosynthesis of various phenols. The first attempt used the dienophile 3 in which the potential... 

Because shikimic acid does not enter into mammalian metabolism, its synthesis and use are clear targets at which to aim selective toxicity. In bacteria, shikimic acid arises by cyclization of the carbohydrate 3-deoxy-2-oxo-D- mAzVzoheptulosonic acid 7-phosphate, which is formed by the condensation of erythrose 4-phosphate and phosphoenolpyruvic acid. Shikimic acid undergoes biosynthesis to chorismic acid (4.55) which is the enolpyruvic ether of raw5-3,4-dihydroxy cyclohexa-1,5-diene-1-carboxylic acid. As its name indicates, this acid sits at a metabolic fork, the branches of which lead to prephenic acid, to phenylalanine (and hence to tyrosine), to anthranilic acid (and hence tryptophan), to ubiquinone, vitamin K, and/ -aminobenzoic acid (and hence folic acid). 

Before Mrs. Mingioli and I started work on the isolation of a new intermediate, its excretion by suitable mutants was studied by Dr. Davis, who selected the strain which showed the best production of the metabolite. Our research led to the isolation in pure form, and to the complete structure proof, of three new intermediates 3-dehydroquinic acid, shikimic acid 3-phosphate, and prephenic acid. Of these, the last one turned out to be a precursor of phenylalanine and tyrosine only. In contrast, the two other compounds were shown to be intermediates in the biosynthesis of all five primary aromatic metabolites. 

The 2,5-cyclohexadienol ring, which I had postulated initially as a working hypothesis capable of rationalizing a few observations made before prephenic acid had even been isolated in pure form, has subsequently assumed significance in the biosynthesis of natural compounds other than prephenic acid. It occurs in several substances not connected with the shikimic acid pathway, and plays a role in the biosynthesis of a number of alkaloids, including morphine. Again, we encounter a relationship that could not have been anticipated in 1953 when the idea of this ring occurred to me. 

Shikimic Acid, Isoshikimic Acid, and Prephenic Acid. As will be seen in the next chapter (Chapter 12), the amino acids phenylalanine, tyrosine, and tryptophan all contain aryl rings. The biosynthesis of the aromatic rings of these amino acids passes through the same seven-carbon carboxylic acid, shikimic acid, which itself is derived from erythrose and phosphoenolpyruvate. As shown in Scheme 11.10, as part of the... 

Thus, from the Solanaceae, hyoscyamine, a tropane alkaloid, which we now know to be derived from ornithine and/or arginine (Arg, R, Scheme 12.7), and nicotine, derived from a combination of nicotinic acid (see Scheme 12.103, et seq.) and ornithine, serve as examples of alkaloids based on nonaromatic amino adds. In the benzylisoquinoline alkaloids, only morphine, arguably the first alkaloid isolated in purified form and known to be derived from tyrosine (Tyr, T), itself a prephenic acid derivative, is discussed. For the indole alkaloids, the single example of this rich family to be illustrated is vinblastine, a (relatively) recently isolated base of some medicinal value and which is shown to be composed of two different expressions of tryptophan (Trp, W) and mevalonate (Scheme 11.40). Finally, caffeine is chosen as the member of the purine alkaloids, a relatively small family of compounds but one of major commercial import and one whose biosynthesis is closely tied to the library of life. 

The next steps in aromatic biosynthesis have not been demonstrated enzymatically. A labile intermediate, prephenic acid, has been isolated the side chain is derived from glucose carbons in a pattern consistent with introduction of phosphopyruvate. This compound is converted to phen-ylpyruvic acid by acid and by an enzyme detected in crude extracts of E. coli. Transamination of phenylpyruvate to give phenylalanine has already been discussed. 

No wonder that nature uses a catalyzed version of this rearrangement in the biosynthesis of prephenic acid, which also documents the very important aspect of charge acceleration for 3,3-sigmatropic reactions [76]. 

Prephenic acid theory of indole alkaloid biosynthesis... 

Much attention has been given to other enzymes of phenolic biosynthesis, and a veritable spate of papers, many of purely physiological interest, has appeared on the topic of phenylalanine ammonia lyase. Full details of the work on the stereochemistry of this enzyme have been published during the year. A paper on the biosynthesis of aromatic amino-acids from shikimate is of general significance, since it indicates that m-carboxy-substituted aromatic acids can be formed without involving prephenic acid as an intermediate. These results are discussed in more detail elsewhere in this volume (see Chapter 3). 

The Shikimate pathway is responsible for biosynthesis of aromatic amino acids in bacteria, fungi and plants [28], and the absence of this pathway in mammals makes it an interesting target for designing novel antibiotics, fungicides and herbicides. After the production of chorismate the pathway branches and, via specific internal pathways, the chorismate intermediate is converted to the three aromatic amino acids, in addition to a number of other aromatic compounds [29], The enzyme chorismate mutase (CM) is a key enzyme responsible for the Claisen rearrangement of chorismate to prephenate (Scheme 1-1), the first step in the branch that ultimately leads to production of tyrosine and phenylalanine. 

Schultz and coworkers (Jackson et a ., 1988) have generated an antibody which exhibits behaviour similar to the enzyme chorismate mutase. The enzyme catalyses the conversion of chorismate [49] to prephenate [50] as part of the shikimate pathway for the biosynthesis of aromatic amino acids in plants and micro-organisms (Haslam, 1974 Dixon and Webb, 1979). It is unusual for an enzyme in that it does not seem to employ acid-base chemistry, nucleophilic or electrophilic catalysis, metal ions, or redox chemistry. Rather, it binds the substrate and forces it into the appropriate conformation for reaction and stabilizes the transition state, without using distinct catalytic groups. 

Aryl side chain containing L-a-amino acids, such as phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp), are derived through the shikimate pathway. The enzymatic transformation of phosphoenolpyr-uvate (PEP) and erythro-4-phosphate, through a series of reactions, yields shikimate (Scheme 2). Although shikimate is an important biosynthetic intermediate for a number of secondary metabolites, this chapter only describes the conversion of shikimate to amino acids containing aryl side chains. In the second part of the biosynthesis, shikimate is converted into chorismate by the addition of PEP to the hydroxyl group at the C5 position. Chorismate is then transformed into prephenate by the enzyme chorismate mutase (Scheme 3). 

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