Phenylalanine from prephenic acid

Otherwise no immediate precursors of tyrosine appear to have been reported. Transamination of p-hydroxyphenylpyruvic acid has been suggested to be the final stage in yeasts (474), and may occur in E. coli (809), and isotopic evidence, discussed later, suggests that even if tyrosine is not formed from phenylalanine, the method of introduction of the tyrosine side chain is very similar to that postulated above for phenylalanine. Formation of p-hydroxyphenylpyruvic acid from prephenic acid can be readily visualized. 

One of the first important derivatives of phenylpyruvic acid is the essential amino-acid phenylalanine, produced with the aid of pyridoxamine (Figure 2.17, with that reaction running in reverse). It is an essential amino-acid because all animals must have it in their diet. Tyrosine in plants is made directly from prephenic acid by oxidation concurrently with decarboxylation. Mammals can make tyrosine from phenylalanine, but insects must obtain it in food. Some other simple phenyl-Cj compounds and derivatives are shown in Figure 8.2. 

The flavonoids, which comprise the largest group of these natural products, are derived from a mixed acetate-shikimate pathway. A shikimate-derived C6-C3 unit combines with a six-carbon polyketide chain to provide the open-chain precursor (685) of the group. The derivation of p-hydroxycinnamic add (p-coumaric acid), the C6-C3 component, from shikimic acid proceeds through chorismic acid, prephenic acid and phenylalanine. 

FIGURE 3.1 The biosynthetic pathway from chorismate to L-phenylalanine in Escherichia coli K12. The mnemonic of the genes involved are shown in parentheses below the enzymes responsible for each step. Compound 1 is L-phenylalanine, 2 is chorisimic acid, 3 is prephenic acid, and 4 is phenylpyruvic acid. 

Among the more interesting reactions involved in making all three of these natural products are the loss of ammonia from phenylalanine to give an alkene and the introduction of extra OH groups around the benzene rings. We know how a para OH of Tyr is introduced directly by the oxidation of prephenic acid before decarboxylation and it is notable that the extra oxygen functionalities appear next to that point. This is a clue to the mechanism of the oxidation. 

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... 

Certain phenylalanine auxotrophs of E. coli were observed to possess the unusual ability of developing a growth factor for themselves. The accumulation of a nutritionally inactive compound, its spontaneous decomposition to phenylpyruvic acid (as the pH of the growth medium dropped), and the conversion of the latter to phenylalanine, were shown to be responsible for this effect. The unstable compound, prephenic acid, was isolated from culture filtrates and was characterized as XVAlthough its half-life at room temperature in N hydrochloric acid was 1 noinute, prephenic acid was relatively stable at neutral pH. Its enzymic conversion... 

At the branching point of chorismic acid, either anthranilic acid, the precursor of tryptophan, or prephenic acid, the precursor of phenylalanine, itself the precursor of tyrosine and dopa (3,4-dihydroxy-phenylalanine), is formed (Fig. 10). Phosphorylation at the 3-position, condensation with phosphoenolpyru-vate, and elimination of phosphoric acid yields choris-mate from shikimate. Chorismate is also the precursor of a number of simple, and very important, aromatic compounds, including salicylic acid, 4-amino-benzoic acid (PABA), a constituent of folic acid, and 2,3-dihydroxybenzoic acid, a key acylating group of enterobactin. 

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. 

Both phenylalanine and tyrosine are derived from chorismic acid, which is itself derived from shikimic acid-3-phosphate through the shikimic acid pathway. In this sequence, chorismic acid is first transformed into prephenic acid by chorismate mutase. If prephenic acid is converted into phenylpyruvic acid by... 

In both bacteria and plants, two additional amino acids, phenylalanine and tyrosine, are formed from chorismic acid. From chorismate, two separate routes diverge and lead to the amino acids L-phenylalanine and L-tyrosine. However, the pathways in bacteria and plants are distinct and involve different intermediates. Both of these pathways pass through the same intermediate, prephenic acid (26) (Fig. 7.9) (Floss,... 

Basically, the shikimic acid pathway involves initial condensation of phosphoenolpyruvate (PEP) from the glycolysis process with erythrose-4-phosphate derived from the oxidative pentose phosphate cycle. A series of reactions leads to shikimic acid, which is then phosphorylated. The phosphorylated shikimic acid combines with a second molecule of PEP to yield prephenic acid via chorismic acid intermediate. Prephenic acid is then decarboxylated to form phenyl-pyruvate or p-hydroxyphenylpyruvate. On transamination, these two compounds yield phenylalanine and tyrosine, respectively. 

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... 

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. 

Amination of chorismic acid 5.10) leads through anthranilic acid 5.13) to tryptophan 5.14). The formation of phenylalanine 5.17) and tyrosine 5.18), on the other hand, proceeds via prephenic acid 5.16), whose formation from chorismic acid 5.10 = 5.15) involves... 

The terminal stage of the synthesis has had some light cast upon it by the results of enzyme studies. It has been shown that prephenic acid is an intermediate in the path from phenylalanine to tyrosine. Moreover, phenylpyruvic acid also lies along the pathway of phenylalanine synthesis. 

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. 

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). 

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. 

Fig. 2 (1)]. A key intermediate in the pathway is chorismate from which branched pathways lead to tryptophan, phenylalanine, tyrosine, 4-amino benzoate, isoprenoid quinones, and metacarboxyphenylalanine. A secondary branch also occurs at prephenate leading to phenylalanine and tyrosine. A representative number of non amino acid compounds in relation to their shikimic acid pathway precursors are shown in Fig. 1. One side branch leads to quinate which participates in the formation of depsides. 

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