From Chorismate to Phenylalanine and Tyrosine

From Chorismate to Phenylalanine and Tyrosine 1442.Box 25-B The Cyanogenic Glycosides... 

Fig. 8. From chorismate to phenylalanine and tyrosine. (PDHY = prephenate dehydratase EC 4.2.1.51 PDH = prephenate dehydrogenase EC 1.3.1.13 TAT = tyrosine aminotransferase EC 2.6.1.5 PREPAT = prephanate aminotransferase PTDH = pretyrosine dehydrogenase.)...

Prephenlc Acid, l-Carboxy.4-hydroxy-< -oxo-2.5-cyclohexadiene-1-propanoic acid l-carboxy-4-hydraxy-2,S-cyclohexadiene-l-pyruvic acid, C,0H1 Ot mol wt 226.18. C 53.10%, H 4.46%, O 42.44%. Non-aromatic biosynthetic in -termediate that represents a secondary branch-point in the pathway from chorismic acid to phenylalanine and tyrosine, q.q.y., in many organisms. Isoln from cultures of mutant Escherichia coti B. D. Davis, Science 118, 251 (1953). 

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. 

Chorismic acid is the key branch point intermediate in the biosynthesis of aromatic amino acids in microorganisms and plants (Scheme 1.1a) [1]. In the branch that leads to the production of tyrosine and phenylalanine, chorismate mutase (CM, chorismate-pyruvate mutase, EC 5.4.99.5) is a key enzyme that catalyzes the isomerization of chorismate to prephenate (Scheme 1.1b) with a rate enhancement of about lO -lO -fold. This reaction is one of few pericyclic processes in biology and provides a rare opportunity for understanding how Nature promotes such unusual transformations. The biological importance of the conversion from chorismate to prephenate and the synthetic value of the Claisen rearrangement have led to extensive experimental investigations [2-43]. 

FIGURE 22-19 Biosynthesis of phenylalanine and tyrosine from chorismate in bacteria and plants. Conversion of chorismate to prephenate is a rare biological example of a Claisen rearrangement. 

The pathway bifurcates at chorismate. Let us first follow the prephenate branch (Figure 24,17). A mutase converts chorismate into prephenate, the immediate precursor of the aromatic ring of phenylalanine and tyrosine. This fascinating conversion is a rare example of an electrocyclic reaction in biochemistry, mechanistically similar to the well-known Diels-Alder reaction from organic chemistry. Dehydration and decarboxylation yield phenylpyruvate. Alternatively, prephenate can be oxidatively decarboxylated to p-hydroxyphenylpyruvate. These a-ketoacids are then transaminated to form phenylalanine and tyrosine. 

The thousands of enzyme-catalyzed chemical reactions in living cells are organized into a series of biochemical (or metabolic) pathways. Each pathway consists of a sequence of catalytic steps. The product of the first reaction becomes the substrate of the next and so on. The number of reactions varies from one pathway to another. For example, animals form glutamine from a-ketoglutarate in a pathway that has two sequential steps, whereas the synthesis of tryptophan by Escherichia coli requires 13 steps. Frequently, biochemical pathways have branch points. For example, chorismate, a metabolic intermediate in tryptophan biosynthesis, is also a precursor of phenylalanine and tyrosine. 

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... 

The largest flux of carbon atoms from chorismate goes into the phenylala-nine/tyrosine pathway, among others leading to lignin and important groups of secondary metabolites such as flavonoids and anthocyanins. The first enzyme in that particular pathway, chorismate mutase (CM, EC 5.4.99.5), catalyzes the conversion of chorismate to prephenate (Fig. 8). Both a cytosolic and a plastidial form have been detected in several plants (e.g., 144-147). The plastidial isoform is inhibited by phenylalanine and tyrosine, and activated by tryptophan the other isoform is not affected by these... 

From chorismic acid, four major pathways lead to essential metabolites tryptophan, phenylalanine and tyrosine, p-aminobenzoic acid and the folate group of coenzymes, and the isoprenoid quinones (Fig. 7.2). Numerous secondary compounds in plants and other organisms are formed from products and intermediates of these pathways. 

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,... 

Bacteria, fungi, and plants share a common pathway for the biosynthesis of aromatic amino acids with shikimic acid as a common intermediate and therefore named after it—the shikimate pathway. Availability of shikimic acid has proven to provide growth requirements to tryptophan, tyrosine, and phenylalanine triple auxotrophic bacterial strains. Chorismate is also the last common precursor in the aromatic amino acid biosynthetic pathway, but the pathway is not named after it, as it failed to provide growth requirements to the triple auxotrophs. The aromatic biosynthetic pathway starts with two molecules of phosphoenol pyruvate and one molecule of erythrose 4-phosphate and reach the common precursor, chorismate through shikimate. From chorismate, the pathway branches to form phenylalanine and tyrosine in one and tryptophan in another. Tryptophan biosynthesis proceeds from chorismate in five steps in all organisms. Phenylalanine and tyrosine can be produced by either or both of the two biosynthetic routes. So phenylalanine can be synthesized from arogenate or phenylpyruvate whereas tyrosine can be synthesized from arogenate or 4-hydroxy phenylpyruvate. 

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. 

Hydroxybenzoic acid 5.35) has been shown to stand as the key intermediate in ubiquinone biosynthesis, in living systems from micro-organisms to mammals. In animals, phenylalanine and tyrosine serve as precursors, but in bacteria chorismic acid 5.10) is the precursor [28, 29]. Interlocking evidence obtained from bacteria in experiments with mutants (and genetic analysis), cell-free preparations, and isolation and identification of intermediates allows clear delineation [25, 29] of the sequence of biosynthesis as that shown in part in Scheme 5.5 beyond 5.35) the intermediates are the... 

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. 

Figure 1. Hypothetical mechanism for shuttling of intermediates of the common aromatic pathway between plastidic and cytosolic compartments. Enzymes denoted with an asterisk (DAHP synthase-Co, chorismate mutase-2, and cytosolic anthranilate synthase) have been demonstrated to be isozymes located in the cytosol. DAHP molecules from the cytosol are shown to be shuttled into the plastid compartment in exchange for EPSP molecules synthesized within the plastid. Abbreviations C3, phosphoenolpyruvate C4, erythrose 4-P DAHP, 3-deoxy-D-arabino-heptulosonate 7-phosphate EPSP, 5-enolpyruvylshikimate 3-phosphate CHA, chorismate ANT, anthranilate TRP, L-tryptophan PPA, prephenate AGN, L-arogenate TYR, L-tyrosine and PHE, L-phenylalanine.

Aromatic amino acid biosynthesis proceeds via a long series of reactions, most of them concerned with the formation of the aromatic ring before branching into the specific routes to phenylalanine, tyrosine, and tryptophan. Chorismate, the common intermediate of the three aromatic amino acids, (see fig. 21.1) is derived in eight steps from erythrose-4-phosphate and phosphoenolpyruvate. We focus on the biosynthesis of tryptophan, which has been intensively studied by both geneticists and biochemists. 

The aromatic amino acids, phenylalanine, tryptophan, and tyrosine, are all made from a common intermediate chorismic acid. Chorismic acid is made by the condensation of erythrose-4-phosphate and phosphoenol pyruvate, followed by dephosphorylation and ring closure, dehydration and reduction to give shikimic acid. Shikimic acid is phosphorylated by ATP and condenses with another phosphoenol pyruvate and is then dephosphorylated to give chorismic acid. 

---