Anthranilic acid biosynthetic pathway

Anthranilic acid (Figure 4.4) is an intermediate in the biosynthetic pathway to the indole-containing aromatic amino acid L-tryptophan (Figure 4.10). 

Chorismic acid (26), thus, represents the first divergence point of pyocyanin from other biosynthetic pathways. The first authentic pyocyanin biosynthetic enzyme is PhzE, which has sequence similarity to anthranilate synthases, which generate anthranilate from chorismate. PhzE is thought to catalyze the conversion of chorismic acid (26) to amine 165. Compound 165 is in turn a substrate for PhzD, an isochorismatase that catalyzes the hydrolysis of the vinyl ether to 166 and pyruvate [188, 189],... 

Benzodiazepine Alkaloids.—Cyclopenin (49) and cyclopenol (50) are isolated from Penicillium cyclopium. Their biosynthesis would appear to involve one molecule of phenylalanine and one of anthranilic acid and indeed the biosynthetic pathway to viridicatin and viridicatinol in Penicillium viridicatum from these two amino-acids has (49) and (50) as intermediates. ... 

The isolation of alkaloids bearing the 8-hydroxyquinoline moiety prompted postulation (17) of a biosynthetic pathway to 1 from the amino acid tryptophan, for which chemical analogies are known (22) (Scheme 2). It should be noted, however, that, in general, quinolines are biosynthesized from anthranilic acid (14). Biosynthesis of broussonetine (4) would involve condensation of two molecules of 1 with a molecule of acetyl-CoA followed by cyclization, as depicted in Scheme 3. 

In the biosynthetic pathway of the alkaloids derived from anthranilic acid, the carbon atom corresponding to the carboxyl carbonyl group is included in the resulting alkaloids, except for those alkaloids with the phen-azine skeleton. The main groups of alkaloids derived from anthranilic acid are classified as quinoline, acridone, and quinazoline alkaloids. These alkaloids are typically isolated from rutaceous plants, except for febrifugine and related alkaloids, which possess the quinazoline skeleton these are isolated from plants of the Saxifragaceae family. 

The biosynthetic pathway to rutacridone, which was isolated from Ruta graveolens (Rutaceae), was studied in detail using the cell culture method [4,5]. According to these results, the biosynthetic precursors of rutacridone are anthranilic acid, acetic acid, and an isopentenyl unit, as in the case of the quinoline alkaloids described in the previous section. However, in the biosynthesis of rutacridone, two additional C2 units participate in the biosynthesis of the acridone nucleus compared with the biosynthesis of quinoline alkaloids. 

In the biosynthesis of rutacridone, it is proposed that 1,3-dihydroxyacri-done is first formed from anthranilic acid and three C2 units, then the N-10 nitrogen is methylated to form 1,3-dihydroxy-N-methylacridone. Next, a C5 unit, IPP or DMAPP, is attached to 1,3-dihydroxy-N-methylacridone to fc>rm glycocitrine II, which is probably oxidized to produce an as-yet-uniden-tified epoxide. The epoxide is cyclized and dehydrated to give rutacridone [4]. Though rutacridone is a small molecule, as in the case of the quinoline alkaloids, three main biosynthetic precursors are involved in the biosynthesis of this alkaloid. Namely, the shikimic acid, the polyketide, and probably the iso-prenoid pathways all provide precursors for the biosynthesis of rutacridone. 

It is interesting that harmine, which is derived from tryptophan (Section 2.6), coexists with vasicine (peganine), which is derived from an anthranilic acid precursor in P. harmala.Th.uSy tryptophan in which the benzene moiety was labeled with was fed in the same way as described above to P. har-mala, and it was found that the was incorporated into vasicine (peganine), although the incorporation rate was low (0.071%). Thus, it was estimated that this plant also possesses the biosynthetic pathway to convert tryptophan into anthranilic acid. 

Benzoxazinones share a major part of the biosynthetic pathway of tr5 tophan. Label from quinic acid and anthranilic acid is incorporated into the aromatic ring of benzoxazinones. The carbon atoms of the heterocyclic ring stem from ribose. C-2 comes from C-2 of ribose, whereas C-3 comes from C-1 of ribose (Niemeyer, 1988). A number of questions concerning the nature of the biosynthetic reaction sequence remain to be answered. Heating these compounds produces benzoxazolin-2-ones and formic acid. Benzoxazinones decompose readily at higher pH (Niemeyer, 1988). 

The alkaloids reported in the Rutaceae can help to confirm the affinity between subfamilies because there is greater chemical diversity. On the other hand, the bacteria and fungi are needed for more substantial chemical studies. When more data become available, it is likely that useful systematic correlations will emerge. More detailed studies regarding the biosynthetic pathways of quinolines and quinazolines in the Rutaceae are needed. Such studies would clarify the differences in the pathways based on their derivation from anthranilic acid in bacteria and in rutaceous plants. 

Figure 1.6a. Structures of some Rutaceae alkaloids. This order is recognized for accumulating alkaloids derived from different biosynthetic pathways, e.g., from anthranilic acid, tyrosine, tryptophan, and histidine.

It was mentioned above that the carboxyl carbon is lost in the conversion of anthranilic acid to indole. Consequently, two additional carbon atoms must be supplied to complete the pyrrole ring of the indole. The observation that various ribose derivatives could be the source of these two carbons provided the clue that led to the elucidation of the mechanism of indole synthesis in the tryptophan biosynthetic pathway (232). Yanofsky determined that sonic extracts of a tryptophan auxotroph of E. cdi (that also grew on anthranilic acid or indole) could utilize ribose, ribose 5-phosphate, and 5-phosphoribosylpyrophosphate to form indole from anthranilic acid. With the two former compounds, ATP was essential for the reaction, with the latter compound it was not. This result made it appear evident that 5-phosphoribosylpyrophosphate was the more immediate reactant in the condensation with anthranilic acid. 

Anthranilic acid (or o-amino-benzoic acid) is an aromatic acid with the formula C H NO, which consists of a substituted benzene ring with two adjacent, or "ortho- functional groups, a carboxylic acid, and an amine (Fig. 14.1). Anthranilic acid is biosynthesized from shikimic acid (for shikimic acid biosynthesis, see Chapter 10) following the chorismic acid-mediated pathway [1]. Based on its biosynthetic mechanism, shikimate is transformed to shikimate 3-phosphate with the consumption of one molecule of ATP, catalyzed by shikimate kinase. 5-Enolpyruvylshikimate-3-phosphate (EPSP) synthase is then catalyze the addition of phosphoenolpyruvate to 3-phospho-shikimate followed by the elimination of phosphate, which leads to EPSP. EPSP is further transformed into chorismate by chorismate synthase. Chorismate reacts with glutamine to afford the final product anthranilate and glutamate pyruvate catalyzed by anthranilate synthase (Fig. 14.1). 

Phenazines compounds based on the phenazine ring system (Table). All known naturally occurring P. are produced only by bacteria, which excrete them into the growth medium. Both six-membered carbon rings of P. are biosynthesized in the shikimate pathway of aromatic biosynthesis, via chorismic acid (not from anthranilate, as reported earlier). The earliest identified biosynthetic intermediate after chorismate is phenazine 1,6-dicarboxylate, which has been isolated from Pseudomonas phenazinium and from non-... 

Of the enzymes associated with aromatic amino acid biosynthesis, there is good evidence that unique isozymes of DAHP (Rubin and Jensen, 1985), chorismate synthetase (d Amato et al 1984), and anthranilate synthase (Brotherton et ai, 1986) are differentially localized within chloroplasts and in the cytoplasm. The regulatory properties of the plastid isozymes are consistent with their involvement in amino acid synthesis. Many of the remaining pathway enzymes have also been detected in plastids, including all those required for the synthesis of EPSP [(1) to (6)] (Mousdale and Coggins, 1985). These results, combined with those obtained during measurements of the biosynthetic capabilities of isolated chloroplasts (Bickel and Schultz, 1979 Buchholz and Schultz, 1980 Schulze-Siebert et ai, 1984), leave little doubt that these organelles are a primary site of aromatic amino acid biosynthesis. 

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