Indole pathway tryptophan

In spite of laborious work, knowledge of the mechanism and of the possible intermediates of the pathway tryptophan —> kynurenic acid made little progress until the isolation of kynurenine (M4). This compound, whose structure (o-aminobenzoylalanine) became clear only many years later (B26), is the intermediate (as shown by S. Kotake), no longer an indole, between tryptophan and kynurenic acid (K12). 

Anthranilic acid and indole are precursors of tryptophan in numerous microorganisms and fungi (e.g., 5, 263, 264, 602, 741, 783, 785, 816, 854, 855, 876), and it is probable that anthranilic acid is derived, with intermediate steps, from the common precursor, CP of diagram 1. The conversion of anthranilic acid to indole and tryptophan has been shown unambiguously in Neurospora with the use of isotopic techniques (93, 663). There may, however, be other pathways for tryptophan biosynthesis (45, 702). Tryptophan can, for example, be formed by transamination of indolepyruvic acid (e.g., 470, 912), which might be formed other than via anthranilic acid. Thus aromatic-requiring mutants have been found which accumulate unidentified indole compounds (307). 

Interest then moved to animals. Both isotopic and nutritional experiments showed that the pathway established in microorganisms applied equally to mammals. Thus hydroxyanthranilic acid was converted to nicotinic acid (9, 604), which it could replace as a growth factor (944), whereas there was no similar conversion of anthranilic acid (343). An outstanding series of isotopic experiments, especially by Heidelberger and co-workers, showed that the 8-carbon atom of the tryptophan side chain became the 8-carbon atom of the kynurenine side chain and that the side chain was lost in conversion of kynurenine to nicotinic acid (369, 371, 427). Moreover the carbon in the 3-position of the indole nucleus became the carboxyl carbon of nicotinic acid (370 this experiment proved conclusively the reality of the tryptophan-nicotinic acid conversion) and the indole nitrogen appeared with only slight dilution in kynurenine, kynurenic acid, and xanthurenic acid (759). All these relations are those to be expected for the pathway tryptophan —+ kynurenine —> hydroxykynurenine (or its phosphate) —> hydroxyanthranilic acid (or its phosphate) — nicotinic acid, illustrated in diagrams 17 and 18. 

FIGURE 10.3 Pathways for degradation of L-tryptophan by (a) tryptophanase, (b) deamination and oxidation, and (c) side-chain oxidation and decarboxylation to indole. 

Alkaloids derived from L-tryptophan hold the indole nucleus in a ring system. The ring system originates in the shikimate secondary compounds building block and the anthranilic acid pathway. It is known that the shikimate block. 

The second part of the reaction requires pyridoxal phosphate (Fig. 22-18). Indole formed in the first part is not released by the enzyme, but instead moves through a channel from the a-subunit active site to the jS-subunit active site, where it condenses with a Schiff base intermediate derived from serine and PLP. Intermediate channeling of this type may be a feature of the entire pathway from chorismate to tryptophan. Enzyme active sites catalyzing different steps (sometimes not sequential steps) of the pathway to tryptophan are found on single polypeptides in some species of fungi and bacte-... 

Figure 25-12 Structures and some biosynthetic pathways for some hormones, indole alkaloids, and other metabolites of tryptophan.

Demonstration That Indole Is Not a True Intermediate in the Tryptophan Biosynthetic Pathway... 

Incorporation studies with isotopes showed that when anthranijate was converted to tryptophan, the carboxyl group df anthranilate was lost as carbon dioxide, but the nitrogen was retained. Because the enzymes in the tryptophan biosynthetic pathway have only a limited specificity, it was possible to substitute 4-methyl-anthranilate in E. coli extracts that could convert anthranilate to indole. This nonisotope label was conserved during the conversion to yield 6-methyl indole. 

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

Indoles are very important in medicinal chemistry. The indole moiety is electron-rich so it is very easily oxidized. One example of severe cleavage of the indole ring is the oxidation of tryptophan to V-formylkynurenine. An example of milder oxidation of indoles is reserpine degradation. Reserpine, an indole alkaloid, spontaneously decomposes in chloroform solution to give oxygenated products (58) (Fig. 11). Reserpine 7-hydroperoxide (XIV) was isolated in the reaction mixture this is the key intermediate in the oxidative pathways of many indoles (59). 

Within the natural products field, the investigation of complete biosynthetic pathways at the enzyme level has been especially successful for plant alkaloids of the monoterpenoid indole alkaloid family generated from the monoterpene gluco-side secologanin and decarboxylation product of tryptophan, tryptamine [3-5]. The most comprehensive enzymatic data are now available for the alkaloids ajmalicine (raubasine) from Catharanthus roseus, and for ajmaline from Indian Rauvolfia serpentina [6] the latter alkaloid with a six-membered ring system bearing nine chiral carbon atoms. Entymatic data exsist also for vindoline, the vincaleucoblastin (VLB) precursor for which some studies on enzymatic coupling of vindoline and catharanthine exist in order to get the so-called dimeric Catharanthus indole-alkaloids VLB or vincristine [7-9] with pronounced anti-cancer activity [10, 11]. 

Overall, the biosynthesis of 160 is characterized by the dimerization of 168 to give the central structure of the molecule. This head-to-tail dimerization strategy is efficient, using the same substrate twice, and is a sensible route, given the existence of the shikimate pathway, which provides, in turn, a precursor to 168. An analogous dimerization route can be seen for the biosynthesis of K252c (1), described in Sect. 5, where two molecules of indole-3-pyruvic acid imine (125), derived in turn from L-tryptophan (123), are dimerized to give an intermediate that leads to chromopyrrolic acid (128). In both cases, the monomer precursors, either 168 or 125, serve as both nucleophiles and electrophiles, and are activated to react by the presence of the appropriate enzymes. 

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