Tryptophan in plants

For further investigation of L-tryptophan biosynthesis, labelled antbranilate waus used. C-labelled anthranilic acid is converted uniformly to L-tryptophan in plant tissues. Furthermore, labelled lAA was formed from 3- 0-L-tryptophan. Both metabolic sequences are subject to interference by indole metabolites (intermediates of lAA biosynthesis) and by phenols. p-Coumario acid - one of the phenolic substances that inhibit cell elongation - depresses the formation of tryptophan from C-anthra-nilic acid, whereas caffeic acid, which enhances cell elongation, stimulates the incorporation of anthranilic acid into tryptophan (Table 2). Thus, one control effect in the action of phenols on plant growth may be influence on L-tryptophan biosynthesis. 

A compound of unsuspected importance was isolated in 1885 from the fruit of Illicium religiosum. To this compound was given the name shikimic acid, a name derived from shikimi-no-ki which is the Japanese name for the plant. Shikimic acid (5.7), it transpired from the very elegant studies of much later investigators, is a key intermediate in the biosynthesis of the aromatic amino acids, L-phenylalanine, L-tyrosine and L-tryptophan, in plants and micro-organisms (animals cannot carry out de novo synthesis using this pathway). These three aromatic amino acids are individually important precursors for numerous secondary metabolites, and so to some extent are earlier biosynthetic intermediates related to shikimic acid, as the ensuing discussion in this chapter and in Chapters 6 and 7 will show. 

Fig. 3. The pathway of de novo purine ribonucleotide biosynthesis. The pathway includes the synthesis of PRPP, which is also used in the synthesis of pyrimidines, pyridine nucleotides, histidine, and tryptophan in plants. The enzymes catalyzing the numbered reactions are (1) PRPP synthetase, (2) PRPP amidotransferase, (3) GAR synthetase, (4) GAR transformylase, (5) FGAR amidotransferase, (6) AIR synthetase, (7) AIR carboxylase, (8) succino-AICAR synthetase, (9) adenylosuccinase, (10) AICAR transformylase, and (11) IMP cyclohydrolase.

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