Amino acids glucosinolate precursors

Glucosinolates are derived from amino acids. The precursor amino acids comprise seven protein amino acids (alanine, valine, leucine, isoleucine, phenylalanine, tyrosine, and tryptophan), a number of chain-elongated methionines (CH3-S-(CH2)2-(CH2)n-CH(NH2)-COOH, n = 1-9), and chain-elongated... 

Figure 3.13 Side-chain elongation of amino acids. In analogy to the conversion of valine to leucine, the methene group is introduced to various other amino acids, which subsequently serve as precursors of glucosinolates.

Glucosinolates are divided into three classes based on the general chemical properties of the amino acid precursors. Aliphatic glucosinolates variously contain a straight carbon chain derived from methionine or a branched chain fix)m isoleucine, leucine or valine. Indole glucosinolates are formed from tryptophan, and the aromatic glucosinolates are derived from phenylalanine or tyrosine. ... 

The precursor substrates and enzymes necessary for the first committed steps often appear to have been recruited from primary metabolic pathways, such as glycolysis, the Krebs cycle, the pentose phosphate pathway and the shikimate pathway [32], For example, the aromatic amino acid s L-phenylalanine and L-tyrosine, produced by the shikimate pathway, are precursors for a wide spectrum of natural products including phenylpropanoids, flavonoids, lignins, coumarins, cyanogenic glycosides, glucosinolates, and alkaloids [33],... 

Thus, indole-3-acetic acid is biosynthesized both in higher plants and microorganisms via different pathways using the amino acid L-tryptophan as precursor. The same hormone is also obtained, for instance, by transformation of glucobrassicin, an indole glucosinolate widely distributed in Brassicaceae and related families (Eef.20). 

Isotopically labeled amino acids have been administered to excised plant parts. As is true for cyanogenic glycosides, incorporation of administered amino acid precursors into glucosinolates is often extremely high (up to 40%) (Larsen, 1981). In one study, plants were fed amino acids, and biosyn-... 

Nonprotein amino acids are usually found in the free state or as part of simple products such as y-glutamyl, acetyl or oxalyl derivatives. They also occur, however, as constituents of small peptides, such as the phalloidins and amanitins (D 23), or as precursors of other secondary products (see formation of glucosinolates, D 9.4). 

Glucosinolates are formed from amino acids (Table 44). Either protein or nonprotein amino acids act as precursors. Relatively often the nonprotein amino acids are homologs of the protein amino acids formed by the pathway of chain lengthening described for the biosynthesis of L-leucine from L-valine (D 14). 

Cyanogenic glycosides (Table 16.22) are present in lima beans and in some other plant foods. Precursors of cyanogenic glycosides are the amino acids listed in Table 16.22. As in the biosynthesis of glucosinolates (cf. 17.1.2.6.5), an aldoxime is initially formed, which is then transformed into... 

Among the most important volatile sulfur compounds are hydrogen sulfide (sulfan), various thiols, sulfides, isothiocyanates and heterocyclic sulfur compounds. Precursors of volatile sulfur compounds are usually non-volatile, sensory indifferent sulfur compounds, especially sulfur-containing amino acids cysteine, cystine, methionine and their derivatives, such as S-alk(en)yl cysteine sulfoxides, glucosinolates, thiamine and other compounds. An important sulfur compound is also sulfur dioxide, which is used as a preservative and an inhibitor of enzymatic browning reactions or the Maillard reaction. It also occurs in a small amount as a metabolite in fermentation processes. Sulfur compounds may be accompanied by their selenium analogues at very low concentrations. 

Two small but distinctive groups of natural products found in higher plants which show a clear and definite relationship to their biosynthetic precursors, the a-L-amino acids, are the cyanogenic glycosides (128)162,163 jjjg mustard oil glucosides or glucosinolates... 

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