Aldoximes biosynthesis

HANSEN, C.H., WITTSTOCK, U., OLSEN, C.E., HICK, A.J., PICKETT, J.A., HALKIER, B.A., Cytochrome P450 CYP79F1 from Arabidopsis catalyzes the conversion of dihomomethionine and trihomomethionine to the corresponding aldoximes in the biosynthesis of aliphatic glucosinolates, J. Biol. Chem., 2001, 276, 11078-11085. 

Glucosinolate biosynthesis further characterization of the aldoxime-forming microsomal monoxygenases in oilseed rape leaves, Plant Physiol., 1995, 109, 299-305. 

For example, the anti (25) and syn (4-hydroxyphenyl)acetaldoximes, 26, are established intermediates in the biosynthesis of the cyanogenic glucoside of sorghum, dhurrin, 27, and the biochemical pathway for its production in the plant was shown to originate in the A -hydroxylation of tyrosine, in the presence of NADPH/O2, as outlined in equation 15". It was further suggested that the Z (syn) isomer, 26, is utilized preferentially over E(anti )-25 in the subsequent biosynthesis of dhurrin, 27. The same authors provided evidence that the biosynthesis of the aldoxime, 25, proceeds via an aci-nitro containing intermediate, R R C=N(0)0H, that is positioned between Af-hydroxytyrosine and anti-25 in the biosynthetic pathway . 

Bak, S., Nielsen, H.L. and Halkier, B.A. (1998) The presence of CYP79 homologues in glucosinolate-producing plants shows evolutionary conservation of the enzymes in the conversion of amino acids to aldoxime in the biosynthesis of cyanogenic glucosides and glucosinolates. Plant Mol. Biol., 38, 725-34. 

Bennett, R.N., Dawson, G.W., Hick, A.J. and Wallsgrove, R.M. (1995b) Glucosinolate biosynthesis further characterization of the aldoxime-forming microsomal monooxygenases in oilseed rape leaves. Plant Physiol., 109, 299-305. 

The Chi synthesis modulators that Rebeiz et al. (13, 14) used in conjunction with ALA could be divided into three categories A) enhancers of ALA conversion to porphyrins (2-pyridine aldoxime, 2-pyridine aldehyde, picolinic acid, 2,2 dipyridyl disulfide, 2,2 -dipyridyl amine, 4,4 dipyridyl, and phenanthridine), B) inducers of ALA biosynthesis and porphyrin accumulation (2,2 -dipyridyl and 1,10-phenanthroline), and C) inhibitors of MV PChlide synthesis (2,3-dipyridyl, 2,4-dipyridyl, 1,7-phenanthroline, and 4,7-phenanthroline). Compounds in group A did not cause significant porphyrin accumulation alone however, they enhanced dark conversion of exogenous ALA to porphyrins. This group was further subdivided into compounds that enhanced conversion of ALA to MV PChlide (2-pyridine aldoxime, 2-pyridine aldehyde, picolinic acid, and 2,2 -dipyridyl disulfide) and those that stimulated conversion to DV PChlide (4,4 dipyridyl, 2,2 dipyridyl amine, and phenanthridine). To qualify as an ALA biosynthesis and porphyrin accumulation inducer (category B), the compound had to cause these effects in the absence of ALA. Compounds in category C had to inhibit accumulation of MV PChlide with or without ALA. In most cases, in conjunction with ALA, the compounds stimulated DV PChlide accumulation compared to the ALA-treated control. 

Aldoximes are the first intermediates of the biosynthetic pathway firmly established. They are probably formed by the reactions shown to proceed in the biosynthesis of cyanogenic glycosides (D 9.3). The first well-defined sulfur-containing intermediates are thiohydroximic acids. The most efficient source of the... 

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... 

Figure 2.4 Summary of glucosinolate biosynthesis. Amino acids or elongated amino acids are converted to aldoximes by the action of enzymes encoded by the CYP79 gene family. The aidoxime conjugates with cysteine, which acts as the sulphur donor, which is then lysed and detoxified by glycosylation and sulphation. The MAM and CYP79 enzymes have a high degree of specificity to the amino acid structure, whereas other enzymes of the pathway have lower specificity.

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