Most cyanogenic glucosides are derived from the five hydrophobic amino acids, valine, leucine, isoleucine, phenylalanine and tyrosine (Poulton, 1990) (see also Dhurrin synthesis and metabolism under Secondary products derived from aromatic amino acids).

Trifolium repens accumulates a cyanogenic glucoside linamarin, derived from valine. Cassava (Manihot esculenta, Crantz) accumulates both linamarin and lotaustralin. The latter is derived from isoleucine (White et al, 1998; Andersen et al, 2000).

The pathway of synthesis of cyanogenic glucosides involves the conversion of an amino acid to an aldoxime, catalyzed by a cytochrome P-450 (green arrows in the reaction scheme below) [this reaction likely proceeds via two successive hydroxylations of the amino group, followed by decarboxylation and dehydration; only the first hydroxylated intermediate is shown in the reaction scheme below]. In cassava, CYP79D1 and CYP79D2 are the enzymes which catalyze the N-hydroxylation of valine and isoleucine, to their respective aldoximes (Andersen et al, 2000). The aldoxime is then converted to a hydroxynitrile through the action of a second cytochrome P-450. This reaction involves dehydration to a nitrile, followed by hydroxylation of the alpha carbon (orange arrows). The final step in cyanogenic glucoside synthesis is the glucosylation of the alpha-hydroxy moiety, catalyzed by a glucosyltransferase.

Cyanogenesis (release of HCN upon tissue disruption) in T. repens is dependent on at least two independent genes, Ac and Li, which encode a glucosyltransferase and linamarase activities, respectively.

Linamarase is a cell-wall associated B-glycosidase. Hydrolysis of linamarin by linamarase yields an unstable hydroxynitrile intermediate, acetone cyanohydrin plus glucose. Acetone cyanohydrin spontaneously decomposes to HCN and acetone at pH of greater than 5.0 or temperatures greater than 35^oC and can be broken down enzymatically by hydroxynitrile lyase (HNL) (White et al, 1998). The hydroxynitrile lyase of cassava has been purified, and shown to be absent in roots, accounting for the high acetone cyanohydrin levels in poorly processed cassava (White et al, 1998).

Andersen MD, Busk PK, Svendsen I, Moller BL 2000 Cytochromes P-450 from cassava (Manihot esculenta Crantz) catalyzing the first steps in the biosynthesis of the cyanogenic glucosides linamarin and lotaustralin. Cloning, functional expression in Pichia pastoris, and substrate specificity of the isolated recombinant enzymes. J. Biol. Chem. 275: 1966-1975.

Poulton JE 1990 Cyanogenesis in plants. Plant Physiol. 94: 401-405.

White WLB, Arias-Garzon DI, McMahon JM, Sayre RT 1998 Cyanogenesis in cassava. The role of hydroxynitrile lyase in root cyanide production. Plant Physiol. 116: 1219-1225.