Biosynthesis of Cyanogenetic Glycosides

4.3.6 Biosynthesis of Cyanogenetic Glycosides

Cyanogenesis is the ability of living organisms that exist freely in the higher plants but instead it is released from the cyanogenetic precursors as a result of the enzymatic action. Now, it has been well established that these precursors are normally glycosides of hydroxynitriles (or cyanohydrins). Once the cellular integrity of a cyanophoric plant tissue is disrupted, the cyanogenetic glycosides in
turn are brought in contact with the respective catabolic enzymes that helps to hydrolyze the glycosides and ultimately give rise to the formation of hydroxynitriles.
There are two important cyanogenetic glycosides, namely: dhurrin nad prunasin which are present in a variety of plant families and genera as stated below*:

dhurrin
The two amino acids Tyrosine and Phenylalanine are considered to have derived from the
Shikimic Acid Pathway and Phenylalanine Pathway as depicted below:
(a) Shikimic acid Pathway: It is based on studies carried out with E. coli as follows:

Shikimic acid Pathway
(b) Phenylalamine Pathway: It starts with Shikimic acid as the starting material as shown under:

Phenylalamine Pathway
(c) Tyrosine from Phenylalanine: It is obtained by the oxidation of phenylalanine.

Tyrosine from Phenylalanine
(d) Dhurrin from Tyrosine: It is obtained from tyrosine as shown below:

Dhurrin from Tyrosine:
(e) Prunasin (or Prulaurasin) from Phenylalanine: It is obtained from phenylalanine as given below: It has been established that labelled shikimic acid and labelled tyrosine were equally effective precursors of the hydroxylated aglycone of Dhurrin, a cyanogenetic glycoside produced by Sorghum vulgare Linn. belonging to family Gramineae.


Prunasin
Likewise, introducing labelled phenylalanine to young cherry laurel plants ie; Prunus
laurocerasus, has proved that the amino acid acts as a precursor of Prunasin in young peach seedlings.
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* Seigler, D.S., Prog, Phytochem., 4, 83-120, 1977.
Seigler, D.S., Rev Latinoam Quim, 12, 39-48, 1981.

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