3. Cyanogenic Glycosides - Introduction
Cyanogenic glycosides are natural plant toxicants.
They are widely distributed in the plant kingdom including
many that are commonly consumed by humans.
Cyanogenic Glycosides are Nitrogen containing Secondary
metabolites.
Amygdalin is the most common of the cyanogenic glycosides.
It occur as Glycosides which are stored in the vacuole.
4. Cyanogenic Glycosides - Occurrence
It is present in abundance in the seeds and kernels of fruits such as
apricot, almond, apple, cherry, plum, peach and nectarine.
When cyanogenic plants are ingested by humans, enzymes produced by
the intestinal microflora are able to hydrolyse intact cyanogenic
glycoside to produce hydrogen cyanide in vivo.
More than 60 cyanogenic glycosides are known which are widely
distributed among plants.
More than 2600 cyanogenic taxa have been reported especially
Rosaceae, Leguminosae, Gramineae and Araceae.
5. Cyanogenic Glycosides - Occurrence
Few insects, like Zygaenidae ( Lepidoptera) feed on plants with
cyanogens and sequester these compounds but are also capable of
synthesising the same cyanogens independently themselves.
Certain bacteria and fungi produce HCN by a mechanism which does
not involve cyanogenic glycosides or cyanolipids.
6. Biochemistry of Cyanogenic Glycosides
• Cyanogenic glycosides mainly composed of alpha- hydroxy nitrile
type aglycone and of sugar moiety (mostly D- glucose).
• Cyanogens are derivatives of 2-hydroxynitriles which often from
glycosides with D- glucose.
Phytochemical analysis (Spectroscopy, Chromatography) of
cyanogens
Easily hydrolyze by dilute acid to nitriles which further
decompose to aldehydes or ketones and HCN.
7. Biochemistry of Cyanogenic Glycosides
• Cyanogenic glycosides ar polar substances
they donot diffuse across biomembranes,
such as the tonoplast by simple diffusion.
• A carrier mediated transport system or
vesicle fusion might exist to facilitate an
exchange from cytoplasm into the vacuole.
10. Biosynthesis of Cyanogenic Glycosides
• Cyanogenic Glycosides derived biogenetically from L-
Aminoacids seems to be catalyzed by a multienzyme complex.
• In the first step, the amino group of L-aminoacids is hydrolyzed
by L-aminoaicd N-monooxygenase.
• Upon oxidative decarboxylation, the N-hydroxy –L-aminoacid is
converted into an aldoxime.
• Aldoxime to nitrile is catalyzed by an aldoxime dehydratase.
11. Biosynthesis of Cyanogenic Glycosides
• Nitrile is then hydroxylated at the C2-position by a
Nitrile monooxygenase to yield the key intermediate
2-hydroxynitrile (or cyanohydrin).
• Glucosyltransferase forms the glucoside using
activated glucose, i.e. UDP-glucose.
14. Functions of Cyanogenic Glycosides
• Cyanogenic glycosides serve as important chemical weapons in the
defence of the plant against herbivores because of the potential to
generate toxic hydrogen cyanide.
• Cyanogenic Glycosides are defense compounds which are activated by
action of Glucosidase in case of emergency, releasing the deterrent:
toxic Cyanide from cyanogens or isothiocyanates from glucosinolates.
• Cyanogens considered as active and potent chemical defense
compounds.
15. Functions of Cyanogenic Glycosides
• When plants are wounded by herbivores or other organisms, the
cellular compartmentation breaks down and cyanogenic glycosides
come into contact with an active glycosidase of broad specifically,
which hydrolyzes them to yield 2-hydroxynitrile (Cyanohydrin).
• 2-Hydroxynitrile is further cleaved into the corresponding aldehyde or
ketone and HCN by Hydroxynitrile lyase.
• HCN is highly toxic for animals or microorganism due to its
inhibition of enzymes of the respiratory chain (i.e. Cytochrome
Oxidases) and its binding to other enzymes containing heavy metal ions.
16. Glucosinolates
• Nitrogen and sulphur containing secondary metabolites.
• It is classified as thioglucosides.
• When hydrolyzed, liberate D-glucose, sulfate and an unstable aglycone.
• It form isothiocyanate (mustard oil) as the main product under certain
conditions.
• It form Thiocyanate, nitrile, cyano-epithioalkane( Responsible for
distinctive, pungent flavor and odour of mustard and horseradish).
21. Occurrence of Glucosinolates
• 80 different glucosinolates have been found
in higher dicotyledonous plants in the orderr
Capparales,
• In plant families of Capparidaceae,
Cruciferae, Resedaceae, Moringaceae,
Tropaeolaceae.
22. Biochemistry of Glucosinolates
• Glucosinolates are polar molecules.
• It is formed in the cytoplasm and stored in vacuoles
• Glucosinolates are fairly stable at neutral pH values.
• It occur as salts with cations such as K+ or sinapine to balance the
negative charges on the glucosinolate anions.
• It is volatile allelochemicals with a pungent smell and taste (Benzyl
isothiocyanates).
• It easily penetrate biomembranes, they can interact with epitermal and
mucosal skin leading to painful irritations.
23. Biosynthesis of Glucosinolates
• Protein and non protein aminoacids serve as precursors for the
biosynthesis of glucosinolates.
• Pathway leading to the corresponding Aldoxime is analogues to that
of cyanogenic glycosides.
• Aldoxime is converted into a thiohydroximic acid a key
intermediate, using L-cysteine as a sulfur donor.
• A Thioglucoside Is formed in the next step with aid of a UDP-
glucose : thiohydroximate glucosyltransferase.
26. Functions of Glucosinolates
• Isothiocyanates are antibiotic; besides making bacterial or fungal
cells leaky.
• Glucosinolates considered as preformed defense chemicals which are
activated in case of emergency.
• It is very important especially wide range of activities in plant
herbivore but also in plant-plant and plant- microbe interactions.
• Homosapiens can be considered a glucosinolate addict, since plants
containing them are used as spices and condiments(Cabbage,
Radish, Horseradish, mustard, flowers).
