Cyanogenic Glycosides

from Wikipedia, the free encyclopedia
Cleavage of cyanogenic glycosides.

Cyanogenic glycosides are widespread plant toxins from the group of glycosides . They consist of a glycoside, i. H. a compound of an alcohol and a carbohydrate ( sugar ), which also has a nitrile group - CN. During the enzymatic cleavage, this results in u. a. of toxic hydrogen cyanide HCN , including hydrogen cyanide (the term therefore called cyanogen ).

Representative

effect

Cyanogenic glycosides themselves have no toxic effect; only when the molecule is split does the hydrogen cyanide (HCN), which is actually the toxic substance, be released. The degradation is initially carried out enzymatically . The sugar component (mostly glucose ) is split off by a more or less specific β-glucosidase . The free cyanohydrin is produced . This decomposes spontaneously or with enzyme catalysis by a hydroxynitrile lyase into a ketone or aldehyde and HCN.

The initial splitting off of the sugar content is decisive for this process, known as cyanogenesis. For example, the enzyme linamarase is required to split the glycoside linamarine . In the intact plant, both substances are present separately in different organelles and the glycoside is retained (compartmentalization). This has the advantage that the toxins are only released when the plant cells are damaged (e.g. by insect damage ) and the poison is thus produced directly (decomposition, plant defense against herbivores ).

Toxicity of hydrogen cyanide

The hydrogen cyanide released by cyanogenic glycosides is highly toxic to almost all animals. The reason for this is that hydrocyanic acid intervenes in the metabolism . The photosynthesis of the plants is not affected. The energy metabolism of the animals, however, is paralyzed within a very short time.

HCN binds to the central iron ion (oxygen binding site) of the enzyme cytochrome c oxidase , which causes a fundamental redox reaction in the respiratory chain to take place. This irreversibly inactivates the enzyme. If a certain proportion of the cytochrome c oxidase is bound by HCN, the respiratory chain fails and the organism receives no more energy.

Cyanogenic plants

Examples of plants whose toxicity is based on the release of hydrogen cyanide from cyanogenic glycosides are many rosaceae such as plum ( Prunus domestica ), blackthorn ( Prunus spinosa ), apricot ( Prunus armeniaca ), almond ( Prunus dulcis ), peach ( Prunus persica ), Sour cherry ( Prunus cerasus ), ferns such as the golden spotted fern ( Phlebodium aureum ), sorghum and cassava ( Manihot esculenta ).

Polymorphisms in white clover and horn clover

A polymorphism of cyanogenic glycosides occurs in Europe in various clover populations. Breeding experiments show that two independent genes are responsible for this.

One of the genes controls the synthesis of the cyanogenic glycosides. The plant can only produce these substances if this is dominant.

The second gene is responsible for the enzyme that is required to break down the glycosides, making them dangerous. There are therefore four different types of clover in terms of their toxicity from cyanogenic glycosides. Toxic effects only occur if both genes are dominant. Otherwise the plants can synthesize the glycosides, but do not allow them to develop. Or the enzymes are present, but no glycosides that could be broken down. In the fourth case, neither glycosides nor enzymes are present.

The genes in the plant can be investigated by chemical tests: you put fresh leaves in a test tube, briefly crush them with a glass rod together with a drop of chloroform and close the tube with a stopper, a piece of which is in picric acid solution dipped filter paper hangs down. A color within an hour indicates that hydrogen cyanide is present and that both genes are dominant.

literature

  • Jeffrey B. Harborne: Ecological Biochemistry. An introduction. Spektrum Akademischer Verlag, Heidelberg / Berlin / Oxford 1995, ISBN 3-86025-245-3 (Description of the effects of plant substances on animals.)