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Glycosides

Glycosides are generally conjugates from sugars, in specific cases the conjugates from monosaccarides. Here are the glycosides of secondary natural products are discussed. These are the glycosides in which the secondary natural products are bound to a sugar. Examples are the vanillosid, the cardiac glycosides or salicin.

Structure and Nomenclature

Glycosides are defined by IUPAC as:

Glycosides were originally defined as mixed acetals (ketals) derived from cyclic forms of monosaccharides.

The formation of the acetal is carried from the hemiacetal, which represent the cyclic form of sugars. The formation of the mixed acetal may result in the formation of oligosaccharides or polysaccharides (such as starch). These are sugar-sugar compounds carry no non-sugar moiety. However, it also non-sugar residues are possibly attached. These are called as opposed to sugar residues aglycone. The sugar residues are then called glycone.

Aglycone and sugar compound

The glycosides can be divided in different sub-classes depending on the atom which is involved in the bonding between glycone and aglycone. There are glycosides (O),thioglycosides (S), N-glycosides (N) or C- glycosides (C). In contrast to the other glycosides the C-glycosides are not longer acetals.

Glycosidic bond

The glycosidic bond is chemically a mixed acetal. By cyclisation the sugar first forms a hemiacetal and a new stereocenter is created (alpha or beta sugar). In contrast to the other OH groups, which are alcohol groups, the OH group is chemically an acetal, which means that it has a higher reactivity.

Reasons for the glycosylation

The main reasons of glycosylation are the change in solubility of aglcone, its stabilization and the attenuation of the reactivity. To obtain the aglycone the cleavage of the glycosidic bond is provided. This can take place by fermentation.

Stabilization of aglycones

Reactive natural products can be stabilized through glycosidation. Even a prefered structure of the aglycone can be formed and for this the sugar can be seen as protecting group. With the glycosidation the aglycone lost one nucleophilic centre and therefore the aglycone has a lower reactivity.

Change in solubility

Often secondary natural substances aren't water soluble. This reduces their bioavailability and the plant isn't able to use the compound. By glycosidation the solubility of the aglycone can be changed. Sugars are highly polar, soluble substances. Thus, it may be possible to convert the formerly insoluble aglycone in soluble glycosides. As glycoside the aglycone can take part in the aqueous substance transport.

Protecting the plant against aglycone

Plants biosynthesizing also toxins or toxic compounds, and also substances that are antimicrobial or cytotoxic. Since these can be toxic to the plants, the toxic aglycone must be stored in a non-toxic form. One strategy is the glycosylation .

Example sinigrin

Sinigrin

Sinigrin is the storage form of allyl isothiocyanate. Allyl isothiocyanate is highly reactive and is classified as toxic and dangerous for the environment. In humans and animals, it is perceived as sharp, pungent and lachrymatory. Allyl isothiocyanate is used for defense against herbivores. For this sinigrin is cleaved by the enzym myrosinase and the aglycone is transformed to allyl isothiocyanate. In contrast, sinigrin is harmless to organisms.

Sinigrin is part of the black mustard (Brassica nigra) and various other types of mustard.

Detoxification

In the human body, in animals and in plants, foreign substances are glycosylated in the metabolism. This is one of the strategies of the organism's transforme and eliminate foreign substances. Reasons for this are the previously mentioned changes in the solubility and the change in reactivity.

Biosynthetic aspects

Biosynthesis of glycosides

In plants, the glycosides of glucose are synthesized on an activated molecule glucose and the aglycone. Starting point of the synthesis is glucose which is phosphorylated by ATP. After isomerization glucose is activated by uridine triphosphate. The result is UDP-glucose, which contains an energy-rich anhydride. In the reaction with the aglycone the various glycosides are formed.

In addition to this general scheme there are other ways of synthesize glycosides.

Examples

Name Exemplary representatives Structural formula
Aucubin Plantain(Plantago lanceolata L.)
Cardic glycosides Digitalis (Digitalis L.)
Salicin Salix
Steviol glycoside Sweet leaf (Stevia rebaudiana)
Vanilloside Vanilla (Vanilla planifolia)

Sources

glycosides.txt ยท Last modified: 10.12.2014 by naturalproductswiki