GLYCOSIDES: FORMATION, FUNCTION AND TYPES / GROUPS - BIOLOGY - 2025

The glycosides are metabolites side plants that are bound to mono- or oligosaccharides through glycosidic linkages, which are metabolites is glycosylated. They belong to the chemical family of glycosides, which include all chemical compounds attached to sugary residues.

In the typical structure of a glycoside molecule, two regions are recognized: algicone and glycone. The region made up of the saccharide residue is called the glycon and the region corresponding to the non-saccharide molecule is known as the aglycone portion.

Structure of a glycoside (Source: Yikrazuul via Wikimedia Commons)

Commonly, the term "glucoside" is used to refer to the fact that glucose molecules are released during the hydrolysis of these compounds, however, members of the same family of molecules have residues of other types of sugar such as rhamnose, galactose or mannose, among others.

The nomenclature of glycosides typically denotes the nature of their aglycone region. Those names with the ending "-ina" are reserved for nitrogenous compounds, while alkaloids are named with the suffix "-ósido".

These suffixes often accompany the root of the Latin name of the botanical origin where the molecules are described for the first time and the prefix "gluco-" is usually added.

The glycosidic bond between the glycon and aglycone portions can occur between two carbon atoms (C-glucosides) or oxygen atoms (O-glucosides) can participate, on which their stability against chemical or enzymatic hydrolysis will depend.

The relative abundance of glycosides in angiosperms is much higher than in gymnosperms and it has been shown that with respect to monocots and dicots, with some exceptions, there is no great difference in the amount and types of glycosides found.

It is important to emphasize the great diversity and heterogeneity of this group of compounds, since the identity of each one will depend on the aglycone portion, which is highly variable.

Training

The biosynthesis or formation of glycosidic compounds (Peng, Peng, Kawagoe, Hogan, & Delmer, 2002) in plants depends on the type of glycoside considered, and in plants, their biosynthesis rates often depend on conditions. environmental

Cyanogenic glycosides, for example, are synthesized from amino acid precursors, including L-tyrosine, L-valine, L-isoleucine, and L-phenylalanine. Amino acids are hydroxylated to form N-hydroxyl amino acids that are subsequently converted to aldoximes, which are then transformed into nitriles.

Nitriles are hydroxylated to form α-hydroxynitriles, which can be glycosylated to form the corresponding cyanogenic glucoside. Two multifunctional cytochromes known as P450 and glycosyltransferase enzymes are involved in this biosynthetic pathway.

For the most part, the biosynthetic pathways of glycosides involve the participation of glycosyltransferase enzymes, which are capable of selectively transferring carbohydrate residues from an intermediate activated by a UDP molecule, to the corresponding aglycone portion.

The transfer of activated sugars, such as UDP-glucose, to an acceptor aglycone moiety helps stabilize, detoxify, and solubilize metabolites in the final steps of secondary metabolite-producing pathways.

Thus, the glycosyltransferase enzymes are responsible for the great variety of glycosides in plants and for this reason they have been extensively studied.

Some in vitro synthetic methods exist to obtain glycoside derivatives from plants that involve reverse hydrolysis or trans glycosylation systems of compounds.

Function

In plants, one of the main functions of flavonoid glycosides, for example, has to do with protection against ultraviolet light, against insects and against fungi, viruses and bacteria. They serve as antioxidants, pollinator attractants, and plant hormone controllers.

Other functions of flavonoid glycosides include stimulation of nodule production by bacterial species of the genus Rhizobium. They can participate in enzyme inhibition processes and as allelopathic agents. Thus, they also provide a chemical defense barrier against herbivores.

Many glycosides, when hydrolyzed, generate glucose residues that can be used by plants as a metabolic substrate for energy production or even for the formation of structurally important compounds in cells.

Anthropocentrically speaking, the function of these compounds is very diverse, since while some are used in the food industry, others are used in the pharmaceutical industry to design drugs for the treatment of hypertension, circulatory disorders, anticancer agents, etc.

Types / groups

The classification of glycosides can be found in the literature based on the non-saccharide portions (aglycones) or on the botanical origin of these. The following is a form of classification based on the aglycone portion.

The main groups of glycosides correspond to the cardiac glycosides, the cyanogenic glycosides, the glucosinolates, the saponins and the anthraquinone glycosides. Some flavonoids also commonly occur as glycosides.

Cardiac glycosides

These molecules are generally composed of a molecule (aglycone region) whose structure is steroid. They are present in plants of the Scrophulariaceae family, particularly in Digitalis purpurea, as well as in the Convallariaceae family with Convallaria majalis as a classic example.

This type of glycoside has a negative inhibitory effect on the sodium / potassium ATPase pumps in cell membranes, which are especially abundant in cardiac cells, so the ingestion of plants with these secondary compounds has direct effects on the heart; hence its name.

Cyanogenic glycosides

They are chemically defined as α-hydroxy nitrile glycosides, which are derived from amino acid compounds. They are present in angiosperm species of the Rosaceae family, particularly in species of the genus Prunus, as well as in the Poaceae family and others.

It has been determined that these are part of the characteristic toxic compounds of some varieties of Manihot esculenta, better known in South America as cassava, yucca or cassava. Likewise, they are abundant in apple seeds and in nuts such as almonds.

The hydrolysis of these secondary metabolites ends in the production of hydrocyanic acid. When hydrolysis is enzymatic, the glycon and aglycone portions are separated, the latter being able to be classified as aliphatic or aromatic.

The glycone portion of cyanogenic glycosides is typically D-glucose, although gentobiose, primeverose, and others have also been seen, mostly linked by β-glucosidic bonds.

Consuming plants with cyanogenic glycosides can have negative effects, including interference with iodine utilization, resulting in hypothyroidism.

Glucosinolates

The base of its aglycone structure is made up of sulfur-containing amino acids, which is why they could also be called thioglycosides. The main family of plants associated with the production of glucosinolates is the Brassicaceae family.

Among the negative effects for the organisms that ingest these plants are the hepatic bioactivation of environmental procarcinogens, which is the product of complex effects on isoforms of cytochrome P450. Additionally, these compounds can irritate the skin and induce hypothyroidism and gout.

Saponins

Many "soap-forming" compounds are glycosides. The aglycone part of the glycosidic saponins consists of pentacyclic triterpenoids or tetracyclic steroids. They are structurally heterogeneous, but have common functional characteristics.

In their structure they have highly hydrophilic glycone portions and strongly hydrophobic aglycone regions, which provide emulsifying properties, so they can be used as detergents.

Saponins are present in a wide range of plant families, among which are the species belonging to the Liliaceae family, exemplified in the Narthecium ossifragum species.

Anthraquinone glycosides

They are less common in the plant kingdom compared to the other glycosides mentioned above. They are present in Rumex crispus and species of the genus Rheum. The effect of its ingestion corresponds to an exaggerated secretion of water and electrolytes accompanied by peristalsis in the colon.

Flavonoids and pro-anthocyanins

Many flavonoids and their oligomers, pro-anthocyanins, occur as glycosides. These pigments are very common in much of the plant kingdom, with the exception of algae, fungi and some hornworts.

They can exist in nature as C- or O-glucosides, depending on the nature of the glycosidic bond that occurs between the glycon and algicone regions, so some are more resistant to chemical hydrolysis than others.

The aglycone structure of the C-glucoside flavonoids corresponds to three rings with some phenolic group that provides them with the characteristic of antioxidants. The union of the saccharide group to the aglycone region occurs through carbon-carbon bonds between the anomeric carbon of the sugar and the C6 or C8 carbon of the aromatic nucleus of the flavonoid.

References

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