Heteropolysaccharides: Characteristics, Structure, Functions

The heteropolysaccharides  or heteroglycans are a group of complex carbohydrates classified within the group of polysaccharides, which include all carbohydrates that are composed of more than 10 monosaccharide units of different types of sugars.

Most heteropolysaccharides that are synthesized in nature usually contain only two different monosaccharides. Meanwhile, synthetic heteropolysaccharides generally possess three or more different monosaccharide units.

Example of the basic unit of a heteropolysaccharide (Source: Ccostell [Public domain] via Wikimedia Commons)

Heteropolysaccharides are macromolecules that fulfill essential functions for life. They are composed of multiple different sugar monomers (monosaccharides), repeatedly linked together by glycosidic bonds of various types.

Among the complex carbohydrates found most frequently in nature are hemicellulose, pectins, and agar-agar, and most of these are polysaccharides of commercial interest to the food industries.

In the medical context, the most studied heteropolysaccharides have been those of connective tissue, those of blood groups, those associated with glycoproteins such as γ-globulin and glycolipids that coat neurons in the central nervous system.

Over the years and with scientific advances, different techniques have been developed for the study of heteropolysaccharides, which generally involve their decomposition into their constituent monosaccharides and their individual analysis.

These separation techniques are different for each carbohydrate and depend on the physical and chemical characteristics of each carbohydrate. However, chromatography is the most widely used techniques for the analysis of heteropolysaccharides.

Article index

  • one

    Characteristics and structure

    • 1.1

      Bacterial carbohydrates

  • two

    Features

  • 3

    Examples

    • 3.1

      Hemicellulose

    • 3.2

      Pectin

    • 3.3

      Heparin

    • 3.4

      Hyaluronic acid

  • 4

    References

Characteristics and structure

Heteropolysaccharides are linear or branched polymers composed of repeating units of two or more different monosaccharides. It should be borne in mind that these monosaccharides may or may not be in the same proportion.

Heteropolysaccharides have complex structures, with a generally branched topology and, in their native state, have an asymmetric and somewhat amorphous morphology.

The repeating units that make up heteropolysaccharides (monosaccharides, disaccharides, or oligosaccharides) are linked together by α- or β-glucosidic bonds. In these units, it is common to observe modifications or substitutions such as methyl and acetyl groups and others, especially in the branches.

Furthermore, the association of certain molecules with heteropolysaccharides can give the latter a net charge, which has important physiological functions in various types of cells.

Bacterial carbohydrates

Microbial heteropolysaccharides are composed of repeating units of three to eight monosaccharides that can be linear or branched. They are generally composed of the monosaccharides D-glucose, D-galactose and L-rhamnose in different proportions.

Fucose, mannose, ribose, fructose, monosaccharides and monosaccharides substituted with glycerol and others can be obtained, although to a lesser extent.

Features

Typically, heteropolysaccharides function as extracellular supports for organisms of all kingdoms, from bacteria to humans. These sugars, together with fibrous proteins, are the most important components of the extracellular matrix in animals and of the intermediate lamina in plants.

Heteropolysaccharides are frequently found in association with proteins to form proteoglycans, glycosaminoglycans, and even mucopolysaccharides. These perform various functions, from regulating the absorption of water, acting as a kind of cellular “cement” and functioning as a biological lubricant, among many others.

Heteropolysaccharides in connective tissues have acid groups in their structures. These act as bridges between the water molecules and the metal ions. The most common heteropolysaccharide in these tissues is uronic acid with sulphated substitutions.

Proteoglycans can be found as structural elements of the plasma membrane, acting as coreceptors in the reception of stimuli on the surface of the cell membrane and stimulating internal response mechanisms.

Globulins are glycoproteins that are part of the immune system of many animals and base their recognition system on the portion of the heteropolysaccharides that they have in their outermost layer.

Heparins have anticoagulant functions and are mucoglacans that use disaccharides with sulfated substituents to reduce their negative charge and interfere with the union between thrombin and platelets, promoting, in turn, the binding of antithrombins and inactivating prothrombins.

Examples

Hemicellulose

This term encompasses a group of heteropolysaccharides that include monosaccharides such as glucose, xylose, mannose, arabinose, galactose, and various uronic acids in their structure. However, the most common structures are linear polymers of xylanes and xyloglycans linked by β-1,4 bonds.

These heteropolysaccharides are very abundant in the cell wall of plants. They are also soluble in concentrated alkaline solutions and some types develop a fibrillar form where they act as cementing agents in plant tissue.

Pectin

Pectins are polysaccharides of the middle sheet between cell walls of primary origin in plants. Its main component is D-galacturonic acid linked by an α-D-1,4 bond, in which some carboxyls can be esterified with methyl groups.

This type of sugar has the ability to easily polymerize on contact with methyl esters and other sugars such as galactose, rabbinose, and rhamnose. They are widely used in the food industry to give firmness to some products such as jams, compotes and sugary gums.

Heparin

It is an anticoagulant that is produced in the blood and in various organs such as the lungs, kidneys, liver and spleen of animals. It is made up of 12 to 50 repeats of D-glucuronic acid or L-iduronic acid and N -acetyl-D-glucosamine. Heparins are polysaccharides of the glycosaminoglycan type with a strong negative charge.

Heparins are of great industrial importance and are obtained artificially from genetic engineering in bacteria or naturally from the lungs of cattle or the intestinal mucous membranes of pigs.

Hyaluronic acid

This is one of the drugs most used in the aesthetics industry as a lubricant due to its viscous, elastic and rheological properties. It is used as an eye lubricant, a shock absorber in the joints and to delay aging processes, since it reduces the activity of cells in the cell cycle.

It is a polymer belonging to the group of glycosaminoglycans and is composed of D-glucuronic acid and N -acetyl-D-glucosamine, linked together by a β-1,3 bond. It is found in almost all prokaryotic and eukaryotic cells, especially in connective tissues and skin of animals.

References

  1. Delgado, LL, & Masuelli, M. (2019). Polysaccharides: Concepts and Classification. Evolution in Polymer Technology Journal , 2 (2), 2–7.
  2. Huber, KC, & BeMiller, JN (2018). Carbohydrates. In Organic Chemistry (pp. 888-928). Elsevier Inc.
  3. Davison, E. (1999). Encyclopaedia Britannica. Retrieved August 14, 2019, from www.britannica.com/science/carbohydrate/
  4. Huber, KC, & BeMiller, JN (2018). Carbohydrates. In Organic Chemistry (pp. 888-928). Elsevier Inc.
  5. The University of Maine. (nd). Retrieved August 14, 2019, from www.umaine.edu

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