9 julio, 2024

Polysaccharides: what they are, characteristics, structure, classification, examples

What are the polysaccharides?

The polysaccharides, often called glycans, are high molecular weight chemical compounds made up of more than 10 individual sugar units (monosaccharides). In other words, they are polymers of monosaccharides linked together through glycosidic bonds.

These are very common molecules in nature, as they are found in all living beings, where they perform a wide variety of functions, many of which are still being studied. They are considered the largest source of renewable natural resources on earth.

The plant cell wall, for example, is made up of one of the most abundant polysaccharides in the biosphere: cellulose.

This compound, made up of repeated units of a monosaccharide called glucose, serves as food for thousands of microorganisms, fungi and animals, in addition to the functions it has in maintaining the structure of plants.

Man, over time, has managed to take advantage of cellulose for practical purposes: he uses cotton to make clothing, the «pulp» of trees to make paper, etc.

Another very abundant polysaccharide, also produced by plants and of great importance to man, is starch, since it is one of the main sources of carbon and energy. It is in the cereal grains, in the tubers, etc.

Characteristics of polysaccharides

– They are macromolecules of very high molecular weight

They are composed mainly of carbon, hydrogen, and oxygen atoms.

– They are very diverse structurally and functionally speaking

– They exist in practically all living things on earth: plants, animals, bacteria, protozoa and fungi

– Some polysaccharides are highly soluble in water and others are not, which usually depends on the presence of branches in their structure

– They function in energy storage, in cell communication, in the structural support of cells and tissues, etc.

– Its hydrolysis generally results in the release of individual residues (monosaccharides)

– They can be found as part of more complex macromolecules, such as the carbohydrate portion of many glycoproteins, glycolipids, etc.

Structure of polysaccharides

Polysaccharides are polymers of more than 10 sugar or monosaccharide residues, which are linked together through glycosidic bonds.

Although they are highly diverse molecules (there are an infinite variety of possible structural types), the most commonly found monosaccharides in the structure of a polysaccharide are pentose and hexose sugars, that is, sugars with 5 and 6 carbon atoms, respectively.

Diversity

The diversity of these macromolecules lies in the fact that, in addition to the different sugars that can form them, each sugar residue can be in two different cyclic forms: furanose or pyranose (only those sugars with 5 and 6 carbon atoms).

In addition, glycosidic bonds can be in the α- or β- configuration and, as if that were not enough, the formation of these bonds can involve the substitution of one or more hydroxyl (-OH) groups on the adjacent residue.

They can also be formed by sugars with branched chains, by sugars without one or more hydroxyl groups (-OH) and by sugars with more than 6 carbon atoms, as well as by different derivatives of monosaccharides (common or not).

Polysaccharides with straight chains generally “pack” better into rigid or inflexible structures and are insoluble in water, unlike branched polysaccharides, which are highly soluble in water and form “pasty” structures in aqueous solutions.

Classification of polysaccharides

The classification of polysaccharides is usually based on their natural occurrence, however, it is increasingly common to classify them according to their chemical structure.

Many authors consider that the best way to classify polysaccharides is based on the type of sugars that compose them, according to which two large groups have been defined: that of homopolysaccharides and that of heteropolysaccharides.

Homopolysaccharides or homoglycans

To this group belong all the polysaccharides that are formed by units of identical sugars or monosaccharides, that is, they are homopolymers of the same type of sugar.

The simplest homopolysaccharides are those with a linear conformation, in which all the sugar residues are linked through the same type of chemical bond. Cellulose is a good example: it is a polysaccharide composed of glucose residues linked by β (1→4) bonds.

However, there are more complex homopolysaccharides and they are those that have more than one type of link in a linear chain and can even present ramifications.

Examples of very common homopolysaccharides in nature are cellulose, glycogen and starch, all made up of repeating units of glucose; This group also includes chitin, which consists of repeating units of N-acetyl-glucosamine, a derivative of glucose.

Then there are others less popular in the literature such as fructans (composed of fructose units), pentosans (composed of arabinose or xylose) and pectins (formed by derivatives of galacturonic acid, derived, in turn, from galactose).

heteropolysaccharides or heteroglycans

Instead, all those polysaccharides that are composed of two or more different types of sugars are classified within this group, that is, they are heteropolymers of different sugars.

The simplest heteropolysaccharides are made up of two dissimilar sugar residues (or sugar derivatives), which may (1) be in the same linear chain or (2) be one forming a main linear chain and the other forming side chains.

However, there may also be heteropolysaccharides formed by more than 2 types of highly branched sugar residues, or not.

Many of these molecules associate with proteins or lipids, forming glycoproteins and glycolipids, which are very abundant in animal tissues.

Very common examples of heteropolysaccharides are those that are part of mucopolysaccharides such as hyaluronic acid, widely distributed among animals and which is formed by glucuronic acid residues linked to N-acetyl-D-glucosamine residues.

Cartilage, present in all vertebrate animals, also contains abundant heteropolysaccharides, especially chondroitin sulfate, which is made up of repeating units of glucuronic acid and N-acetyl-D-galactosamine.

A general fact about the nomenclature

Polysaccharides are named with the generic term glycan, so the most precise nomenclatures use the prefix of the “parent sugar” and the ending “-ane” to give a name. For example, a polysaccharide based on glucose units can be called a glucan.

Examples of polysaccharides

Throughout the text we have cited the most common examples that undoubtedly represent this large group of macromolecules. Next, we will develop a little more some of them and we will mention others equally important.

Cellulose and chitin

Cellulose, a polymer of glucose residues, is, along with chitin, a polymer of N-acetyl-glucosamine residues, one of the most abundant polymers on earth.

The former is an essential part of the cell wall covering plant cells, and the latter is in the cell wall of fungi and the exoskeleton of arthropods, incredibly diverse and abundant invertebrate animals, including insects and insects. crustaceans, for example.

Both homopolysaccharides are equally important, not only for man, but for all biosphere ecosystems, since they form a structural part of the organisms that are at the base of the food chain.

glycogen and starch

Polysaccharides, among their multiple functions, serve as energy reserve material. Starch is produced in plants and glycogen is produced in animals.

Both are homopolysaccharides composed of glucose residues, which are linked through different glycosidic bonds, presenting numerous branches in quite complex patterns. With the help of some proteins, the two types of molecules can form more compact granules.

Starch is a complex formed by two different glucose polymers: amylose and amylopectin. Amylose is a linear polymer of glucose residues linked by α(1→4) bonds, while amylopectin is a branched polymer that is linked to amylose through α(1→6) bonds.

Glycogen, on the other hand, is also a polymer of glucose units linked by α(1→4) bonds and with numerous branches connected by α(1→6) bonds. This presents a number of ramifications significantly higher than starch.

heparin

Heparin is a glycosaminoglycan associated with sulfate groups. It is a heteropolysaccharide composed of glucuronic acid units, many of which are esterified, and N-glucosamine sulfate units that have an additional sulfate group on their carbon 6 linked by α(1→4) bonds.

This compound is commonly used as an anticoagulant, usually prescribed for the treatment of heart attacks and unstable angina pectoris.

Other polysaccharides

Plants produce many substances rich in complex heteropolysaccharides, including gums and other adhesive or emulsifying compounds. These substances are often rich in glucuronic acid polymers and other sugars.

Bacteria also produce heteropolysaccharides which, many times, they release into the environment that surrounds them, which is why they are known as exopolysaccharides.

Many of these substances are used as gelling agents in the food industry, especially those synthesized by lactic acid bacteria.

References

De Vuyst, L., & Degeest, B. (1999). Heteropolysaccharides from lactic acid bacteria. FEMS microbiology reviews, 23(2), 153-177.
Aspinall, G.O. (Ed.). (2014). The polysaccharides. Academic Press.
The Editors of Encyclopaedia Britannica (2019). Encyclopaedia Britannica. Retrieved April 18, 2020, from www.britannica.com/science/polysaccharide
Dische, ZACHABIAS (1955). Sugars in polysaccharides. In Methods of biochemical analysis (Vol. 2, pp. 313-358). Interscience New York.
Brown Jr, RM (2004). Cellulose structure and biosynthesis: what is in store for the 21st century? Journal of Polymer Science Part A: Polymer Chemistry, 42(3), 487-495.
Roach, P.J. (2002). Glycogen and its metabolism. Current molecular medicine, 2(2), 101-120.al of Polymer Science Part A: Polymer Chemistry, 42(3), 487-495.

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