Glucans: Structure, Characteristics And Functions

The glucans are perhaps the most abundant carbohydrates in the biosphere. Most make up the cell wall of bacteria, plants, yeasts, and other living organisms. Some make up the reserve substances of vertebrates .

All glucans are made up of one type of repeating monosaccharide: glucose. However, these can be found in a great variety of forms and with a great variety of functions.

The name glucan has its main origin from the Greek word ” glykys “, which means “sweet.” Some textbooks refer to glucans as non-cellulosic polymers made up of glucose molecules linked by β 1-3 bonds (when saying “non-cellulosic”, those that are part of the cell wall of plants are excluded from this group) .

However, all polysaccharides composed of glucose, including those that make up the cell wall of plants, can be classified as glucans.

Many glucans were among the first compounds to be isolated from different life forms to study the physiological effects they had on vertebrates, especially on the immune system of mammals.

Structure

Glycans have a relatively simple composition, despite the great diversity and complexity of structures that can be found in nature. All are large glucose polymers linked by glycosidic bonds, the most frequent bonds being α (1-3), β (1-3) and β (1-6).

These sugars, like all saccharides that have glucose as their base, are fundamentally composed of three types of atoms: carbon (C), hydrogen (H) and oxygen (O), which form cyclic structures that can be joined together. yes forming a chain.

Most of the glucans consist of straight chains, but those that present branches are joined to these through glucosidic bonds of type α (1-4) or α (1-4) in combination with α (1-6) bonds.

It is important to mention that most of the glucans with “α” bonds are used by living beings as an energy supply, metabolically speaking.

The glucans with the highest proportion of “β” bonds are more structural carbohydrates. These have a more rigid structure and are more difficult to break by mechanical or enzymatic action, so they do not always serve as a source of energy and carbon.

Types of glucans

These macromolecules vary according to the anomeric configuration of the glucose units that compose them; the position, type and number of branches that join them. All variants have been classified into three types of glucans:

– β-glucans (cellulose, lichenine, cymosan or zymosan, etc.)

– α, β-glucans

– α-glucans (glycogen, starch, dextran, etc.)

Α, β-Glucans are also known as “mixed glucans”, since they combine different types of glycosidic bonds. They have the most complex structures within carbohydrates and generally have structures that are difficult to separate into smaller carbohydrate chains.

Generally, glucans have high molecular weight compounds, with values ​​that vary between thousands and millions of daltons.

Characteristics of glucans

All glucans have more than 10 glucose molecules linked together and the most common is to find these compounds made up of hundreds or thousands of glucose residues forming a single chain.

Each glucan has special physical and chemical characteristics, which vary depending on its composition and the environment where it is found.

When glucans are purified they do not have any color, aroma or taste, although purification is never as precise as to obtain a single isolated single molecule and they are always quantified and studied “approximately”, since the isolate contains several different molecules.

Glycans can be found as homo- or heteroglycans.

– Homoglycans are composed of only one type of glucose anomer

– Heteroglycans are made up of different anomers of glucose.

It is common for heteroglycans, when dissolved in water, to form colloidal suspensions (they dissolve more easily if they are subjected to heat). In some cases, heating produces ordered structures and / or gels.

The union between the residues that form the main structure of glucans (the polymer) occurs thanks to glucosidic bonds. However, the structure is stabilized through “hydrostatic” interactions and a few hydrogen bonds.

As in plants, in bacteria and fungi, a network of glucan fibers represents the molecules that make up the rigid cell wall that protects the plasma membrane and the cytosol that is found inside the cells.

In vertebrate animals the main reserve molecule is glycogen. This is a glucan made up of many glucose residues joined repeatedly, forming a chain, which branches throughout the structure.

Generally, glycogen is synthesized in the liver of all vertebrates and a part is stored in the tissues of the muscles.

In short, glucans not only have structural functions, but are also important from an energy storage point of view. Any organism that has the enzymatic apparatus to break down the bonds and separate the glucose molecules to use them as “fuel” uses these compounds to survive.

Applications in industry

Glucans are widely used in the food industry around the world, as they have very varied characteristics and most do not have toxic effects for human consumption.

Many help stabilize the structure of food by interacting with water, creating emulsions or gels that provide greater consistency to certain culinary preparations. An example can be starch or cornstarch.

Artificial flavors in foods are generally the product of the addition of sweeteners, most of which are composed of glucans. These have to go through very extreme conditions or long periods of time to lose their effects.

The high melting point of all glucans serves to protect many of the low temperature sensitive compounds in foods. Glucans “sequester” water molecules and prevent ice crystals from breaking down the molecules that make up the other parts of food.

In addition, the structures formed by glucans in food are thermoreversible, that is to say that by increasing or decreasing the temperature inside the food, they can recover their flavor and texture at the appropriate temperature.

References

1. Di Luzio, NR (1985, December). Update on the immunomodulating activities of glucans. In Springer seminars in immunopathology (Vol. 8, No. 4, pp. 387-400). Springer-Verlag.
2. Nelson, DL, & Cox, MM (2015). Lehninger: principles of biochemistry.
3. Novak, M., & Vetvicka, V. (2009). Glucans as biological response modifiers. Endocrine, Metabolic & Immune Disorders-Drug Targets (Formerly Current Drug Targets-Immune, Endocrine & Metabolic Disorders), 9 (1), 67-75.
4. Synytsya, A., & Novak, M. (2014). Structural analysis of glucans. Annals of translational medicine, 2 (2).
5. Vetvicka, V., & Vetvickova, J. (2018). Glucans and Cancer: Comparison of Commercially Available β-glucans – Part IV. Anticancer research, 38 (3), 1327-1333.