Introduction

Polysaccharides are long-chain polymers of monosaccharide units, joined together by glycosidic linkages. They are also known as glycans. They are the most abundant carbohydrates found in food.

Carbohydrates are vital macromolecules required for the essential functions of organisms. They are classified into the following three classes depending on two criteria, whether they undergo hydrolysis, and if they do, the number of products they form.

1.   Monosaccharides

They are composed of a single unit of polyhydroxy aldehyde and ketone. These are simple sugars with general chemical formula (CH2O)n. Examples of monosaccharides include glucose, fructose, galactose, ribose, deoxyribose, etc. Glucose is the most abundant monosaccharide in nature. They serve as an energy source in respiration and act as building blocks for large molecules that are essential to life.

2.   Disaccharides

They are composed of two monosaccharide units joined together by an O-glycosidic linkage. Some examples of disaccharides include maltose, lactose, sucrose, trehalose, melibiose, etc. After a hydrolysis reaction, they split into two sugar molecules. Different disaccharides have different functions to perform. For example, lactose is a major animal energy source, trehalose is a major circulatory sugar in insects, and sucrose is a primary sugar that transports carbon within a plant.

3.   Polysaccharides

They are ubiquitous. When many monomer units are joined together by a condensation reaction, they form a polymer called polysaccharides. They are involved in the mechanical stability of cells, organs, organisms, and act as carbohydrate stores in some other organisms.

This article presents you with the fundamentals of polysaccharides, their definition, classification, and functions in different organisms. It also introduces you to the physical and chemical properties.

Classification and Functions of Polysaccharides

Polysaccharides are an essential class of biological polymers. They are mainly involved in the structural or storage functions of the organism. One key information about polysaccharides is if a sugar molecule contains 3-10 monomeric units, it’s considered oligosaccharides, and if it’s composed of 11 or more monomeric units, they are considered true polysaccharides.

The physical and biological properties of polysaccharides depend on the components and the architecture of the molecule, and its interaction with the available enzymatic machinery. They are classified either based on the functions they perform, the type of monosaccharide units they contain, or their origin.

Polysaccharides are categorized into two classes based on the type of monosaccharide units they contain: homopolysaccharides and heteropolysaccharides.

1. Homopolysaccharides

They are composed of repeating units of only one type of monomer. The major homopolysaccharides include cellulose, chitin, starches (amylose and amylopectin), glycogen, and xylans.

a.   Cellulose

It is a linear, unbranched polymer of glucose units joined by beta 1-4 linkages. It was discovered by a French scientist named Anselme Payen. It’s one of the most abundant organic compounds in the biosphere and it’s a basic structural component of the plant cell wall. Though cellulose forms a part of the human diet, it’s indigestible in them because of the absence of the enzymes required to hydrolyze cellulose in the human body.

Celluloses are easily digestible by animals. These herbivore animals retain food for a longer time, where digestion is slowly performed by a group of microorganisms present in their alimentary tract.Figure: The image shows repeating units of cellulose with hydrogen bond interaction.

Properties

  1. Cellulose is tasteless and odorless.
  2. Cellulose is insoluble in water and soluble in organic solvents.
  3. It’s a crystalline solid with a white powdery appearance. It requires a temperature of 320°C and pressure of 25 MPa to become amorphous in water.
  4. The plant cellulose exhibits 60% water retention values, whereas bacterial cellulose exhibit 1000% water retention of the cellulose sample weight.
  5. Cellulose has a high tensile strength (like steel) resulting from an alternate arrangement of glucose molecules in its structure.
  6. It can be broken down into glucose monomer units by treating it with concentrated mineral acids at high temperatures.
  7. The process of degradation of cellulose is known as cellulolysis and the breakdown of cellulose, when exposed to high temperature, is called thermolysis.

Functions

  1. In plants, it helps in maintaining the stability and rigidity of the structures.
  2. In bacteria, it helps in flocculation and attachment to plants. It also helps maintain their shape and structure or maintain a favorable environment.
  3. It’s one of the major components of the cell walls of algae and fungi.

Applications

  1. Cellulose is used to produce paperboard, paper, cardboard, cardstocks, etc.
  2. It is used to make electrical insulation paper.
  3. It is used to make gun powder and bio-fuel.
  4. It’s used as a stabilizer in drugs.

b. Chitin

It is a linear, long-chain polymer of N-acetyl-D-glucosamine (a derivative of glucose) residues/units. These units are joined together by glycosidic beta 1-4 glycosidic linkages. Chitin is the second most abundant natural biopolymer after celluloses. It’s biodegradable in the natural environment, in which the reaction is catalyzed by the enzyme, chitinases, secreted by some bacteria and fungi. Chitin is closely related to cellulose, which is a linear unbranched chain of glucose units, rather than N-acetyl-glucosamine present in chitin.Figure: The structure of repeating units of chitin.

Properties

  1. The N-deacetylation of chitin results in the formation of chitosan, which is a crystalline, cationic, and hydrophilic polymer. Chitosans have excellent gelation and film-forming properties.
  2. Chitin is a polymorphic polysaccharide, present in three different crystalline modifications alpha, beta, and gamma.

Functions

  1. Chitins naturally occur in the exoskeleton of arthropods like shrimps, insects, lobsters, crabs, and the cell walls of certain fungi and molds.
  2. It helps in the stability and rigidity of the cell wall structures of organisms in which it is present.
  3. It’s an integral part of an insect’s peritrophic membrane (present in its midgut). The membrane helps insects in digestion processes, protects them from mechanical damages, toxins, and pathogenic attacks.

Applications

  1. It’s used to make fertilizers that are organic, non-toxic, and increases crop productivity by enhancing soil health.
  2. Chitins in the diet helps to reduce cholesterol absorption efficiency.
  3. Chitins are used to make a surgical thread that facilitates wound healing.

c. Starch

It’s made of repeating units of D-glucose that are joined together by alpha-linkages. Starch is one of the most abundant polysaccharides found in plants. It’s composed of a mixture of amylose (15-20%) and amylopectin (80-85%). The breakdown of starch is done by the enzyme, amylase. It’s present in both humans and animals, and that’s why both of them can easily digest starch.Figure: The structure of a section of amylose and amylopectin.

Properties

  1. Starch is a white tasteless powder that’s insoluble in cold water, alcohol, or other solvents.
  2. The basic formula to deduce starch is (C6H10O5)n, where n is the number of glucose molecules in the chain.
  3. The breakdown of starch by dry heat forms pyrodextrins which are responsible for the browning of bread.

Functions

  1. In plants, starch serves as a reserve for food supply.
  2. In humans and animals, starch is broken down by amylase into its sugar molecules, which act as a reserve of energy supply.

Applications

  1. Starch is used in paper industries for the strengthening and surface sizing of paper.
  2. It’s used in brewing industries.
  3. Starch is used in kitchens for nutritional purposes or as thickening agents in baked foods.
  4. Starch is used to manufacture paperboard, paper bags, paper boxes, gummed paper, and tape.
  5. Corn starch is used as a lubricant in surgical gloves.

d. Xylan

It’s a polysaccharide composed of repeating units of xylose (a pentose sugar) joined together by beta-1, 4-linkages. Xylan is the third most abundant polymer on Earth and it is found in the cell walls of plants. Xylans account for 25-30% dry biomass of woody tissues of dicots and lignified tissues of monocots. In some cereals and grasses, it accounts for up to 50% of their weight.Figure: The structure of xylan in hardwood.

Properties

  1. After acidic hydrolysis or enzymatic hydrolysis, using xylanases, xylans are converted into xylooligosaccharide.
  2. Xylans interact with other polysaccharides through non-covalent interactions, like hydrogen bonding. But, a covalent interaction is involved when they interact with lignins and other phenolic compounds.

Functions

  1. Xylans provide rigidity to the plant’s cell wall and make it recalcitrant towards enzymatic digestion.
  2. Xylans help plants against herbivores and pathogenic attacks.

Applications

  1. It has a significant role in the bread industry – it enhances its quality and toughness.
  2. The main constituent of xylan is converted into xylitol that is used as a sugar substitute for diabetic patients.

2. Heteropolysaccharides

They are composed of two or more repeating units of different types of monomers. Some of the heteropolysaccharides include glycosaminoglycans, agarose, and peptidoglycans. Naturally, most of the heteropolysaccharides are connected with peptides, proteins, and lipids.

a.   Glycosaminoglycans (GAGs)

They are negatively charged unbranched heteropolysaccharides. They are composed of repeating units of disaccharides that include acidic and amino sugars. The formula for GAG structure is n.
GAGs are found in animals and bacteria but are absent in plants. The amino sugar in GAGs is either N-acetylglucosamine or N-acetylgalactosamine. And the acidic sugar is usually a uronic acid (like glucuronic acid). The chart given below shows the building units of some major glycosaminoglycans.

S. No. GAGs Acidic sugar Amino sugar
1 Hyaluronic acid D-Glucuronic acid N-acetylglucosamine
2 Chondroitin sulfate D-Glucuronic acid N-acetylgalactosamine
3 Heparan sulfate D-Glucuronic acid or L-iduronic acid N-acetylglucosamine
4 Heparin D-Glucuronic acid or L-iduronic acid N-acetylglucosamine
5 Dermatan sulfate D-Glucuronic acid or L-iduronic acid N-acetylgalactosamine
6 Keratan sulfate D-Galactose N-acetylglucosamine

Figure: The repeating units of Hyaluronic acid.

Properties

  1. It’s composed of amino sugar and uronic acids.
  2. Except for hyaluronic acids, all GAGs are sulfated, either as O-esters or N-sulfate.
  3. Except for hyaluronic acids, all GAGs are covalently attached to some proteins forming proteoglycans.
  4. The sulfate groups present on the surface of glycosaminoglycans make them negatively charged. The charged compound creates an osmotic pressure that causes the tissue to imbibe water and swell. This enhances the tissue’s ability to bear the load.

Functions

  1. Hyaluronic acids are an essential component of the vitreous humor in the eye and synovial fluid (a lubricant fluid, present in the joints of the body). It is also involved in other developmental processes like tumor metastasis, angiogenesis, and blood coagulation.
  2. Heparin acts as a natural anticoagulant that prevents blood from clotting.
  3. Keratan sulfate is present in the cornea, cartilage, and bones. In joints, it acts as a cushion to absorb mechanical shocks.
  4. Chondroitin is an essential component of cartilage that provides resistance against compression.
  5. Dermatan sulfate is involved in wound repair, blood coagulation regulation, infection responses, and cardiovascular diseases.

Applications

  1. Chondroitin sulfate is used in alternative medicine as dietary supplements to treat osteoarthritis.
  2. Heparin is a major drug to treat diseases like venous thromboembolism (VTE) and atherothrombotic syndromes.
  3. Hyaluronic acid has been shown to decrease the inflammation related to cystic fibrosis in mice models.
  4. Chondroitin sulfate and heparin are considered promising molecules in antitumor therapeutics.

b.   Peptidoglycans

It is a heteropolymer of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acids (NAM), linked together by beta-1,4-glycosidic linkage. The 40-90% dry weight of the cell wall of gram-positive bacteria is composed of peptidoglycans whereas, in gram-negative bacteria, it only accounts for 10% of the dry weight.Figure: The structure of typical bacterial cell wall (peptidoglycan).

Properties

  1. It is degraded by an enzyme, lysozyme, that causes the hydrolysis of beta-1,4-linkage between N-acetylglucosamine (NAG) and N-acetylmuramic acids (NAM).
  2. The production of peptidoglycans can be interfered with by using penicillin.
  3. N-Acetylmuramoyl-L-alanine amidases are enzymes that cause the hydrolysis of peptidoglycans and the dissolution of their structure.

Functions

  1. It is an essential component of bacterial cell walls. It provides strength to the cell wall and participates in binary fission during bacterial reproduction.
  2. It also protects bacterial cells from bursting by counteracting the osmotic pressure of the cytoplasm.

Applications

  1. The presence of peptidoglycans in the cell wall of bacteria helps clinicians to identify the nature of the bacteria, whether it’s gram-positive or negative.

c.   Agarose

It’s a polysaccharide composed of repeating units of agarobiose. Agarobiose is a disaccharide consisting of D-galactose and 3,6-anhydro-L-galactopyranose. It is generally extracted from red seaweed.Figure: The structure of repeating units of agarose.

Properties

  1. The molecular weight of a polymer chain of agarose is about 120,000.
  2. It’s a white powder that dissolves in nearly boiling water and forms a gel after it’s cooled down.
  3. The 3-D structure of agarose is held together by hydrogen bonds, which breaks after heating, leading to the liquid state of agarose.
  4. The melting and gelling temperature of agarose depend on the type of agarose. For example, agarose extracted from Gelidium has a gelling temperature of 34–38°C (93–100°F) and a melting temperature of 90–95°C (194–203°F). Whereas, the agarose extracted from Gracilaria has a gelling temperature of 40–52°C (104–126°F) and melting temperature of 85–90°C (185–194°F).

Functions

  1. Agarose provides a supporting structure in the cell wall of marine algae.

Applications

  1. Agarose is used in labs for the separation of large molecules like DNA (deoxyribonucleic acids). The lower degree of complexity of agarose makes them less interactive with biomolecules.
  2. In some lab experiments, agarose is used to measure microorganisms’ motility and mobility.

Conclusion

Polysaccharides are long-chain polymers of repeating units of sugars. They perform different life-supporting functions in different organisms. Some polysaccharides, because of their strong long polymer chain structure, elasticity, and rigidity, are used in several biomedical applications. Some of them, like agars and gellan gums, provide nutritional value and are used in the food industry.

The study of polysaccharides is still an ongoing research area. Many natural polysaccharides are being explored for their use as nanoformulation, microparticles, tablets, aerogels, hydrogels, and transdermal drug-delivery systems.

The natural physico-chemical properties of polysaccharides pose several challenges for their use in pharmaceuticals. However, scientists are working towards chemically modifying the physico-chemical properties of natural polysaccharides to develop novel semisynthetic natural polysaccharides that support their application in biomedical and pharmaceutical areas.

References

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  18. Xylan. Retrieved from https://en.wikipedia.org/wiki/Xylan#
  19. Keratan Sulfate. Retrieved from https://en.wikipedia.org/wiki/Keratan_sulfate
  20. Dermatan sulfate. Retrieved from https://en.wikipedia.org/wiki/Dermatan_sulfate
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