Polysaccharides Overview

Polysaccharides

• Polysaccharides, also known as glycans, are composed of more than 10 monosaccharide units or their derivatives joined by glycosidic linkages.

Types of Polysaccharides

• Homopolysaccharides (homoglycans): Contain only one type of monosaccharide. Examples include starch, glycogen, dextran, dextrin, cellulose, and inulin.
• Heteropolysaccharides (heteroglycans): Contain more than one type of monosaccharide. Examples include glycosaminoglycans (GAGs), such as mucopolysaccharides, and agar.

Classification of Polysaccharides

• Storage polysaccharides: Used for energy storage. Examples include starch (plants), glycogen (animals), dextran, and inulin.
• Structural polysaccharides: Provide structural support. Examples include cellulose (plants), agar.

Homopolysaccharides (Detailed)

• 1. Starch:

  • Structure: Composed of two layers:
    • Amylose: Linear, non-branching, glucose units linked by α-1,4 glycosidic bonds. Iodine reacts with amylose to produce a blue color.
    • Amylopectin: Highly branched, glucose units linked by α-1,4 and α-1,6 glycosidic bonds. Iodine reacts with amylopectin to yield a red-violet color.
  • Function: The primary storage polysaccharide in plants.
  • Hydrolysis: Amylase enzymes (salivary and pancreatic) rapidly hydrolyze both amylose and amylopectin to yield maltose and dextrins.

• 2. α-Dextrin:

  • Structure: Results from the partial hydrolysis of starch by acids or enzymes. Consists of several glucose units. Hydrolyzed by α-dextrinase.
  • Function: Used as mucilage.

• 3. Dextran:

  • Structure: Highly branched α-glucose units. Primarily α-1,6 glycosidic bonds, with occasional α-1,4 bonds.
  • Function: Storage polysaccharide produced by microorganisms (yeasts, bacteria). Used intravenously as a plasma volume expander to treat hypovolemic shock.

• 4. Glycogen (animal starch):

  • Structure: Highly branched α-glucose units. Branches every 8-12 glucose units(higher branching than amylopectin). Iodine reacts with glycogen to give a red-violet color.
  • Function: The primary storage polysaccharide in animals and humans. Found in the liver and skeletal muscles.
    • In fasting states, glycogen is hydrolyzed to glucose to maintain blood glucose levels.

• 5. Inulin:

  • Structure: Formed from fructose.
  • Function: Found in plants. Hydrolyzed by inulinase. Used in inulin clearance tests to evaluate glomerular filtration rate. Potentially a dietary component for those with diabetes.

• 6. Cellulose:

  • Structure: Composed of β-D-glucose units.
  • Function: The primary structural component of plant cell walls. Extremely resistant to hydrolysis by acids and digestive enzymes. Can stimulate peristalsis and prevent constipation. Certain bacteria in the gut of ruminant animals can hydrolyze cellulose.

Heteropolysaccharides (Detailed)

• Glycosaminoglycans (GAGs):

  • Structure: Long, linear, unbranched chains of repeating disaccharides. Disaccharides contain either N-acetyl-glucosamine or N-acetyl-galactosamine and either glucuronic or iduronic acid. Amino sugars are often acetylated or sulfated.

  • Proteoglycans: Most GAGs (except hyaluronic acid) are covalently linked to proteins to form proteoglycan monomers.

    • Examples (with brief characteristics):
      • 1-Heparin: Anticoagulant; found in mast cells.
      • 2-Heparan sulfate: Component of cell surfaces; found in extracellular matrix.
      • 3-Hyaluronic acid: Non-sulfated; found in synovial fluid, connective tissue, lubricating joints, etc.
      • 4- Keratan sulfate: Found in cornea; contributes to corneal transparency.
      • 5- Chondroitin sulfate: Found in cartilage; responsible for its elasticity and compressibility.
      • 6- Dermatan sulfate: Found in skin, blood vessels; involved in blood coagulation, wound repair, etc.

• Agar:

  • Structure: A polysaccharide isolated from marine red algae, composed of agarose and agaropectin.
  • Function: Used as a gel in gel chromatography, gel electrophoresis, and in microbiological culture media preparation.