Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
7.2 Polysaccharides
This chapter describes polysaccharides, or glycans, which are carbohydrate polymers of medium to high molecular weight. They differ in monosaccharide composition, chain length, glycosidic linkages, and degree of branching. Polysaccharides are categorized as homopolysaccharides (one type of sugar) or heteropolysaccharides (two or more kinds). They perform diverse biological functions — storage, structure, and extracellular support.
Homopolysaccharides and Heteropolysaccharides
• Homopolysaccharides consist of only one kind of monosaccharide unit, such as starch, glycogen, cellulose, or chitin.
• Heteropolysaccharides contain multiple sugar types and often form structural matrices, such as bacterial peptidoglycan or animal extracellular matrix components.
• Unlike proteins, polysaccharides do not have defined lengths or templates for synthesis. Their polymerization is controlled by enzyme specificity rather than a genetic template.
Storage Polysaccharides: Starch and Glycogen
• Starch (in plants) and glycogen (in animals) are the principal storage forms of glucose, occurring as intracellular granules that are heavily hydrated due to exposed hydroxyl groups.
• Starch consists of two glucose polymers: amylose (unbranched α(1→4) chains) and amylopectin (branched α(1→4) chains with α(1→6) branch points every 24–30 residues).
• Glycogen resembles amylopectin but is more highly branched, with α(1→6) branches every 8–12 residues, forming compact spherical granules.
• Glycogen is abundant in liver and muscle; it contains numerous nonreducing ends for rapid enzymatic mobilization of glucose.
• Glucose is stored in polymeric form because if stored freely, the high osmotic concentration (~0.4 M) would cause cellular rupture; glycogen avoids this osmotic stress.
Structural Polysaccharides: Cellulose and Chitin
• Cellulose is a linear homopolysaccharide of β(1→4)-linked D-glucose units, forming long, unbranched chains (10,000–15,000 residues) with strong interchain hydrogen bonding that gives high tensile strength.
• It is the main structural component of plant cell walls and is the basis for products like wood, cotton, paper, and textiles.
• Chitin is a β(1→4)-linked polymer of N-acetylglucosamine, similar to cellulose but with acetylated amino groups that make it more hydrophobic and water-resistant.
• Chitin forms the exoskeletons of insects, crustaceans, and other arthropods, ranking as the second most abundant polysaccharide on Earth after cellulose.
Three-Dimensional Structures of Polysaccharides
• Polysaccharide folding follows the same principles as polypeptide folding — stabilized by hydrogen bonds, van der Waals interactions, and hydrophobic effects.
• Steric hindrance limits rotation around glycosidic bonds, defining dihedral angles (ϕ and ψ) that determine chain conformation.
• Starch and glycogen adopt tightly coiled helical structures stabilized by intrachain hydrogen bonds. Each helical turn contains six glucose residues, and amylose forms blue complexes with iodine — a classical starch test.
• Cellulose adopts an extended, linear conformation with alternating chair orientations, maximizing inter- and intrachain hydrogen bonding, forming strong, insoluble fibers.
Peptidoglycan in Bacterial Cell Walls
• Peptidoglycan is a heteropolymer of alternating β(1→4)-linked N-acetylglucosamine and N-acetylmuramic acid, cross-linked by short peptide chains that provide rigidity and prevent osmotic lysis.
• Lysozyme hydrolyzes the β(1→4) linkages, acting as an antibacterial enzyme found in tears and egg whites.
• Penicillin and related antibiotics inhibit cross-link formation, weakening bacterial cell walls and causing lysis.
Glycosaminoglycans (GAGs): Extracellular Heteropolysaccharides
• Glycosaminoglycans are linear polymers of repeating disaccharides containing an amino sugar (N-acetylglucosamine or N-acetylgalactosamine) and a uronic acid (D-glucuronic or L-iduronic acid).
• They carry numerous sulfate and carboxylate groups, giving them a high negative charge density that forces extended conformations and promotes water retention and mechanical resilience.
• GAGs combine with proteins to form proteoglycans, essential components of the extracellular matrix (ECM) that provide structure, lubrication, and selective permeability.
Major Glycosaminoglycans
• Hyaluronan (Hyaluronic Acid): A non-sulfated polymer of D-glucuronic acid and N-acetylglucosamine with up to 50,000 repeating units; contributes to joint lubrication, vitreous humor viscosity, and cartilage elasticity.
• Chondroitin Sulfate: Composed of D-glucuronic acid and N-acetylgalactosamine; provides tensile strength to cartilage, tendons, and heart valves.
• Dermatan Sulfate: Similar to chondroitin sulfate but with L-iduronic acid replacing some D-glucuronic residues; contributes to skin pliability and vascular integrity.
• Keratan Sulfate: Lacks uronic acid; found in cornea, cartilage, bone, hair, nails, and horns.
• Heparan Sulfate and Heparin: Highly sulfated polymers interacting with many proteins; heparin binds antithrombin to inhibit blood coagulation and is used therapeutically as an anticoagulant.
Structural and Functional Summary
• Starch and glycogen store energy in α-linked glucose chains with varying degrees of branching.
• Cellulose and chitin provide rigidity through β-linkages forming straight, hydrogen-bonded fibers.
• Peptidoglycan confers bacterial cell wall strength, while glycosaminoglycans form hydrated ECM scaffolds providing mechanical support and biochemical signaling functions.
In a Nutshell
Polysaccharides are vital carbohydrate polymers with diverse roles in storage, structure, and cellular architecture. Homopolysaccharides like starch, glycogen, cellulose, and chitin function in energy metabolism or mechanical support, while heteropolysaccharides such as glycosaminoglycans build extracellular matrices. Their distinct linkages and conformations define biological properties, from flexible energy granules to rigid cell walls and tissues, illustrating the molecular versatility of carbohydrates in living systems.