BIMI 200 Lecture 2 - Polysaccharides

BIMI 200 Lecture 2 Notes

Introduction to Polysaccharides (Glycans)

  • Polysaccharides consist of 10 or more monosaccharide units that are covalently linked.

    • Major classes of polysaccharides:

    • Homopolysaccharides

    • Heteropolysaccharides

Homopolysaccharides

Fuel Storage

  • Types of homopolysaccharides:

    • Starch and Glycogen

Starch

  • Definition: Fuel storage molecule in plants and a major nutrient in animals.

  • Structure of Starch:

    • Contains two components:

    • Amylose: Unbranched, consists of long chains of glucose molecules.

    • Amylopectin: Somewhat branched structure, also contains glucose but with a branched configuration.

    • Visual representation shows:

    • Long glucose chains in branched and unbranched forms.

Glycogen

  • Definition: Fuel storage molecule in animals, primarily stored in the liver and muscle tissues.

  • Characteristics:

    • Glycogen is extensively branched compared to starch.

  • Function: Glucose is released from glycogen in response to insulin to regulate blood sugar levels.

Structural Homopolysaccharides

Cellulose

  • Function: Primary structural component of plant cell walls.

  • Composition: Linear homopolymer of up to 15,000 units of D-glucose linked by β(1→4) glycosidic bonds.

Chitin

  • Function: Main structural component of exoskeletons in invertebrates such as crustaceans, insects, and spiders.

  • Composition: Linear homopolymer of N-acetyl-D-Glucosamine units linked by β(1→4) glycosidic bonds.

Visualization of Starch, Glycogen, and Cellulose Structures

  • Includes structural representations showing:

    • Amylose (unbranched) and Amylopectin (branched) in starch, stored in plastids (e.g., potato tuber cell).

    • Glycogen granules in muscle tissue.

    • Cellulose microfibrils in plant cell walls with hydrogen bonding.

Heteropolysaccharides

  • Definition: Yields a mixture of constituent monosaccharides and derived products upon enzymatic or acid hydrolysis.

  • Types of Heteropolysaccharides:

    • Glycosaminoglycans (mucopolysaccharides)

    • Proteoglycans

    • Peptidoglycans

    • Glycoproteins

    • Glycolipids

Glycosaminoglycans (Mucopolysaccharides)

  • Composition: Mainly of unbranched polysaccharides of alternating uronic acid and hexosamine residues.

  • Example: Hyaluronic Acid

    • Binds water to form a viscoelastic gel.

    • Important constituent of synovial fluid in joints.

Further Importance of Hyaluronic Acid

  • Present in:**

    • Vitreous humor of the eye, contributing to the structure and function of ocular components (pupil, cornea, lens).

  • Ground substance in connective tissues along with various cell types (e.g., collagen fibers, fibroblasts).

Other Glycosaminoglycans

  • Chondroitin-4-sulfate: An important structural component of cartilage; used as a dietary supplement for osteoarthritis treatment.

  • Keratan sulfate: Found in cornea, cartilage, and bone; cushions joints to absorb mechanical shocks.

  • Heparin: Prevents blood clotting.

Mucopolysaccharidoses (MPSs)

  • Group of disorders caused by errors in carbohydrate metabolism, leading to:

    • Corneal opacification, retinal degeneration, and optic atrophy.

    • Accumulation of glycosaminoglycans in multiple organ systems.

    • Patients with MPS I (Hurler's syndrome) and MPS VI (Maroteaux-Lamy syndrome) exhibit corneal opacification affecting vision and ocular evaluation.

Classification of Corneal Clouding

  • GRADE I: Normal cornea.

  • GRADE II: Mild opacity; iris and fundus details visible.

  • GRADE III: Opacity interferes with vision of iris & fundus details.

  • GRADE IV: Anterior chamber and fundus cannot be evaluated.

Proteoglycans (Mucoproteins)

  • Aggregate structures made up of proteins and glycosaminoglycans.

  • Location: Found in cartilage.

    • Subunits: Core proteins to which glycosaminoglycans (like keratin sulfate and chondroitin sulfate) are covalently linked.

Peptidoglycans

  • Key structural components in bacterial cell walls.

  • Structure includes:

    • Outer membrane, cytoplasm, and periplasmic space in Gram-negative bacteria.

Glycoproteins and Glycolipids

  • Most oligosaccharides and polysaccharides are covalently linked to:

    • Proteins (Glycoproteins): Linked through O or N linkages.

    • Lipid molecules (Glycolipids): Critical for maintaining cell membrane stability and cellular recognition.

Functions of Glycoproteins

  • Protection: Involved in immune responses (e.g., antibodies like IgM and IgG).

  • Transport: Such as transferrin (iron transport).

  • Host Recognition: O-antigens on pathogenic gram-negative bacteria.

  • Vision: Role in visual receptors, such as rhodopsin.

  • Growth Promotion: Granulocyte-macrophage colony stimulating factor (GM-CSF) promotes stem cell production of white blood cells.

Glycolipids

  • Role: Maintain the stability of cell membranes and facilitate cellular recognition against other cells or pathogens.

Structural Elements of Glycoproteins and Glycolipids

  • Illustrations demonstrate relationships between carbohydrate and protein components, highlighting various protein structures (integral proteins, peripheral proteins, channel proteins).

End of Lecture Notes.