Disaccharides and Polysaccharides Overview

Disaccharides

• Two monosaccharides joined together by a glycosidic bond
• Yield two simple sugars upon hydrolysis
• Types: Maltose, Lactose, Sucrose, Lactulose

Maltose

• Glucose + glucose
• Malt sugar

Lactose

• Glucose + galactose
• Main carbohydrate in milk

Sucrose

• Glucose + fructose
• Does not contain a free carbonyl carbon, making it non-reducing

Lactulose

• β-galactose + fructose
• Joined by a β-1-4 glycosidic bond between C1 of Galactose and C4 of Fructose
• Importance:
  • Laxative: Not absorbable from the intestine, creates bulk in the stool due to its osmotic effect, treats constipation
  • Beneficial for patients with liver disease: Prevents intestinal bacteria from producing ammonia, which diseased livers cannot process

Polysaccharides

• Complex carbohydrates formed by many monosaccharide units linked together
• Classified as:
  • Homopolysaccharides: Composed of one type of monosaccharide (e.g., starch, glycogen, cellulose)
  • Heteropolysaccharides: Composed of different types of monosaccharides (e.g., hyaluronic acid, heparin)

Homopolysaccharides

• Starch: Major storage form of carbohydrates in plants.
  • Structure: Composed of two chains:
    • Amylose: Straight chain of glucose units linked by α-1-4 glycosidic bonds
    • Amylopectin: Branched chain of glucose units linked by α-1-4 glycosidic bonds and α-1-6 glycosidic bonds at branching points
• Glycogen: Major storage form of carbohydrates in animals.
  • Structure: Branched polymer of glucose units linked by α-1-4 glycosidic bonds and α-1-6 glycosidic bonds at branching points
• Cellulose: Forms the wall of plant cells.
  • Structure: Composed of repeated glucose units linked together by β-1-4 glycosidic bonds.
  • Significance: Not digested in the human body due to the lack of β-glycosidase enzymes, prevents constipation, forms bulk of stool

Heteropolysaccharides (Complex Polysaccharides)

• Nonsulfated glucosaminoglycans (GAGs) or mucopolysaccharides
  • Hyaluronic acid: Composed of alternating residues of D-glucuronic acid and N-acetylglucosamine.
    • Functions:
      • Forms a clear, highly viscous solution that acts as a lubricant in synovial fluid of joints
      • Forms the vitreous humor, which fills the eyeball
      • Forms the extracellular matrix of cartilage and tendons
  • Hyaluronic acid Defect: Can lead to osteoarthritis
• Sulfated Glucosaminoglycans
  • Heparin
  • Heparan sulfate
  • Keratan sulfate
  • Chondroitin sulfate
  • Dermatan sulfate

Heparin

• An anticoagulant found in mast cells
• Structure: Repeated disaccharide units of glucosamine-so4 + glucuronic acid (variable) or iduronic acid

Heparan Sulfate

• Similar structure to Heparin, but more acetylated and less sulfated

Keratan Sulfate

• Present in:
  • Type I: Cornea
  • Type II: Cartilage
• Structure: Repeated disaccharide units of N-acetylglucosamine-so4 + galactose

Mucopolysaccharidosis

• Autosomal recessive inherited diseases caused by lysosomal enzyme defects responsible for metabolizing membrane-associated glucosaminoglycans
• Excess accumulation of glucosaminoglycans in lysosomes of affected cells
• Consequences: Skeletal deformities, mental retardation, and early death

Glycoconjugates

• Carbohydrates covalently bound to:
  • Proteins (Proteoglycans & Glycoproteins)
  • Lipids (Glycolipids “Lipopolysaccharides”)

Proteoglycans

• Macromolecules containing glucosaminoglycan chains covalently bound to a membrane or secreted protein
• Glycan (CHO) part makes up the larger portion
• GAGs are composed of amino sugars and uronic acids
• Major component of cartilage, contributing to its strength
• Provide points of adhesion, recognition, and information transfer between cells or between cells and the extracellular matrix
• Glucosaminoglycans attach to the protein core via a trisaccharide linker composed of:
  • Galactose
  • Galactose
  • Xylose
  • S (sulfate)
• The protein core is coupled to the trisaccharide linker via its serine residue by an O-glycosidic bond.

Glycoproteins

• Oligosaccharides are smaller than the proteins
• Present on cell surfaces (cell membranes) or extracellularly (plasma proteins, except albumin)

Glycolipids and Lipopolysaccharides

• Oligosaccharides are covalently bound to lipids
• Gangliosides: Membrane lipids in eukaryotic cells, complexed with oligosaccharides, play a role in determining blood groups
• Lipopolysaccharides: Glycolipids found on the outer surface of bacterial cell walls in gram-negative bacteria
  • Major target of antibodies produced by the immune system against these bacteria
  • Some lipopolysaccharides are toxic to humans and are called bacterial endotoxins
  • Endotoxins cause:
    • Lowering blood pressure
    • Toxic shock accompanying gram-negative bacterial infections

This quiz covers the essential aspects of disaccharides and polysaccharides, including their structures, types, and functions. Learn about common disaccharides like maltose, lactose, and sucrose, as well as the significance of polysaccharides in biological systems. Test your knowledge on these important carbohydrates!