Disaccharides and Polysaccharides Overview

Disaccharides

• Two monosaccharides linked by a glycosidic bond
• Hydrolyzed to form two simple sugars

Types of Disaccharides

• Maltose: Glucose + Glucose
  • Malt sugar
• Lactose: Glucose + Galactose
  • Main carbohydrate in milk
• Sucrose: Glucose + Fructose
  • Non-reducing sugar (no free carbonyl carbon)
• Lactulose: β-galactose + fructose
  • β-1-4 glycosidic bond between C1 of galactose and C4 of fructose

Importance of Lactulose

• Laxative:
  • Not absorbed by the intestine - creates bulk in the stool due to its osmotic effect
  • Used to treat constipation
• Liver Disease:
  • Prevents bacterial production of ammonia in the intestines - beneficial for patients with liver disease who cannot deal with ammonia

Polysaccharides

• Large polymers of monosaccharides
• Two main types:
  • Homopolysaccharides: Same type of monosaccharide
  • Heteropolysaccharides: Different types of monosaccharides

Homopolysaccharides

• Starch: Major storage form of carbohydrates in plants
  • Two chains:
    • Straight chain (amylose)
    • Branched chain (amylopectin)
  • Formed by repeated glucose units linked by α 1-4 glycosidic linkages and α 1-6 glycosidic linkages at branch points
• Glycogen: Major storage form of carbohydrates in animals
  • Branched polymer of glucose
  • Formed by repeated glucose units linked together by α 1-4 glycosidic linkages and α 1-6 glycosidic linkages at branch points
• Cellulose: Forms the cell wall of plants
  • Formed by repeated glucose units linked together by β 1-4 glycosidic linkages
  • Not digested in the human body due to the lack of β-glycosidase enzyme
  • Helps prevent constipation and forms the bulk of stool

Heteropolysaccharides (Complex Polysaccharides)

• Nonsulfated Glucosaminoglycans
  • Hyaluronic acid
• Sulfated Glucosaminoglycans
  • Heparin
  • Heparan sulfate
  • Keratan sulfate
  • Chondroitin sulfate
  • Dermatan sulfate

Hyaluronic Acid (Nonsulfated GAG)

• Composed of alternating residues of D-glucuronic acid and N-acetylglucosamine
• Clear, highly viscous solution
• 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

Defects in Hyaluronic Acid:

• Osteoarthritis

Heparin (Sulfated GAG)

• Anticoagulant
• Found in mast cells
• Repeated disaccharide units:
  • Glucosamine-SO4 + Glucuronic acid ± Iduronic acid (variable)

Heparan Sulfate (Sulfated GAG)

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

Keratan Sulfate (Sulfated GAG)

• Found in two types:
  • Type I: cornea
  • Type II: cartilage
• Repeated disaccharide units:
  • N-acetylglucosamine-SO4 + Galactose

Mucopolysaccharidosis

• Autosomal recessive disorder
• Caused by defects in lysosomal enzymes responsible for metabolism of membrane-associated glucosaminoglycans
• Leads to accumulation of glucosaminoglycans in lysosomes of affected cells
• Symptoms: skeletal deformities, mental retardation, and early death

Glycoconjugates

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

Proteoglycans

• Large macromolecules composed of glucosaminoglycan chains covalently bound to a membrane or secreted protein
• Glycan (CHO) constitutes the larger proportion
• GAGs are formed of amino sugars + uronic acids
• Major component of cartilage: confers strength and provides adhesion points, recognition signals, and information transfer between cells or between cells and the extracellular matrix
• Glucosaminoglycan attaches to the protein core via a trisaccharide linker composed of: Galactose, Galactose, Xylose, and Sulfur
• The protein core is coupled to the trisaccharide linker via its serine residue by an O-glycosidic bond

Glycoproteins

• Smaller oligosaccharide portion than proteins
• Present on cell surfaces (cell membranes) or extracellularly (plasma proteins, except albumin)

Glycolipids and Lipopolysaccharides

• Oligosaccharides are covalently bound to lipids
• Gangliosides are membrane lipids in eukaryotic cells complexed with oligosaccharides
• Some of these complexes determine blood groups
• Lipopolysaccharides are glycolipids present on the outer surface of bacterial cell walls of some Gram-negative bacteria
• Main target of antibodies produced by the immune system against these bacteria
• Lipopolysaccharides of some bacteria are toxic to humans and are called bacterial endotoxins
• Endotoxins cause:
  • Decreased blood pressure
  • Toxic shock, which accompanies infection with Gram-negative bacteria