Disaccharides and Polysaccharides Overview

Disaccharides

• Disaccharides are composed of two monosaccharides linked together by a glycosidic bond.
• Disaccharides yield two simple sugars upon hydrolysis.
• Maltose is a disaccharide composed of glucose and glucose. It is also known as malt sugar.
• Lactose is a disaccharide composed of glucose and galactose. It is the main carbohydrate found in milk.
• Sucrose is a disaccharide composed of glucose and fructose. It does not contain a free carbonyl carbon, making it a non-reducing sugar.
• Lactulose is a disaccharide composed of β-galactose and fructose linked by a β-1-4 glycosidic bond.
  • Lactulose is not absorbed by the intestine and is used to treat constipation due to its osmotic effect.
  • It is also helpful for patients with liver disease because it prevents intestinal bacteria from producing excess ammonia.

Polysaccharides

• Polysaccharides are polymers composed of multiple monosaccharide units linked together by glycosidic bonds.
• Homopolysaccharides are composed of a single type of monosaccharide.
  • Starch is a branched polymer of glucose, and it is the main storage form of carbohydrates in plants.
    • Starch is composed of two chains: amylose (straight) and amylopectin (branched).
  • Glycogen is a branched polymer of glucose, and it is the main storage form of carbohydrates in animals.
    • Glycogen is composed of glucose units linked by α 1-4 glycosidic linkages and α 1-6 linkages at the branch points.
  • Cellulose is a linear polymer of glucose linked by β 1-4 glycosidic linkages. It forms the cell walls of plants.
    • Cellulose is not digested in the human body due to the absence of the enzyme β-glycosidase. It helps prevent constipation and forms the bulk of stool.
• Heteropolysaccharides are composed of different types of monosaccharides.
  • Glucosaminoglycans (GAGs) are a type of heteropolysaccharide.
    • They are also known as mucopolysaccharides and are major components of the extracellular matrix.
    • Hyaluronic Acid is a non-sulfated GAG composed of alternating residues of D-glucuronic acid and N-acetylglucosamine.
      • Hyaluronic acid is found in the synovial fluid of joints as a lubricant and in the vitreous humor of the eye.
      • It is also found in the extracellular matrix of cartilage and tendons, providing strength and structure.
      • Defects in hyaluronic acid can lead to osteoarthritis.
    • Sulphated GAGs include heparin, heparan sulfate, keratan sulfate, chondroitin sulfate, and dermatan sulfate.
      • Heparin is an anticoagulant found in mast cells. It is composed of repeated disaccharide units of glucosamine-SO4 + glucuronic acid or iduronic acid.
      • Heparan Sulphate is structurally similar to Heparin but is more acetylated and less sulphated.
      • Keratan Sulphate is found in the cornea and cartilage. It is composed of repeated disaccharide units of N-acetylglucosamine-SO4 + galactose.
  • Mucopolysaccharidoses are a group of autosomal recessive inherited diseases caused by defects in the lysosomal enzymes responsible for the metabolism of GAGs.
    • This leads to the accumulation of GAGs in lysosomes, resulting in skeletal deformities, mental retardation, and early death.

Glycoconjugates

• Carbohydrates can be covalently linked to proteins or lipids, forming glycoconjugates.
• Proteoglycans are macromolecules formed of glucosaminoglycan chains covalently bound to a membrane or secreted protein.
  • The glycan component is the larger proportion.
  • They are major components of cartilage, providing strength and points of adhesion, recognition, and information transfer between cells or between cells and the extracellular matrix.
  • Glucosaminoglycans attach to the protein core through a trisaccharide linker composed of galactose, galactose, and xylose.
  • The protein core is coupled to the trisaccharide linker via its serine residue by an O-glycosidic bond.
• Glycoproteins are molecules with smaller oligosaccharide portions than proteins.
  • They are found on cell surfaces (cell membranes) and in the extracellular matrix (plasma proteins, except albumin).
• Glycolipids and Lipopolysaccharides are molecules with oligosaccharides covalently bound to lipids.
  • Gangliosides are membrane lipids in eukaryotic cells that are complexed with oligosaccharides. Some of these complexes determine blood groups.
  • Lipopolysaccharides are found on the outer surface of bacterial cell walls of some gram-negative bacteria.
    • They are the main targets of antibodies produced against these bacteria, and some are toxic to humans.
    • Bacterial endotoxins are lipopolysaccharides that can lower blood pressure and cause toxic shock.
    • They are responsible for the toxic shock that accompanies infections with gram-negative bacteria.

This quiz covers the essential concepts of disaccharides and polysaccharides, focusing on their structures and functions. Learn about key examples like maltose, lactose, and sucrose, and explore the significance of polysaccharides in biology. Test your knowledge of carbohydrate chemistry!