Biochemistry BCH 261 Module 8 Notes

Overview of Carbohydrates

  • Focus on disaccharides and polysaccharides (BCH 261, Module 8)

Key Terminology

  • Carbohydrate Stereochemistry:
    • Enantiomers: Stereoisomers that are mirror images.
    • Diastereomers: Stereoisomers not mirror images.
    • Anomers: Stereoisomers that differ at the anomeric carbon.
    • Epimers: Stereoisomers differing at a single carbon other than the anomeric carbon.

Oligosaccharides and Disaccharides

  • Definition: Oligosaccharides are polymers of monosaccharides.
    • Examples include: glycogen, amylopectin, amylose, cellulose, and chitin.
    • Functional oligosaccharides: glycoaminoglycans, glycoproteins, glycolipids.

Glycosidic Bond Formation

  • Mechanism:
    • Formed via condensation reaction between hydroxyl groups of two monosaccharides, resulting in the liberation of water.
    • Example: Galactose + Glucose → Lactose (via beta-1,4 glycosidic bond).
    • Reducing Sugars vs. Non-reducing Sugars:
    • A disaccharide with a free anomeric carbon is a reducing sugar.
    • Example: Lactose is reducing; Trehalose (two reducing ends engaged) is non-reducing.

Synthesis of Disaccharides

  • Formation Examples:
    • Maltose: Alpha-D-Glucose + Beta-D-Glucose is joined via a 1,4 linkage, creating a reducing sugar.
    • Trehalose: Two Alpha-D-Glucose molecules joined via an 1,1 glycosidic linkage, resulting in a non-reducing sugar.

Polysaccharides

  • Types:
    • Homopolysaccharides: Comprised of one type of monosaccharide.
    • Heteropolysaccharides: Comprised of different types of monosaccharides.
    • Molecular weight varies significantly.

Examples of Polysaccharides

  • Glycogen:
    • Glucose polymer, branched structure (alpha-1,4 linkages and branches via alpha-1,6 linkages).
    • Energy storage in animals; typical size: several thousand units.
  • Starch:
    • Mixture of amylose (unbranched, alpha-1,4) and amylopectin (branched, alpha-1,4 and alpha-1,6).
    • Main storage polysaccharide in plants; higher molecular weights.
  • Cellulose:
    • Homopolymer of beta-linked glucose, unbranched.
    • Structural carbohydrate in plants, stabilized by hydrogen bonding; contributes to toughness and insolubility.

Glycoconjugates and the Glyco Code

  • Glycoconjugates: Modifications on proteins and lipids via carbohydrates.
    • Functions: Cell signaling, cellular development, immune response.
    • Importance of structural diversity:
    • Enables diverse molecular recognition.
    • Carbohydrates surpass proteins and nucleic acids in combinatorial diversity.

Protein Modifications

  • Glycoproteins:
    • O-linked (serine/threonine) vs. N-linked (asparagine).
  • Structural variations play significant roles in cell interactions and functionality.

Summary of Important Points

  • Glycosidic bonds are essential for energy storage in carbohydrates.
  • Glycogen and starch serve as primary forms of glucose storage in animals and plants, respectively.
  • Extensive hydrogen bonding in cellulose makes it structurally robust.
  • Carbohydrates function as information molecules, with implications in disease and cellular function.

Important Structures

  • Familiarize with:
    • Chair forms: glucose, mannose, N-acetylglucosamine, glucuronic acid.
    • Haworth forms: glucose, fructose, ribose.
  • Understand the mutarotation reaction, reducing sugar reactions, and glycosidic bond formation leading to acetals or ketals.