Carbohydrates: Structure and Function

Learning Objectives

  • Convert 5- and 6-carbon monosaccharides from the Fischer projection to the Haworth projection.
  • Identify the anomeric carbon and the alpha (α) or beta (β) form of the monosaccharide, and describe the role of mutarotation in cyclic structure.
  • Predict the products of oxidation and reduction reactions on monosaccharides.
  • Predict the products of reactions between monosaccharides and alcohols.
  • Recognize and predict the products of hydrolysis reactions of polysaccharides and phosphorylation reactions of monosaccharides.

The Cyclic Form of Monosaccharides

  • Formation of Hemiacetals: An aldehyde reacts with an alcohol to form a hemiacetal.
    • Reaction: RCHO + R'OH
      ightarrow R-C(OH)-OR'
  • When the aldehyde and alcohol are parts of the same molecule, a stable cyclic hemiacetal forms.

Anomeric Carbon

  • The carbon involved in forming the hemiacetal becomes a new chirality center termed the anomeric carbon.
  • Cyclization results in two forms (α and β) of the same sugar:
    • α-Anomer: Hydroxyl group on the anomeric carbon points down.
    • β-Anomer: Hydroxyl group on the anomeric carbon points up.

Mutarotation

  • The equilibrium between the α and β anomers in solution leads to a phenomenon known as mutarotation.
    • At equilibrium, the concentration of α-D-glucose is approximately 37%, while β-D-glucose is about 63%.

Oxidation and Reduction of Monosaccharides

  • Oxidation:

    • Aldehydes can oxidize to form carboxylic acids: RCHO
      ightarrow RCOOH.
    • Reducing sugars: Aldoses can be oxidized, but ketoses cannot directly oxidize unless they rearrange to aldoses in basic conditions.

  • Reduction:

    • The carbonyl group of an aldose can be reduced to a primary alcohol using hydrogen gas (H₂) and palladium (Pd).
    • Resulting products: Sugar alcohols (alditol) such as D-glucose
      ightarrow D-glucitol.

Glycosides and Disaccharides

  • Glycosides: Formed when cyclic hemiacetals react with alcohols, resulting in acetals with two -OR groups.
  • Disaccharides: Comprising two monosaccharides linked by a glycosidic bond (acetal formation).
    • Examples:
    • Maltose: Hydrolysis yields two glucose molecules, linked by an extα−1,4ext{α-1,4} glycosidic bond.
    • Lactose: Comprised of galactose and glucose, linked by a extβ−1,4ext{β-1,4} glycosidic bond.
    • Sucrose: Composed of α-D-glucose and β-D-fructose, linked by an extα−1,β−2ext{α-1,β-2} glycosidic bond.

Polysaccharides

  • Polysaccharides: Large polymers of monosaccharides (D-glucose), such as:
    • Cellulose: Composed of β-D-glucose, providing structure to plant cell walls.
    • Starch: Contains amylose (unbranched, extα−1,4ext{α-1,4} linkages) and amylopectin (branched, extα−1,4ext{α-1,4} and extα−1,6ext{α-1,6} linkages).
    • Glycogen: Highly branched polysaccharide for energy storage in animals.

Modified Monosaccharides

  • Various monosaccharides have modifications, leading to different compounds with specific functions:
    • Glycosaminoglycans (GAGs): Composed of alternating amino sugars and glucuronate units.
    • Chitin: A polysaccharide derived from N-acetyl-D-glucosamine, contributing to the structure of fungi and insect exoskeletons.

Conclusion

  • Understanding the structure and chemistry of carbohydrates is essential for their roles in biological systems, including energy storage, structural integrity, and metabolic activities. There are various reactions involving monosaccharides and oligosaccharides that define their biological functionalities.