Lecture on Carbohydrates and Glycosidic Linkages

Overview

• Cyclic monosaccharides like glucose can react with alcohols to form acetals and ketals.
• Alcohols involved in these reactions can sometimes be other carbohydrates, leading to the formation of glycosidic linkages.
• A glycosidic linkage between two monosaccharides forms a disaccharide.

Disaccharides

Glycosidic Linkage

• Typically forms between the anomeric carbon (C1) of the first sugar and C4 of the second sugar (1,4 glycosidic linkage).
• Specificity:
  • Alpha linkage: OR group is trans to the sixth carbon.
  • Beta linkage: OR group is cis to the sixth carbon.

Common Disaccharides

1. Lactose

   • Found in milk (both human and cow).
   • Consists of galactose and glucose.
   • Linked by a Beta 1,4 glycosidic bond.
   • Not appreciably sweet compared to other sugars.

2. Maltose

   • Composed of two glucose units.
   • Linked by an Alpha 1,4 glycosidic bond.
   • A common disaccharide with a sweet taste.

3. Sucrose

   • Commonly found as table sugar.
   • Composed of glucose and fructose.
   • Linked by their anomeric carbons resulting in two acetals.
   • Non-reducing sugar due to lack of hemiacetals.

Polysaccharides

• Cellulose

  • Found in plant cell walls and cotton.
  • Composed of glucose units linked by Beta 1,4 glycosidic bonds.
  • Forms a straight, unbranched chain.
  • Humans cannot digest cellulose due to lack of enzymes.

• Starch

  • Consists of glucose units linked by Alpha 1,4 glycosidic bonds.
  • Used as an energy source since humans can digest it.

• Glycogen

  • Similar to starch but highly branched.
  • Composed of glucose with Alpha 1,4 linkages and branching at Alpha 1,6 linkages.
  • Functions as energy storage in humans, allowing quick release of glucose.

Important Concepts

• Reducing sugars (e.g., lactose, maltose) have hemiacetal groups allowing for chain extension and further acetal formation.
• Non-reducing sugars (e.g., sucrose) cannot be further reduced due to full acetal formation.