Lecture on Carbohydrates: Disaccharides

Introduction

• Focus: Connecting monosaccharides to form larger carbohydrate molecules.
• Disaccharides: Carbohydrates consisting of two monosaccharides connected by an O-glycosidic bond.
• Key disaccharides: Maltose, Lactose, Sucrose.

Maltose

• Structure: Two α-D-glucose molecules in cyclic form.
  • Connection: Carbon 1 of the first glucose and Carbon 4 of the second glucose.
  • Bond: α(1→4) glycosidic linkage.
• Formation: Produced when starch is broken down.
• Digestion: Enzyme maltase in the small intestine breaks down maltose into glucose.
• Type: Reducing sugar (can open to form an aldehyde group in open chain form).

Lactose

• Structure: β-D-galactose and α-D-glucose.
  • Connection: Carbon 1 of galactose to Carbon 4 of glucose.
  • Bond: β(1→4) glycosidic linkage.
• Found in: Milk.
• Digestion: Enzyme lactase in humans and β-galactosidase in bacteria.
• Type: Reducing sugar.

Sucrose

• Structure: α-D-glucose and β-D-fructose.
  • Connection: Carbon 1 of glucose to Carbon 2 of fructose.
  • Bond: α(1→2) glycosidic linkage.
• Source: Cane and beet plants.
• Digestion: Enzyme sucrase in the small intestine.
• Type: Non-reducing sugar
  • Due to bond between anomeric carbons, cannot form open chain aldehyde or ketone.

Reducing vs Non-reducing Sugars

• Reducing sugars (e.g., Maltose, Lactose): Can open into a form with an aldehyde group, react with oxidizing agents.
• Non-reducing sugars (e.g., Sucrose): Cannot form an open chain with aldehyde or ketone group, do not react with oxidizing agents.

Key Points

• Disaccharides are formed by linkages between specific carbons.
• Enzymes specific to each disaccharide facilitate digestion into monosaccharides in the small intestine.
• Reducing and non-reducing sugars differ in their ability to participate in redox reactions.