lecture recording on 11 February 2025 at 16.40.17 PM

Disaccharides Overview

• Definition: Disaccharides consist of two monosaccharides linked together through a dehydration reaction.

  • Example: Lactose forms from beta d-galactose and alpha d-glucose.

Formation of Disaccharides

• Glycosidic Bond: The bond that links monosaccharides in a disaccharide.

  • Formed when the hydroxyl group at the anomeric carbon (carbon 1 of galactose) reacts with a hydroxyl group (position 4 of glucose).

  • A water molecule is released during this reaction.

Specific Disaccharides

• Lactose: Formed from beta d-galactose and alpha d-glucose.

  • Notation: beta d-gal 1 -> 4 d-glucose indicates the bond formation between the specific carbons.

  • Anomeric configurations:

    • The configuration of galactose retains beta while glucose can exist as either alpha or beta due to free anomeric OH.

Other Key Disaccharides

• Sucrose: Formed from alpha d-glucose and beta d-fructose.

  • Notation: alpha d-glucose 1 -> 2 beta d-fructose, illustrating the linkage between anomeric sugars.

  • Both anomeric positions are involved in glycosidic bonds, locking their configurations.

Polysaccharides Overview

• Definition: Long chains of monosaccharides.

• Glycogen: The storage form of glucose in animals.

  • Structure: Composed of glucose units linked by alpha 1 -> 4 linkages with branches every 10 units defined by alpha 1 -> 6 linkages.

Plant Polysaccharides

• Starch: Major plant energy storage, consisting of two types:

  • Amylose: Unbranched polyglucose chain linked by alpha 1 -> 4 linkages.

  • Amylopectin: Branched polymer also linked by alpha 1 -> 4 with branches every 30 residues.

• Cellulose: Structural polysaccharide found in plant cell walls, composed of beta 1 -> 4 linked glucose monomers, creating a rigid structure due to hydrogen-bonding between chains.

Chitin

• A polysaccharide similar to cellulose, found in insect exoskeletons.

  • Contains N-acetylglucosamine units linked by beta 1 -> 4 linkages.

Glycoproteins and Glycosaminoglycans

• Glycoproteins: Proteins with carbohydrate attachments (polysaccharides), important for cell recognition and signaling.

  • Types:

    • O-linked: Attachment via serine or threonine residues.

    • N-linked: Attachment via asparagine residues.

• Glycosaminoglycans: Polysaccharides composed of repeating disaccharide units of amino sugars, serving as shock absorbers in joints due to their ability to retain water and form gel-like substances.

• Examples: Hyaluronic acid, important in skin care, is a glycosaminoglycan.

Differences Between Structural and Energy Storage Polysaccharides

• Cellulose: Arranged in linear chains with hydrogen bonds between chains resulting in rigidity.

  • Opposite directional orientations (flipping each unit) enhance stabilization through additional hydrogen bonding.

Summary of Important Structures

• Distinct structures and linkages (alpha vs beta) affect functionality in biological systems.

• Understanding glycosidic bonds and structural arrangements helps in identifying different carbohydrate types and their roles in biological systems.