Disaccharides Structure and Bonds

Carbohydrates

• Represent 50-60% of human food and are the most abundant biomolecules on Earth
• Functions:
  • Rapid source of energy
  • Energy storage
  • Structural components in cells and biomolecules
  • Cell surface recognition

Classification of Carbohydrates

• Divided into four major groups:
  • Monosaccharides (simple sugars)
  • Disaccharides (two sugars)
  • Oligosaccharides (a few sugars)
  • Polysaccharides (many sugars)

Monosaccharides

• Also known as simple sugars
• Have at least 3 carbons
• General formula: (CH2O)n
• Have a carbonyl group (C=O) and hydroxyl groups (-OH)
• Classified into:
  • Aldoses (C=O at carbon 1)
  • Ketoses (C=O at other carbons)
• Differ in the number of carbons and hydroxyl groups
• Can exist as anomers (isomers that differ in the configuration of one carbon)

Disaccharides

• Formed by linking two monosaccharides through a glycosidic bond
• Can have different glycosidic bonds
• Examples:
  • Lactose (milk sugar)
  • Sucrose (table sugar)

Polysaccharides

• Long polymer chains of monosaccharides
• Used for energy storage or structure
• Can be linear or branched
• Examples:
  • Starch (energy storage in plants)
  • Glycogen (energy storage in animals)
  • Cellulose (structural component in plants)
  • Chitin (structural component in insects and crustaceans)

Glycoconjugates

• Carbohydrates covalently linked to proteins or lipids

• Examples:

  • Proteoglycans (core protein with covalently attached glycosaminoglycans)
  • Glycoproteins (carbohydrate-protein conjugates with branched oligosaccharides)
  • Glycosphingolipids (sphingolipids with covalently bound oligosaccharides)### Carbohydrate Structures and Functions

• Starch and glycogen form helical structures with intrachain hydrogen bonding

• Cellulose and chitin form long, straight strands that interact with neighbouring strands

Glycoconjugates

• Glycoconjugates have oligo- or polysaccharides covalently attached to a protein or lipid
• Exposed on the outside of the plasma membrane
• Involved in signalling, transport within cells, recognition, and information transfer between cells and the extracellular matrix or parasites

Monosaccharides and Anomers

• Haworth projection formulas represent the cyclic form of sugars and show the distinction between alpha and beta anomers
• Fructose is a ketose
• A hemiacetal is a cyclic structure of a ketose
• In the open-chain form of a sugar, the highest numbered chiral centre becomes the anomeric carbon in the cyclic form

Glycosidic Linkages and Polymers

• Glycosidic linkages between monosaccharide units vary based on the anomeric form and the -OH group involved
• Linear polymers are more water-soluble than branched ones
• Branched polymers have one non-reducing end and many reducing ends
• Amylopectin and glycogen have a high degree of alpha(1-6) branching linkages

Glycoproteins and Biological Roles

• Glycoproteins are proteins covalently bound to sugars
• Carbohydrates play various roles in biology, including cell-cell communication, energy storage, and structural elements in cell walls

Specific Carbohydrates and Their Roles

• Cellulose: most abundant organic substance on earth, structural element in plant cell walls
• Chitin: exoskeleton structure in insects
• Glycogen: energy storage in animals, extensively branched
• Starch: energy storage in plants, less branched than glycogen
• Sucrose: not the most abundant organic substance on earth