Disaccharides

Glycoside Formation

• Formation of Glycosides:
  • Hemiacetals and hemiketals react with alcohols to yield acetal and ketal.
  • In the case of cyclic hemiacetals or hemiketals from monosaccharides, the reaction with alcohol results in O-glycosidic linkage, turning the compound into a glycoside.

Disaccharides

• Common Disaccharides:
  • Acetals derived from glucose and fructose are termed glucoside and fructoside, respectively.
  • When a hemiacetal hydroxyl of one monosaccharide connects with the hydroxyl of another monosaccharide, a disaccharide is formed.
  • Polysaccharides consist of numerous monosaccharides joined together through acetal linkages.

Types of Polysaccharides

• Polysaccharides (Glycans):
  • Can range from hundreds to thousands of monosaccharide subunits.
  • Structures can be linear or branched.
  • Classified into homoglycans and heteroglycans.

Homoglycans vs. Heteroglycans

• Homoglycans:

  • Composed of one type of monosaccharide.
  • Examples: Starch, glycogen, cellulose, chitin (glucose monomers).
  • Function: Starch and glycogen serve as energy storage, while chitin and cellulose provide structure.

• Heteroglycans:

  • Composed of multiple types of monosaccharides.
  • Typically branched structures.

Starch

• Composition: Starch consists of two homopolysaccharides of glucose:
  • Amylose: Unbranched polymer with α(1→4) linked glucose residues.
  • Amylopectin: Branched polymer with α(1→6) branch points every 20-25 residues.
  • Amylopectin can reach a molecular weight of up to 200 million.
  • Function: It is the primary storage polysaccharide in plants and is a significant carbohydrate source in human diet.

Glycogen

• Structure and Function:
  • Glycogen is the storage form of carbohydrates in vertebrates, chiefly found in liver and muscle cells.
  • Comprises about 8–10% of liver cells and 2–3% in muscle cells.
  • Structurally similar to amylopectin but features more α(1→6) branch points (every 8-12 residues) making it more compact and readily mobilized.

Cellulose

• Structure:

  • A polymer of D-glucopyranosides connected via β(1→4) glycosidic bonds.
  • This is the most crucial structural polysaccharide in plants and the most abundant organic compound on Earth.

• Microfibrils:

  • Composed of pairs of unbranched cellulose molecules (each about 12,000 glucose units) held together by hydrogen bonds, forming strong, sheet-like structures.
  • These bundles exhibit considerable tensile strength.
  • Important applications: dietary fiber, and materials like wood, paper, and textiles.

Cellulose Metabolism

• Substrate Challenges:
  • The fibrous, insoluble nature of cellulose makes it hard to digest.
• Digestive Enzymes:
  • Certain fungi, bacteria, and protozoa produce cellulase, allowing them to utilize wood for glucose.
  • Most animals lack the enzyme to hydrolyze β(1→4) linkages in cellulose, making cellulose unutilizable as a fuel source.
  • Some animals, such as ruminants and termites, rely on symbiotic microorganisms to produce cellulase.

Glycoproteins

• Definition:
  • Proteins with carbohydrate components are called glycoproteins, which are crucial in various biological roles.
• Classes of Glycoproteins:
  • Glycoproteins: Where protein is the predominant component.
  • Proteoglycans: Here, proteins are associated with a specific polysaccharide called glycosaminoglycan. In these, carbohydrates constitute the major portion by weight and serve various structural or lubrication roles.

Glycoprotein Linkages

• Attachment Sites:
  • Carbohydrates can link to proteins either through asparagine (N-linkage) or through serine/threonine (O-linkage).
  • N-linked polysaccharides contain a pentasaccharide core comprising three mannoses and two N-acetylglucosamines, with additional monosaccharides that can be attached.

Summary of the Pathways

• N-linked and O-linked Glycoproteins:
  • Diagram describes the mechanisms of glycosidic bonds between proteins (Asn for N-linked and Ser for O-linked) and carbohydrates.