Lecture 16. Carbohydrates: Polysaccharides
Thursday 21 March 2019
Complex carbohydrates: Starches, cellulose and chitin. Polysaccharide structure. Heteropolysaccharides of bacterial and algal cell walls and extracellular matrix. Glycobiology.
Reading: Lehninger - Ch.7, pp.252-272.
Summary
Reading summary. §7.2 Polysaccharides.
Homopolysaccharides and heteropolysaccharides. Some homopolysaccharides are storage forms of fuel.
Starch (amylose and amylopectin) and glycogen. Dextrans.
Some homopolysaccharides serve structural roles. Cellulose and chitin.
Steric factors and hydrogen bonding influence homopolysaccharide folding.
Bacterial and algal cell walls contain structural heteropolysaccharides.
Glycosaminoglycans are heteropolysaccharides of the extracellular matrix.
§7.3 Glycoconjugates: Proteoglycans, glycoproteins,
and glycosphingolipids.
Proteoglycans are glycosaminoglycan-containing macromolecules of the cell surface and
extracellular matrix.
Box 7-3 -Medicine- Defects in the synthesis or degradation of sulfated glycosaminoglycans
can lead to serious human disease.
Glycoproteins have covalently attached oligosaccharides.
Glycolipids and lipopolysaccharides are membrane components.
§7.4 Carbohydrates as informational molecules: The sugar code.
Lectins are proteins that read the sugar code and mediate many biological processes.
Lectin-carbohydrate interactions are highly specific and often multivalent.
***
Carbohydrates are the prime example of metabolic "fuel" for cells. The stepwise degradation of the monosaccharide glucose is a nearly universal pathway that yields biochemical energy in the form of adenosine triphosphate (ATP). Plants, in carrying out photosynthethesis, harvest the energy of sunlight by using it to convert carbon dioxide and water into carbohydrates. Glucose, fructose, and ribose are among the most common monosaccharides in biochemistry, a monosaccharide being a carbohydrate that cannot be broken down into simpler carbohydrate fragments. Polymeric forms of such simple carbohydrates are the complex carbohydrates. Furthermore, carbohydrate moeities are linked to or incorporated in other biomolecules, forming nucleotides, glycolipids, and glycoproteins. Glycobiology examines the roles of carbohydrates, glycoproteins, and other glycoconjugates in cellular and multicellular structure and function.