Chapter 7 Carbohydrates and Glycobiology

1. Chapter 7 Carbohydrates and Glycobiology

2. Carbohydrates • Carbohydrates • The most abundant biomolecules on Earth • Many carbohydrates: (CH2O)n • Polyhydroxy aldehydes or ketones • Glycoconjugates • Carbohydrate polymers attached to proteins or lipids 1. Energy source 2. ECM 3. Lubricant 4. Protection barrier 5. Cell-cell communication 6. Intracellular trafficking

3. Carbohydrates • Classes of carbohydrates • Monosaccharides (simple sugar) • Oligosaccharides • Disaccharides • Polysaccharides Functional group: Aldose and Ketose # of carbons: triose (C3) tetrose (C4) pentose (C5) hexose (C6) heptose (C7)

4. 7.1 Monosaccharides and Disaccharides

5. Aldoses and Ketoses • Monosaccharide • Unbranched carbon chain with 3 to 7 carbons • Triose, tetrose, pentose, hexose, heptose • One carbonyl C & other carbons with hydroxyl group • Aldose : carbonyl group is at the end  aldehyde group • Ketose : carbonyl group is not at the end  ketone

6. Chiral Centers of Monosaccharides • One or more asymmetric (chiral) carbon centers •  Optically active isoforms; optical isomers or enantiomers • # of stereoisomers; 2n (n = # of chiral centers)

7. Chiral Centers of Monosaccharides • L- or D-isomer • Depending on the configuration of the reference carbon • D-isomer • Reference carbon configuration is the same as D-glyceraldehyde • -OH on the reference carbon; the right in the projection formula • most hexoses of living organism

8. Chiral Centers of Monosaccharides

9. Chiral Centers of Monosaccharides • Numbering C from the C nearest the carbonyl group • Nomenclature of 4- and 5-carbon ketoses • Inserting ‘ul’ into the name of a corresponding aldose • Ribose  ribulose • Epimer • Two sugars that differ only in the configuration around one C

10. Cyclic Structure of Monosaccharides • Formation of cyclic structure • Monosaccharides with 5 or more carbons (+ aldotetrose) • Formation of hemiacetals or hemiketals • Reaction between alcohol and aldehyde or ketone

11. Cyclic Structure of Monosaccharides • Generation of a or b isomers • Anomers • Isomers differ only in the configuration of hemiacetal or hemiketal carbon • Mutarotation • Interconversion of a and b anomers in aqueous solution • D-glucose • 1/3 a-D-glucose • 2/3 b-D-glucose • Very small amount of linear and glucofuranose • Types of ring structure • Pyranose : 6-membered ring • Furanose : 5-membered ring

12. Conformation and Configuration • Conformation of 6-membered pyranose • Not planar  2 “chair” conformations • Interconvertible without the breakage of covalent bonds

13. Hexose Derivatives • Glucosamine, galactosamine, mannosamine • C-2 OH is replaced with amino group • N-acetylglucosamine, N-acetylmuramic acid • Bacterial cell wall structure • C-6 deoxy sugars • L-galactose L-fucose (glycoproteins, glycolipids) • L-mannose L-rhamnose (plant polysaccharides) • Acidic sugars • Aldonic acid : oxidation of the carbonyl carbon • Gluconic acid  gluconate (ionization at pH 7) • Uronic acid : oxidation of the other end of the C chain • Glucuronic acid, galacturonic acid, mannuronic acid • Lactone • Stable intramolecular ester formation of aldonic and uronic acids • N-acetylneuraminic acid (sialic acid) • Component of many glycoproteins and glycolipids in animals • Phosphorylated sugar • Trap the sugar inside the cell • Activation of sugars for subsequence chemical transformation

14. Hexose Derivatives

15. Monosaccharide are Reducing Agents • Reducing sugar • Sugars capable of reducing Fe3+ or Cu2+ • Oxidation of carbonyl to carboxyl group • Only linear forms can be oxidized • Detection of glucose levels • Fehling’s reaction • Measuring the amount of oxidizing agent reduced by reducing sugar • Using glucose oxidase Red cuprous oxide precipitate under alkaline conditions

16. Disaccharides • Disaccharide (maltose, lactose, sucrose) • O-glycosidic bond • Covalent bond between monosaccharides • Reaction of –OH (alcohol) with anomeric carbon of another sugar (hemiacetal) to form acetal • Hydrolysis by acid • N-glycosyl bond • Anomeric carbon joined to N • Reducing end • A free anomeric carbon Glc (a14)Glc

17. Disaccharides • Reducing Disaccharides • Maltose, lactose • Nonreducing disaccharides (glycosides) • Sucrose • Major intermediate product of photosynthesis • Trehalose • Major constituent of the circulating fluid (hemolymph) of insects energy-storage

18. 7.2 Polysaccharides

19. Polysaccharides (Glycan) 1. monosaccharide unit 2. chain length 3. type of bond 4. degree of branching

20. Polysaccharides (Glycan) • Types of polysaccharide • Homopolysaccharides • Contain single type of monomer • Storage of monomer : glycogen, starch • Structural elements : cellulose, chitin • Heteropolysaccharides • Contain two or more types of monomer • Extracellular support • Bacterial cell wall • Extracellular matrix of animal • Synthesis of polysaccharides • Enzymatic polymerization without template • No specific stopping point

21. Homopolysaccharides as Stored Forms of Fuel • Storage polysaccharide • Starch (plant) and glycogen (animal) • Exist as intracellular clusters or granules • Heavily hydrated • Why do cells store glucose in the form of glycogen? • Glycogen in hepatocyte : 0.01mM equivalent to 0.4M glucose • Little contribution to the osmolarity of cytosol • Problem with glucose uptake

22. Glycogen and starch • Starch • Amylose; D-glucose connected by a14 linkages • Amylopectin; a14 chains with a16 branches • Glycogen • a14 chains with extensive a16 branches • More compact than starch • Stored in liver (7% of wet weight) and skeletal muscle • Degradation from nonreducing ends

23. Homopolysaccharides Playing Structural Roles • Cellulose • Component of plant cell wall • Fibrous, tough, water-insoluble • Linear D-glucose chains (unbranched) • 10,000 to 15,000 glc units • b14 linkages; different structural and physical properties from amylose • Cellulase • Produced by Trichonympha living in termites, wood-rot fungi, bacteria • Animals produce only a-amylase • Chitin • Exoskeleton of arthropods • Linear N-acetylglucosamine chains connected by b14 linkages

24. Structural difference between cellulose and amylose

25. Folding of Homopolysaccharides • Factors affecting folding of homopolysaccharides • Weak interactions • Hydrogen bonding • Rotation of glycosidic bonds • Restriction by steric hindrance

26. Folding of Homopolysaccharides • Stable structure • Starch and glycogen • Tightly coiled helix stabilized by H bond • Amylose • Helical structure with 6 residues/ turn • Cellulose • Straight extended chain • Extensive intra- and inter-chain H bond • Stable fiber of great tensile strength • Low water content

27. Heteropolysaccharides of Bacterial Cell Walls (Peptidoglycan) • Bacterial cell wall • Alternating N-acetylglucosamine and N-acetylmuramic acid; b14 • Polysaccharide chains are linked by bacterial specific peptide linkage • Prevention of cell wall formation • Lysozyme : hydrolysis of b14 linkage • Tears, bacterial viruses • Penicillin : preventing peptide-mediated cross-linking

28. Heteropolysaccharides of Bacterial Cell Walls

29. Heteropolysaccharides of Algal Cell Walls • Cell walls of red algae • Agar • A mixture of sulfated heteropolysaccharides - D-glactose and L-glactose derivatives (3,6-ether link) • Agarose (Mr ~ 120,000) • Unbranched polymer of D-Gal(b14)3,6-anhydro-L-Gal2S • Formation of gel after heating-cooling in water • Applications of agar and agarose • Surface to grow bacteria • Capsule for drug delivery • Used as matrix for DNA electrophoresis

30. Glycosaminoglycans • Extracellular matrix • Gel-like material filling the extracellular space in the tissues • Functions • Holding cells together • Providing porous pathways for nutrients and signaling molecules • Composition • Heteropolysaccharide: glycosaminoglycan • Fibrous proteins: collagen, elastin, fibronectin, laminin • Glycosaminoglycan • Linear polymer of repeating disaccharides • N-acetylglucosamine or N-acetylgalactosamine • Uronic acid : D-glucuronic or L-iduronic acid • Unique to animal and bacteria (not plant) • Addition of sulfate groups - High density of negative charges extended conformation - Specific recognition by protein ligands - Attached to extracellular protein  proteoglycan

31. Repeating disaccharides of Glycosaminoglycans • Hyaluronic acid : up to 50,000 repeat • Clear, highly viscous solutions • Lubricants in joints • Vitreous humor of vertebrate eye • ECM of cartilage and tendons  tensile strength and elasticity • Hyaluronidase • Hydrolysis of glycosidic linkage of hyaluronate • Pathogenic bacteria or sperm invasion • Chondroitin sulfate • Tensile strength of cartilage, tendons, ligaments, and the walls of the aorta • Dermatan sulfate • Pliability of skin, blood vessel, heart valves • GlcA in chondroitin  IodA (iduronate) Hyaluronate, ~ 50,000 Chondroitin 4-sulfate, 20-60

32. Glycosaminoglycans • Keratan sulfate • Cornea, cartilage, bone • Horny structures : horn, hair, hoofs, nails, claws • Heparin • Anticoagulant made in mast cells • Binding to antithrombin • Causes antithrombin to bind to and inhibit thrombin (protease)  prevent blood clotting • Highest negative charge density in biological macrobmolecules Keratan sulfate, ~ 25 Heparin, 15-90