1. Carbohydrates and Glycobiology
Chapter 7
Carbohydrates and Glycobiology

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

3. 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) Reducing end
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 Glycogen
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)
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 Glycosaminoglycan
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