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Chapter 7
- 2. 2 Chapter 7: Carbohydrates and Glycobiology keystone concepts: • All carbohydrates are made up of C, H, and O • The three elements that make up all carbohydrates are arranged as alcohols, aldehydes, or ketones • Monosaccharides are the monomeric subunits of di-, oligo-, and polysaccharides • Polysaccharides vary in composition, type of glycosidic bond, chain length, degree of branching, and biological function • Glycoconjugates including proteoglycans, glycoproteins, and glycolipids and are hybrid carbohydrate molecules with protein and lipid components • Carbohydrates function in energy storage, structural support, and intercellular signaling
- 3. 3 Monosaccharide classification • Monosaccharides have the general formula CnH2nOn, where n varies from 3 to 8. • Aldose: a monosaccharide containing an aldehyde group. • Ketose: a monosaccharide containing a ketone group.
- 4. • all monosaccharides (except dihydroxyacetone) contain 1+ chiral carbon atom – occur in optically active isomeric forms • enantiomers = two different optical isomers that are mirror images • in general, a molecule with n chiral centers can have 2n stereoisomers Monosaccharides have chiral carbons
- 5. • are used to represent three-dimensional sugar structures on paper • bonds drawn horizontally indicate bonds that project out of the plane of the paper • bonds drawn vertically project behind the plane of the paper • The carbon chain is written vertically with the most oxidized carbon at the top. • Carbohydrates can have multiple chiral carbons; the configuration of groups around each carbon atom determines how the compound interacts with other biomolecules. Fischer Projections
- 6. • The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group • in a standard Fischer projection if the hydroxyl group is on the right the molecule is a D sugar (dextro), otherwise it is an L sugar (levo). • Assignment is compared to glyceraldehyde D Isomers and L Isomers
- 7. What Makes Sugar Sweet? • TAS1R2 and TAS1R3 encode sweet-taste receptors • binding of a compatible molecule generates a “sweet” electrical signal in the brain – requires a steric match
- 8. D-Aldoses
- 9. D-Ketoses
- 10. Epimers • epimers = two sugars that differ only in the configuration around one carbon atom
- 11. Galactose is in many plant gums and pectins • component of the disaccharide lactose Fructose is the sweetest of all the naturally occurring sugars • honey, fruits • component of the disaccharide sucrose Monosaccharides
- 12. Other monosaccarides -components of DNA and RNA
- 13. • in aqueous solution, all monosaccharides with 5 or more backbone carbons occur as cyclic structures • The carbonyl group of a straight- chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom • To form a hemiacetal or hemiketal, forming a heterocyclic ring with an oxygen bridge between two carbon atoms. • Rings with five and six atoms are called furanose and pyranose respectively. 13 The Common Monosaccharides Have Cyclic Structures
- 14. Hemiacetals and Hemiketals • hemiacetals or hemiketals are the derivatives formed by a general reaction between alcohols and aldehydes or ketones • acetal or ketal = product of the second alcohol molecule addition – forms a glycosidic bond
- 15. α versus β • The carbon atom containing the carbonyl oxygen is called the anomeric carbon • The oxygen atom may take a position either above or below the plane of the ring. • The resulting possible pair of stereoisomers are called anomers. In the α anomer, the - OH substituent on the anomeric carbon rests on the opposite side (trans) of the ring from the CH2OH side branch.
- 16. Pyranoses and Furanoses • pyranoses = six-membered ring compounds – form when the hydroxyl group at C-5 reacts with the keto group at C-1 • furanoses = five-membered ring compounds – form when the hydroxyl group at C-5 reacts with the keto group at C-2
- 17. Organisms Contain a Variety of Hexose Derivatives
- 18. Symbols and Abbreviations for Monosaccharides and Derivatives
- 19. Haworth Perspective Formulas • Haworth perspective formulas = more accurate representation of cyclic sugar structure than Fischer projections – six-membered ring is tilted to make its plane almost perpendicular to that of the paper – bonds closest to the reader are drawn thicker than those farther away
- 20. Sugars That Are, or Can Form, Aldehydes Are Reducing Sugars • reducing sugars = undergo a characteristic redox reaction where free aldehyde groups react with Cu2+ under alkaline condition – reduction of Cu2+ to Cu+ forms a brick-red precipitate
- 21. O-Glycosidic Bonds to form dissacharides • O-glycosidic bond = covalent linkage joining two monosaccharides – formed when a hydroxyl group of one sugar molecule reacts with the anomeric carbon of the other
- 22. The Reducing End • formation of a glycosidic bond renders a sugar nonreducing • reducing end = the end of a disaccharide or polysaccharide chain with a free anomeric carbon Free anomeric carbon
- 23. Naming Reducing Oligosaccharides • step 1: with the nonreducing end on the left, give the configuration (α or β) at the anomeric carbon joining the first unit to the second • step 2: name the nonreducing residue using “furano” or “pyrano” • step 3: indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers • step 4: name the second residue and repeat for additional residues
- 24. Three Common Disaccharides • lactose is a reducing disaccharide • sucrose and trehalose are nonreducing sugars
- 25. Polysaccharides - a carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds. Starch -2/3 of the human diet -Potatoes, rice, wheat, cereal grains -mixture of amylose and amylopectin Glycogen -only storage for glucose in the body -liver and muscle -similar in structure to amylopectin but, more branched
- 26. 26 polysaccharides (glycans) Differ by: • Monosaccharide identity • Length • Glycosidic bond type • Branching - a carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds. - most carbohydrates in nature occur as polysaccharides (Mr > 20,000) heteropolysaccharides = contain 2+ kinds of monomers – provide extracellular support homopolysaccharides = contain only a single monomeric sugar species - serve as storage forms and structural elements
- 27. Storage: Starch and Glycogen • starch = contains two types of glucose polymer, amylose and amylopectin – amylose = long, unbranched chains of D-glucose residues connected by (α1→4) linkages – amylopectin = larger than amylose with (α1→4) linkages between glucose residues and highly branched due to (α1→6) linkages • glycogen = polymer of (α1→4)-linked glucose subunits, with (α1→6)-linked branches – more extensively branched – more compact than starch
- 28. Starch • Starch: a polymer of D-glucose. – Starch can be separated into amylose and amylopectin. – Amylose is composed of unbranched chains of up to 4000 glucose units joined by a-1,4- glycosidic bonds. – Amylopectin contains chains up to 10,000 D- glucose units also joined by a-1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by a-1,6-glycosidic bonds.
- 31. Glycogen • Glycogen is the energy-reserve carbohydrate for animals. – Glycogen is a branched polysaccharide of approximately 106 glucose units joined by a-1,4- and a- 1,6-glycosidic bonds. – The total amount of glycogen in the body of a well- nourished adult human is about 350 g, divided almost equally between liver and muscle.
- 32. 32 Glycogen • Especially abundant in liver and muscle • Why is it essential that glycogen be highly branched? • Glucose removal from nonreducing ends is more rapid • Why not store glucose in monomeric form? • Osmolarity • Hydration
- 33. 33 Cellulose • Linear, unbranched homopolysaccharide of glucose that is found in plants (stalks, stems, trunks) • Fibrous, tough, water insoluble Interchain and intrachain hydrogen bonds produce a supramolecular fiber with great tensile strength
- 34. Cellulose • Cellulose is a linear polysaccharide of D-glucose units joined by b-1,4-glycosidic bonds. – It has an average molecular weight of 400,000 g/mol, corresponding to approximately 2200 glucose units per molecule. – Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds. – This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength. – It is also the reason why cellulose is insoluble in water.
- 35. Cellulose • Cellulose (cont’d) – Humans and other animals cannot use cellulose as food because our digestive systems do not contain b-glucosidases (a cellulase), enzymes that catalyze hydrolysis of b-glucosidic bonds. – Instead, we have only a-glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen. – Many bacteria and microorganisms have b-glucosidases and can digest cellulose. – Termites have such bacteria in their intestines and can use wood as their principal food. – Ruminants (cud-chewing animals) and horses can also digest grasses and hay.
- 37. 37 Chitin • Linear homopolysaccharide of N-acetylglucosamine in b linkage • C-2 hydroxyl is replaced with an acetylated amino group • Not digestible • Exoskeletons of arthropods, lobsters etc. • Second most abundant polysaccharide in nature (cellulose is #1)
- 38. 38 Bacterial cell wall: Peptidoglycans • Heteropolysaccharide of N- acetylglucosamine (NAG) and N-acetylmuramic (NAM) acid in b1→4 linkage • Linear polymers lie side by side and are crosslinked by small peptides • Strong sheath prevents swelling and lysis • Lysozyme breaks the b1→4 glycosidic bond • Penicillin prevents synthesis of the peptide crosslinks
- 39. 39 Extracellular matrix: glycosaminoglycans • Extracellular matrix is a porous substance in animal and bacteria that holds cells together while allowing diffusion of small molecules • Composed of repeating disaccharides with fibrous proteins (collagen, elastin, fibronectin and laminin) • One of the monosaccharides is either NAG or NAM • Hydroxyls sometimes esterified with sulfate Joints, vitreus humor in eye Cartilage, tendons, ligaments Cartilage, cornea, horns
- 41. 41 Glycoconjugates: Proteoglycans • Glycogonjugates are informational carbohydrates that are covalently linked to proteins or lipids • Macromolecules of the cell surface or extracellular matrix or secreted • One or more sulfated glycosaminoglycan covalently attached to a core protein Proteoglycans can aggregate (ex: aggrecan of the extracellular matrix). Interact with collagen gives strength
- 42. 42 Interactions between cells and the extracellular matrix • Cellular and extracellular interactions: – Anchor cells to the extra- cellular matrix – Direct cell migration – Signal transduction
- 43. 43 Lipids may contain covalently bound oligosaccharides • Gangliosides – membrane lipid - cell recognition (blood typing) • Lipopolysaccharides – Prominent feature ing outer membrane of gram negative bacteria – Antibody recognition Lipopolysaccharide of Salmonella
- 44. 44 Lectins bind carbohydrates • Proteins that bind carbohydrates with varying specificity • Serve in cell recognition, signaling and adhesion • Useful to detect and separate glycoproteins Selectins: type of lectin in the plasma membrane Lectins also mediate bacterial adhesion to host cells as in the case of H. pylori
- 45. 45 Oligosaccharides: recognition and adhesion at the cell surface • Oligosaccharides, as part of glycoproteins and glycolipids, interact with lectins in the extracellular space • Viruses, bacterial toxins, and some bacteria bind to glycoproteins to begin the infection/disease process • Lectins in the plasma membrane mediate cell-cell interactions • Lysosomal enzymes are directed into the lysosome through oligosaccharide recognition