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Chapter 8 - Carbohydrates

Chapter 8 - Carbohydrates. Carbohydrates (“hydrate of carbon”) have empirical formulas of (CH 2 O) n , where n ≥ 3 Mono saccharides one monomeric unit Oligosaccharides ~2-20 monosaccharides Polysaccharides > 20 monosaccharides Glyco conjugates linked to proteins or lipids

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Chapter 8 - Carbohydrates

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  1. Chapter 8 - Carbohydrates • Carbohydrates (“hydrate of carbon”) have empirical formulas of (CH2O)n , where n ≥ 3 • Monosaccharides one monomeric unit • Oligosaccharides ~2-20 monosaccharides • Polysaccharides > 20 monosaccharides • Glycoconjugates linked to proteins or lipids • Trioses - 3 carbon sugars • Tetroses - 4 carbon sugars • Pentoses - 5 carbon sugars • Hexoses - 6 carbon sugars Chapter 8

  2. Fig. 8.3 • Trioses - 3 carbon sugars

  3. Tetroses - 4 carbon sugars Fig. 8.3

  4. Pentoses - 5 carbon sugars Fig. 8.3

  5. Pentoses - 5 carbon sugars Fig. 8.3

  6. Hexoses - 6 carbon sugars Fig 8.3

  7. Hexoses - 6 carbon sugars Fig 8.3

  8. Enantiomers and epimers • D-Sugars predominate in nature • Enantiomers- pairs of D-sugars and L-sugars • Epimers- sugars that differ at only one of several chiral centers • Example: D-galactose is an epimer of D-glucose

  9. Fig 8.6 (a) Pyran and (b) furan ring systems • (a) Six-membered sugar ring is a “pyranose” • (b) Five-membered sugar ring is a “furanose”

  10. Fig 8.8 Cyclization of D-glucose to form glycopyranose In aqueous solution hexoses and pentoses will cyclize, forming alpha (a) and beta (b) forms Chapter 8

  11. Fig 8.10 Cyclization of D-ribose to form a- and b-D-ribopyranose and a- and b-D-ribofuranose

  12. 8.4 Derivatives of Monosaccharides • Many sugar derivatives are found in biological systems • Some are part of monosaccharides, oligosaccharides or polysaccharides • These include sugar phosphates, deoxy and amino sugars, sugar alcohols and acids

  13. Sugar Phosphates Fig 8.14 Some important sugar phosphates

  14. Deoxy Sugars • In deoxy sugars an H replaces an OH • Fig 8.15 Deoxy sugars Chapter 8

  15. Amino Sugars • An aminogroup replaces a monosaccharide OH • Amino group is sometimes acetylated • Amino sugars of glucose and galactose occur commonly in glycoconjugates

  16. Sugar Alcohols (polyhydroxy alcohols) • Sugar alcohols: carbonyl oxygen is reduced • Fig 8.17 Several sugar alcohols

  17. Sugar Acids • Sugar acids are carboxylicacids Fig 8.18 Sugar acids derived from glucose

  18. Sugar Acids • L-Ascorbic acid (Vitamin C) is derived from D-glucuronate L-Ascorbic acid (Vitamin C) Fig 8.18 L-Ascorbic acid Chapter 8

  19. Disaccharides and Other Glycosides • Glycosidic bond - primary structural linkage in allpolymers of monosaccharides • Glucosides- glucose provides the anomeric carbon Fig 8.20 Structures of disaccharides (a) maltose, (b) cellobiose

  20. Fig 8.20 Structures of disaccharides (c) lactose, (d) sucrose

  21. Polysaccharides • Homoglycans - homopolysaccharides containing only one type of monosaccharide • Heteroglycans - heteropolysaccharides containing residues of more than one type of monosaccharide • Lengths and compositions of a polysaccharide may vary within a population of these molecules Chapter 8

  22. Starch • D-Glucose is stored intracellularly in polymeric forms • Plants and fungi store glucose as starch • Starch is a mixture of amylose (unbranched) and amylopectin (branched every 25 sugars) • Amylose is a linear polymer • Figure 8.22 • Amylopectin is a branched polymer • Figure 8.23 Chapter 8

  23. Amylose and Amylopectin form helical structures in starch granules of plants Chapter 8

  24. Starch is stored by plants and used as fuel. Chapter 8

  25. Glycogen is is stored by animals and used as fuel. Chapter 8

  26. Glycogen Glycogen is the main storage polysaccharide of humans. Glycogen is a polysaccharide of glucose residues connected by a -(1-4) linkages with a -(1-6) branches (one branch per 10 sugars). Glycogen is present in large amounts in liver and skeletal muscle. Chapter 8

  27. Cellulose, a structural polysaccharide in plants has b-(1-4) glycosidic bonds Fig 8.25 Structure of cellulose

  28. Fig 8.26 Cellulose fibrils • Intra- and interchain Hydrogen bonds give strength

  29. Humans digest starch and glycogen ingested in their diet using amylases, enzymes that hydrolyze a-(1-4) glycosidic bonds. Humans cannot hydrolyze b-(1-4) linkages of cellulose. Therefore cellulose is not a fuel source for humans. It is fiber. Certain microorganisms have cellulases, enzymes that hydrolyze b-(1-4) linkages of cellulose. Cattle have these organisms in their rumen. Termites have them in their intestinal tract. Chapter 8

  30. Fig 8.27 Structure of chitinThe exoskeleton of arthropods • Repeating units of b-(1-4)GlcNAc residues GlcNAc = N-acetylglucosamine

  31. Glycoconjugates • Heteroglycans appear in 3 types of glycoconjugates: • 1. Proteoglycans • 2. Peptidoglycans • 3. Glycoproteins Chapter 8

  32. Proteoglycans • Proteoglycans - glycosaminoglycan-protein complexes • Glycosaminoglycans - unbranched heteroglycans of repeating disaccharides of amino sugars (D-galactosamine or D-glucosamine) Chapter 8

  33. Fig 8.28 Repeating disaccharide of hyaluronic acid, a glycosaminoglycan • GlcUA =D-glucuronate • GlcNAc= N-acetylglucosamine

  34. Fig 8.29 Proteoglycan aggregate of cartilage

  35. Peptidoglycans - heteroglycan chains linked to peptides • Major component of bacterial cell walls • Heteroglycan composed of alternating N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) • b-(1-4) linkages connect the units Peptidoglycans Chapter 8

  36. Fig 8.30 Glycan moiety of peptidoglycan

  37. Fig 8.31 Structure of the peptidoglycan of the cell wall of Staphylococcusaureus (a) Repeating disaccharide unit, (b) Cross-linking of the peptidoglycan macromolecule

  38. Penicillin inhibits a transpeptidase involved in bacterial cell wall formation • Fig 8.32 Structures of penicillin and -D-Ala-D-Ala • Penicillin structure resembling -D-Ala-D-Ala is shown in red

  39. Glycoproteins • Proteins that contain covalently-bound oligosaccharides, either to serine (O-Glycosidic linkage) or asparagine (N-glycosidic linkage) • Oligosaccharide chains exhibit great variability in sugar sequence and composition Fig. 8.33 O-Glycosidic and N-glycosidic linkages

  40. Fig 8.34 and 8.35. Types of glycosidic linkages

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