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Principles of BIOCHEMISTRY Third Edition

Principles of BIOCHEMISTRY Third Edition

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Principles of BIOCHEMISTRY Third Edition

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  1. Principles ofBIOCHEMISTRYThird Edition HORTON MORAN OCHS RAWN SCRIMGEOUR Chapter 8

  2. 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 Chapter 8

  3. 8.1 Most Monosaccharides are Chiral Compounds • Aldoses - polyhydroxy aldehydes • Ketoses- polyhydroxy ketones • Most oxidized carbon: aldoses C-1, ketoses usually C-2 • Trioses (3 carbon sugars) are the smallest monsaccharides Chapter 8

  4. Aldoses and ketoses • Aldehyde C-1 is drawn at the top of a Fischer projection • Glyceraldehyde (aldotriose) is chiral (C-2 carbon has 4 different groups attached to it) • Dihydroxyacetone (ketotriose) does not have an asymmetric or chiral carbon and is not a chiral compound Chapter 8

  5. Fig 8.1 Fischer projections of: (a) L- and D-glyceraldehyde, (b) dihydroxyacetone Chapter 8

  6. Fig 8.2 Stereo view of L- and D-glyceraldehyde (L) (D) Chapter 8

  7. Fig 8.3 Fisher projections of 3 to 6 carbon D-aldoses • D-sugars have the same configuration as D-glyceraldehyde in their chiral carbon most distant from the carbonyl carbon • Aldoses shown in blue (next slide) are most important in biochemistry Chapter 8

  8. Fig. 8.3 Chapter 8

  9. Fig. 8.3 (continued) Chapter 8

  10. Fig 8.3 (continued) Chapter 8

  11. 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 at C-4 Chapter 8

  12. Fig 8.4 Fisher projections of L- and D-glucose Chapter 8

  13. Fig 8.5 Fisher projections of the 3 to 6 carbon D-ketoses (blue structures are most common) Chapter 8

  14. Fig. 8.5 (continued) Chapter 8

  15. Fig 8.5 (continued) Chapter 8

  16. 8.2 Cyclization of Aldoses and Ketoses Fig. 8.6 Reaction of an alcohol with: (a) An aldehyde to form a hemiacetal (b) A ketone to form a hemiketal Chapter 8

  17. Fig 8.7 (a) Pyran and (b) furan ring systems • (a) Six-membered sugar ring is a “pyranose” • (b) Five-membered sugar ring is a “furanose” Chapter 8

  18. Fig 8.8 Cyclization of D-glucose to form glycopyranose • Fischer projection (top left) • Three-dimensional figure (top right) • C-5 hydroxyl close to aldehylde group (lower left) Chapter 8

  19. Fig. 8.8 (continued) • Reaction of C-5 hydroxyl with one side of C-1 gives a, reaction with the other side gives b Chapter 8

  20. Fig 8.9 Cyclization of D-ribose to form a- and b-D-ribopyranose and a- and b-D-ribofuranose Continued on next slide Chapter 8

  21. Fig. 8.9 (continued) Continued next slide Chapter 8

  22. Fig 8.9 (continued) Chapter 8

  23. 8.3 Conformations of Monosaccharides Fig. 8.10 Conformations of b-D-ribofuranose Chapter 8

  24. Fig 8.11 Conformations of b-D-glucopyranose Haworth projection Chair conformation Boat conformation (b) Stereo view of chair (left), boat (right) Chapter 8

  25. Fig 8.12 Conformations of b-D-glucopyranose • Top conformer is more stable because it has the bulky hydroxyl substituents in equatorialpositions (less steric strain) Chapter 8

  26. 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 Chapter 8

  27. Table 8.1 Chapter 8

  28. A. Sugar Phosphates Fig 8.13 Some important sugar phosphates Chapter 8

  29. B. Deoxy Sugars • In deoxy sugars an H replaces an OH • Fig 8.14 Deoxy sugars Chapter 8

  30. C. Amino Sugars • An aminogroup replaces a monosaccharide OH • Amino group is sometimes acetylated • Amino sugars of glucose and galactose occur commonly in glycoconjugates Chapter 8

  31. Fig 8.15 Several amino sugars • Amino and acetylamino groups are shown in red Chapter 8

  32. Fig. 8.15 (continued) Chapter 8

  33. D. Sugar Alcohols (polyhydroxy alcohols) • Sugar alcohols: carbonyl oxygen is reduced • Fig 8.16 Several sugar alcohols Chapter 8

  34. E. Sugar Acids • Sugar acids are carboxylicacids • Produced from aldoses by: • (1) Oxidation of C-1 to yield an aldonic acid • (2) Oxidation of the highest-numbered carbon to an alduronic acid Chapter 8

  35. Fig 8.17 Sugar acids derived from glucose Chapter 8

  36. Fig. 8.17 (continued) Chapter 8

  37. F. Ascorbic Acid (Vitamin C) • L-Ascorbic acid is derived from D-glucuronate • Fig 8.18 L-Ascorbic acid Chapter 8

  38. 8.5 Disaccharides and Other Glycosides • Glycosidic bond - primary structural linkage in allpolymers of monosaccharides • An acetal linkage - the anomeric sugar carbon is condensed with an alcohol, amine or thiol • Glucosides- glucose provides the anomeric carbon Chapter 8

  39. Fig 8.19 Glucopyranose + methanol yields a glycoside Chapter 8

  40. A. Structures of Disaccharides Fig 8.20 Structures of (a) maltose, (b) cellobiose Chapter 8

  41. Fig. 8.20 (continued) Structures of (c) lactose, (d) sucrose Chapter 8

  42. B. Reducing and Nonreducing Sugars • Monosaccharides and most disaccharides are hemiacetals (contain a reactive carbonyl group) • Called reducingsugars because they can reduce metal ions (Cu2+, Ag+) • Examples: glucose, maltose, cellobiose, lactose Chapter 8

  43. C. Nucleosides and Other Glycosides • Anomeric carbons of sugars can form glycosidic linkages with alcohols, amines and thiols • Aglycones are the groups attached to the anomeric sugar carbon • N-Glycosides - nucleosides attached via a ring nitrogen in a glycosidic linkage Chapter 8

  44. Fig 8.21 Structures of three glycosides Chapter 8

  45. 8.6 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

  46. Chapter 8

  47. A. Starch and Glycogen • D-Glucose is stored intracellularly in polymeric forms • Plants and fungi - starch • Animals - glycogen • Starch is a mixture of amylose (unbranched) and amylopectin (branched) Chapter 8

  48. Fig 8.22 Structure of amylose (a) Amylose is a linear polymer (b) Assumes a left-handed helical conformation in water Chapter 8

  49. Fig 8.23Structure of amylopectin Chapter 8

  50. Fig 8.24 Action of a- and b-amylase on amylopectin • a-Amylase cleaves random internal a-(1-4) glucosidic bonds • b-Amylase acts on nonreducing ends Chapter 8