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Chemistry of Carbohydrates

Chemistry of Carbohydrates. Lecture for MBBS by, Dr. Nivedita L. Rao Prof. of Biochemistry. LEARNING OBJECTIVES. By completing this course on carbohydrate chemistry, you will learn about their-. Definition Biomedical importance Classification and examples

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Chemistry of Carbohydrates

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  1. Chemistry of Carbohydrates Lecture for MBBSby, Dr. Nivedita L. Rao Prof. of Biochemistry

  2. LEARNING OBJECTIVES By completing this course on carbohydrate chemistry, you will learn about their- • Definition • Biomedical importance • Classification and examples • Structures, properties and functions 02/49

  3. Introduction: Carbohydrates are the most abundant class of biomolecules 06/49

  4. Definition: Carbohydrates are polyhydroxy alcohols with free or potentially free aldehyde or keto groups. Carbohydrates are polyhydroxy alcohols with aldehyde/ keto groups or compounds which yield these on hydrolysis. 07/49

  5. Medical importance: • Some disorders associated withcarbohydrates- Diabetes mellitus Galactosemia Lactose intolerance Glycogen storage diseases Mucopolysaccharidoses • Cardiac glycosides are important in pharmaceutical practice.

  6. Classification: Carbohydrates Oligosaccharides Monosaccharides Polysaccharides Homo polysaccharides Hetero polysaccharides Aldoses Disaccharides Trioses Tetroses Trisaccharides Ketoses Pentoses Other Hexoses etc. 04/49

  7. Classification: Carbohydrates Oligosaccharides Monosaccharides 2-10 monosaccharide units Polysaccharides Simplest, basic units > 10 monosaccharide units 04/49

  8. Monosaccharides – Classification 1. Based on the functional group present: Monosaccharides Aldoses Ketoses Contain aldehyde group Contain keto group 04/49

  9. Monosaccharides – Classification 2. Based on the number of carbon atoms present Monosaccharides Trioses 3C Heptoses 7C Tetroses 4C Hexoses 6C Pentoses 5C 04/49

  10. Monosaccharides - Examples 14/49

  11. Oligosaccharides – Classification Based on the number of monosaccharide units they contain (2-10). Oligosaccharides Disaccharide Other Trisaccharide 04/49

  12. Oligosaccharides- Examples 14/49

  13. Polysaccharides – Classification Polysaccharides Heteropolysaccharides Homopolysaccharides Contain only one type of monosaccharide Contain more than one type of monosaccharide 04/49

  14. Polysaccharides- Examples 14/49

  15. Chemistry of Monosaccharides sTRUCTURE Physical and chemical PROPERTIES FUNCTIONS 13/49

  16. Monosaccharides (Simple sugars): Consist of a single polyhydroxy aldehyde or ketone unit. Empirical formula: (CH2O)n suggests that they are carbon “hydrates” e.g: Empirical formula of glucose is C6H12O6 which can also be written (CH2O)6

  17. Monosaccharides – Structure aldehyde group C1 anomeric carbon 1 keto group 2 2 C2 anomeric carbon 3

  18. Ring structures of Monosaccharides (Glucose) (Fructose)

  19. a. b. c. Open chain (a) and pyranose ring structure of glucose Pyranose ring structure of glucose can be written in 2 different ways (b and c)

  20. Learning check point: ??? Identify the monosaccharide among the following. a. Maltose b. Fructose c. Sucrose d. Cellulose 

  21. Isomerism in Monosaccharides • Aldose-ketose isomerism (Structural isomerism) 2. Stereoisomerism

  22. 1. Aldose-ketose isomerism H C=O Aldehyde group C=O Ketogroup 17/49

  23. 2. Stereoisomerism Stereoisomers, • have same structural formulae but different configuration • have asymmetric carbon atoms • are optically active; dextrorotatory/d- / + or levorotatory/ l-/ -

  24. 3 types of stereoisomerism – • D - L Stereoisomerism • Epimerism • Anomerism

  25. a) D-L Stereoisomerism • Depends on configuration around the last asymmetric C atom (C5 for glucose) Enantiomers (mirror images) More common in nature OH OH

  26. b) Epimerism Epimers differ in configuration at a single asymmetric carbonother than anomeric and penultimate C atoms. 18/49

  27. c) Anomerism Occurs due to cyclization which makes anomeric carbon asymmetric –OH group attached to anomeric carbon atom is below the plane of the ring in -anomers and above the plane in -anomers. 21/49

  28. Physical Properties of Monosaccharides • Highly water soluble • Sweet in taste (sugars) • Mutarotation

  29. Mutarotation- change in the optical activity on standing a freshly prepared sugar solution 1/3rd 2/3rd 22/49

  30. Some Important Reactions of Monosaccharides I. Reactions Due to Carbonyl Group II. Reactions Due to Hydroxyl Group

  31. I. Reactions Due to Carbonyl Group: • Carbonyl group (C=O) in all monosaccharides is free so, all monosaccharides are – • reducing in nature • form osazones with phenylhydrazine • form glycosides with alcohols • exhibit mutarotation 26/49

  32. Glycosides and Glycosidic Linkage: Glycoside H2O + HO–R O – R alcohol / phenol α- Glycosidic Bond 27/49

  33. Some Medically Important Glycosides:

  34. II. Reactions Due to Hydroxyl Group: Dehydration Carbohydrate Furfural/ -H2O furfural derivative CondensationPhenols Colored products Tests: • Molisch test(a general test for carbohydrates) • Seliwanoff’s test(test for ketosugars)

  35. Monosaccharides of physiologic importance

  36. Physiologically Important Monosaccharide derivatives:  • Deoxyribose (DNA) • Amino sugars (hexosamines) D-glucosamine, D-galactosamine • Sugar acids(uronic acids) Glucuronic acid, Iduronic acid • Sugar alcohols (polyols) Sorbitol, Ribitol • Sugar phosphates: Glucose-6-phosphate, fructose-6-phosphate

  37. Learning check point: ??? • The following sugars are epimers of glucose except: a. Fructose b. Galactose c. Mannose 

  38. Oligosaccharides: • They contain 2 to 10 monosaccharide units combined together by glycosidic linkages. 11/49

  39. Chemistry of Disaccharides sTRUCTURE Physical and chemical PROPERTIES FUNCTIONS 13/49

  40. Disaccharides: • They consist of two monosaccharide units combined by glycosidic linkage. • The important disaccharides are- Maltose, Lactose and Sucrose 10/49

  41. Maltose α- D-Glucose α- D-Glucose Free α-1,4 glycosidic linkage 32/49

  42. Lactose β –D-Galactose β –D-Glucose β-1,4 glycosidic linkage 33/49

  43. Sucrose 1 α- D-Glucose α-β-1,2 β –D-Fructose 2 34/49

  44. Sources and importance of Disaccharides

  45. Learning check point: ??? • Table sugar contains- a.Glucose + Glucose b. Glucose + Galactose c. Glucose + Fructose d. Glucose + Mannose 

  46. Reactions Of Disaccharides: 1. Reducing property: • Lactose, maltose and isomaltose are reducing sugars- anomeric carbon atoms are free • Sucrose and trehalose are non-reducing sugars- anomeric carbon atoms not free/ involved in glycosidic linkage • sucrose and trehalose do not form osazones, do not exhibit mutarotation

  47. 2. Hydrolysis (digestion) of Disaccharides: • by- • acid and heat • specific digestive enzymes- Maltase • Maltose Glucose + Glucose Lactase • Lactose Galactose + Glucose Sucrase (Invert sugar) • Sucrose Glucose + Fructose levorotatory dextrorotatory

  48. Other Oligosaccharides • contain 3 -10 monosaccharide units • rarely occur freely in nature • mostly occur in- Glycoproteins-covalently linked to proteins and Glycolipids- covalently linked to lipids

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