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FOUNDATION MODULE 2012 Isomerism and Monosaccharide Derivatives LEARNING OBJECTIVES By the end of the lecture, the student will be able to : Explain isomerism in carbohydrates Describe the monosaccharide derivatives (sugar acids, sugar alcohols, aminosugars and de-oxysugars) Discuss the clinical importance of monosaccharide derivatives (GAGs, detoxification, cataract, diabetes mellitus, hexosamines, DNA) ISOMERS Compounds that have the same chemical but have different structural formula For Example : Fructose, Glucose, mannose, and galactose are all isomers of each other, having the same chemical formula , C6 H12 O6. Carbon satisfied with four different atoms/groups of atoms is called chiral/asymmetric carbon D, L Isomers • Position of OH group around carbon adjacent to primary alcohol carbon Epimers Enantiomers Enantiomers are molecules that are mirror images of each other. The two members of the pair are designated as a D – and an L – sugar. Ring Structure In aqueous solutions sugar molecule acquires a ring structure 5 membered furanose (4C) 6 membered pyranose (5C) - and - Anomers Functional group carbon becomes asymmetric in ring form known as Anomeric carbon -configuration/anomer: the OH group on anomeric carbon is below the plane of the ring. -configuration/anomer: the OH group on anomeric carbon is above the plane of the ring. Optical Isomers Plane polarized light is rotated upon passage through the sugar solution Dextrorotatory (d, +) To the right Levorotatory (l,-) Inverse sugar Dextrorotatory nature of sucrose changes to levorotary upon hydrolysis because of fructose is naturally occurring D(-) sugar Oxidation Reactions of Monosaccharides Oxidized to acids Aldoses and ketoses give positive tests when treated with Benedict’s reagent Benedict’s reagent (an alkaline solution of cupric citrate complex) gives a brick red precipitate of Cu2O The Sugar Acids Selective oxidation of the aldehyde group of an aldose to the corresponding carboxylic acid An aldose becomes an Aldonic acid Selective oxidation of the aldehyde group of carbon number 6 produces Uronic Acids Dilute nitric acid oxidizes both the aldehyde and primary hydroxyl groups of an aldose to an Aldaric acid Significance Sugar acids are present in glycosaminoglycans. Detoxification reactions Drugs Bilirubin Glycosaminoglycans Reduction Reactions of Monosaccharides Aldoses and ketoses can be reduced to alditols Significance High blood sugar level Sugar enters in the cell Reduced to sorbitol Osmotically active Drags water in the medium E.g. Cataract Diabetic neuropathy Therapeutic use: Mannitol for cerebral edema Deoxy Sugars Oxygen is removed from the hydroxyl group Dexoribo-Nucleotides Deoxyribo-nucleic acids Significance Amino-Sugars Carbonyl group reacts with amino group to form amino sugars Glycosaminoglycans References Lippincott’s Biochemistry . chapter 7. Pg83-89 Harper chapter “carbohydrates of physiologic significance” Pages 113-120 _____________________________XXXXXXXXXXXXXXXXXXXXXXXXX______________________________