820 likes | 970 Views
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
E N D
Chemistry of Carbohydrates Lecture for MBBSby, 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 • Structures, properties and functions 02/49
Introduction: Carbohydrates are the most abundant class of biomolecules 06/49
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
Medical importance: • Some disorders associated withcarbohydrates- Diabetes mellitus Galactosemia Lactose intolerance Glycogen storage diseases Mucopolysaccharidoses • Cardiac glycosides are important in pharmaceutical practice.
Classification: Carbohydrates Oligosaccharides Monosaccharides Polysaccharides Homo polysaccharides Hetero polysaccharides Aldoses Disaccharides Trioses Tetroses Trisaccharides Ketoses Pentoses Other Hexoses etc. 04/49
Classification: Carbohydrates Oligosaccharides Monosaccharides 2-10 monosaccharide units Polysaccharides Simplest, basic units > 10 monosaccharide units 04/49
Monosaccharides – Classification 1. Based on the functional group present: Monosaccharides Aldoses Ketoses Contain aldehyde group Contain keto group 04/49
Monosaccharides – Classification 2. Based on the number of carbon atoms present Monosaccharides Trioses 3C Heptoses 7C Tetroses 4C Hexoses 6C Pentoses 5C 04/49
Oligosaccharides – Classification Based on the number of monosaccharide units they contain (2-10). Oligosaccharides Disaccharide Other Trisaccharide 04/49
Polysaccharides – Classification Polysaccharides Heteropolysaccharides Homopolysaccharides Contain only one type of monosaccharide Contain more than one type of monosaccharide 04/49
Chemistry of Monosaccharides sTRUCTURE Physical and chemical PROPERTIES FUNCTIONS 13/49
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
Monosaccharides – Structure aldehyde group C1 anomeric carbon 1 keto group 2 2 C2 anomeric carbon 3
Ring structures of Monosaccharides (Glucose) (Fructose)
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)
Learning check point: ??? Identify the monosaccharide among the following. a. Maltose b. Fructose c. Sucrose d. Cellulose
Isomerism in Monosaccharides • Aldose-ketose isomerism (Structural isomerism) 2. Stereoisomerism
1. Aldose-ketose isomerism H C=O Aldehyde group C=O Ketogroup 17/49
2. Stereoisomerism Stereoisomers, • have same structural formulae but different configuration • have asymmetric carbon atoms • are optically active; dextrorotatory/d- / + or levorotatory/ l-/ -
3 types of stereoisomerism – • D - L Stereoisomerism • Epimerism • Anomerism
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
b) Epimerism Epimers differ in configuration at a single asymmetric carbonother than anomeric and penultimate C atoms. 18/49
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
Physical Properties of Monosaccharides • Highly water soluble • Sweet in taste (sugars) • Mutarotation
Mutarotation- change in the optical activity on standing a freshly prepared sugar solution 1/3rd 2/3rd 22/49
Some Important Reactions of Monosaccharides I. Reactions Due to Carbonyl Group II. Reactions Due to Hydroxyl Group
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
Glycosides and Glycosidic Linkage: Glycoside H2O + HO–R O – R alcohol / phenol α- Glycosidic Bond 27/49
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)
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
Learning check point: ??? • The following sugars are epimers of glucose except: a. Fructose b. Galactose c. Mannose
Oligosaccharides: • They contain 2 to 10 monosaccharide units combined together by glycosidic linkages. 11/49
Chemistry of Disaccharides sTRUCTURE Physical and chemical PROPERTIES FUNCTIONS 13/49
Disaccharides: • They consist of two monosaccharide units combined by glycosidic linkage. • The important disaccharides are- Maltose, Lactose and Sucrose 10/49
Maltose α- D-Glucose α- D-Glucose Free α-1,4 glycosidic linkage 32/49
Lactose β –D-Galactose β –D-Glucose β-1,4 glycosidic linkage 33/49
Sucrose 1 α- D-Glucose α-β-1,2 β –D-Fructose 2 34/49
Learning check point: ??? • Table sugar contains- a.Glucose + Glucose b. Glucose + Galactose c. Glucose + Fructose d. Glucose + Mannose
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
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
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