Carbohydrates Chemistry- Part I PDF

BIOCHEMISTRY [CHM 203] CHEMISTRY of CARBOHYDRATES-I By Dr. Maha M Salah Assistant Professor of Medical Biochemistry and Molecular Biology  Chemical Nature of Carbohydrates ❑ Carbohydrates are organic substances composed essentially of Car...

BIOCHEMISTRY [CHM 203] CHEMISTRY of CARBOHYDRATES-I By Dr. Maha M Salah Assistant Professor of Medical Biochemistry and Molecular Biology  Chemical Nature of Carbohydrates ❑ Carbohydrates are organic substances composed essentially of Carbon, Hydrogen and Oxygen, which are represented with general formulae Cn(H2O)n Or Cn(H2O)m and hence called hydrates of carbons. ❑ They are Simple Sugars or their polymers; Simple sugars are polyhydroxy-aldehydes or polyhydroxy- ketones.  Importance of Carbohydrates ❑ Carbohydrates are the most abundant organic molecules in nature. ❑ They have a wide range of functions, including: a) Providing a significant fraction of the dietary calories for most organisms. b) Acting as a storage form of energy in the body. c) Serving as cell membrane components that mediate some forms of intercellular communication. d) Structural component of many organisms, e.g. cell walls of bacteria, exoskeleton of many insects, and fibrous cellulose of plants. CHM 203  Classification of Carbohydrates ❑ Carbohydrates are classified according to the hydrolysis products into four main groups as follows: I. Monosaccharides: contain one saccharide unit. II. Disaccharides: contain 2 monosaccharide units per molecule. III. Oligosaccharides: contain from 3 to 10 monosaccharide units per molecule. IV. Polysaccharides: contain more than 10 monosaccharide units per molecule. CHM 203  Monosaccharides ❑ They are the simplest group. ❑ They have the general formula Cn(H2O)n. ❑ They are classified by Two Methods: According to the According to Number of Functional Carbon Atoms Groups CHM 203  A] Classification according to the number of Carbon Atoms: - The number of carbons in the molecule either: 1. Trioses (3 carbons) 2. Tetroses (4 carbons) 3. Pentoses (5 carbons) 4. Hexoses (6 carbons) 5. Heptoses (7 carbons). B] Classification according to the presence of aldehyde or ketone group into Aldoses and Ketoses. CHM 203  I – Aldoses ❑ The mother compound of all aldoses is the aldotriose Glyceraldehyde. ❑ Other aldoses can be formed theoretically by insertion of secondary alcohol groups (H-C-OH) below the aldehydic group of glyceraldehyde. ❑ All sugars derived from D-glyceraldehyde are called D-sugars, as indicated by having the hydroxyl group of the penultimate carbon atom (the carbon atom before the last one) to the right. ❑ While those derived from L-glyceraldehyde are called L-sugars (contain hydroxyl group on the left side of the penultimate carbon atom). ❑ Most of the naturally occurring monosaccharides are of the D type. CHM 203  D-Glyceraldehyde L-Glyceraldehyde CHM 203 CHM 203  II – Ketoses ❑ All ketoses have two terminal primary alcohol groups (CH2OH) and have one ketone group (C=O). ❑ The simplest ketose (Mother Compound) is Dihydroxyacetone (C3). ❑ Other ketoses are formed, theoretically, by the insertion of Secondary Alcohol groups below the ketonic group. CHM 203 CHM 203  Forms of Isomerism of Monosaccharides ❑ Stereoisomers are isomeric molecules that have the same molecular formula and the sequence of bonded atoms (constitution) but differ in the three-dimensional orientations of their atoms in space. Asymmetric carbon atoms are those attached to four different atoms or groups. All monosaccharides contain one or more asymmetric carbon atoms except Dihydroxyacetone. All monosaccharides are optically active (each one has its specific and characteristic degree of rotation of PPL) except, Dihydroxyacetone. CHM 203  Optical Activity is the ability of a substance (containing asymmetric carbon atom) to rotate the plane polarized light (PPL). The number of optical isomers depends on the number of asymmetric carbon atoms. The number of these isomers = 2n (n = the number of asymmetric carbon atoms in the molecule). The number of asymmetric carbon atoms of Aldoses= Total number of carbons- 2 The number of asymmetric carbon atoms of Ketoses= Total number of carbons- 3 CHM 203  Examples: Aldotrioses contain one asymmetric carbon atom, so they have (2) isomers. Aldotetrose contain two asymmetric carbon atoms, so they have (4) isomers. Aldopentoses contain three asymmetric carbon atoms, so they have (8) isomers. Aldohexoses contain four asymmetric carbon atoms, so they have (16) isomers. - In case of ketoses, the number of asymmetric carbon atoms is less by one compared to the corresponding group of aldoses. Ketotetroses contain one asymmetric carbon atom, so they have 2 isomers (D and L). Ketopentoses contain two asymmetric carbon atoms, so they have 4 isomers. Ketohexoses contain three asymmetric carbon atoms, so they have 8 isomers. Also, half of these isomers is in the D form and the other half in the L-form. CHM 203  A - Enantiomers: They are the D- and L- forms of the same compounds (mirror images) e.g.: D and L-glucose. B - Epimers: They are compounds which have more than one asymmetric carbon and differ only in the configuration around one carbon e.g.: ✓ D-Glucose and D-mannose are epimers at C2. ✓ D-Glucose and D-galactose are epimers at C4. CHM 203 CHM 203  C- Aldose-Ketose Isomers (Functional Group Isomerism): ✓ They have the same molecular formulae but differ in their functional groups. ✓ For example: 1. Fructose is a functional group isomer of glucose, galactose or mannose. 2. Trioses: Glyceraldehyde and Dihydroxyacetone. D- Anomers (Cyclic Form of Monosaccharides): During ring formation (cyclization), a new asymmetric carbon is created called anomeric carbon and the two possible isomers are known as anomers. CHM 203  ✓ In solution, the functional aldehyde group of glucose combines with hydroxyl group of 5th carbon atom. As a result, a 6 membered heterocyclic pyranose ring structure containing 5 carbons and one oxygen is formed. The linkage between the aldehyde group and alcohol group is called ‘hemiacetal’ linkage. ✓ Similarly, a 5 membered furanose ring structure is formed from fructose when its keto group combines with hydroxyl group of 5th carbon atom. The linkage between the keto group and alcohol group is called ‘hemiketal’ linkage. CHM 203  ✓Cyclization creates a carbon with two possible orientations of the hydroxyl group around it (-form and β-form). ✓In case of D-sugars, the -form has the OH group to the right of the anomeric carbon and the β- form has the OH group to the left of the anomeric carbon. ✓Anomers are epimers at C1 of cyclic aldoses and C2 of cyclic ketoses. CHM 203 CHM 203 ❑ Important Monosaccharides: Trioses: Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are intermediates during glucose oxidation in living cells. Tetroses: Erythrose 4-phosphate is formed during glucose oxidation in living cells. Pentoses: - D-ribose is a component of many nucleosides and nucleotides and ribonucleic acids (RNA). - 2-deoxyribose is a component of deoxyribonucleic acid (DNA). CHM 203 Hexoses: - D-glucose ( Grape sugar) is the main sugar present in blood and is present in all animal and plant cells, honey and fruits. It enters in the formation of many disaccharides and polysaccharides. It is also termed dextrose because it is dextrorotatory. - D-fructose (Fruit sugar) is present in honey, fruits and semen. It is a component of sucrose and inulin. It is also termed levulose because it is levorotatory. - D-galactose is a component of lactose which is present in milk. It is also found in glycosaminoglycans (GAGS), glycolipids and glycoproteins. CHM 203  Reducing Properties of Monosaccharides All monosaccharides are reducing due to the presence of a free aldehyde or ketol group in their structure. If the oxygen on the anomeric carbon (the carbonyl group) of a sugar is not attached to any other structure, that sugar is a reducing sugar. CHM 203  Derivatives of Monosaccharides (A) Sugar acids: 1- Aldonic Acids: The aldehyde group of aldoses is oxidized to form the corresponding Aldonic Acid e.g.: ✓ Glucose is oxidized to form gluconic acid ✓ Galactose is oxidized to form galactonic acid. Gluconic Acid CHM 203  2- Uronic Acids : The primary alcohol group of monosaccharides is oxidized to form the corresponding uronic acid. ✓ Glucose is oxidized to form Glucuronic acid (GlcUA). ✓ Galactose is oxidized to form Galacturonic acid (GalUA). CHM 203  3- Aldaric Acids: ✓ These are monosaccharides in which both the aldehyde and primary alcohol groups are oxidized to form the corresponding Aldaric Acid e.g. Glucose gives glucaric acid. 4- L-Ascorbic Acid ( Vitamin C): ✓ It is formed in some animals (not humans) from glucose. It is considered as a sugar acid. CHM 203 (B) Sugar Alcohols: - These are sugars in which the carbonyl group is reduced to alcohol group, e.g.: ✓ Sorbitol is the alcohol of Glucose. ✓ Dulcitol is the alcohol of Galactose. ✓ Mannitol is the alcohol of Mannose. CHM 203  - The reduction of ketones produces 2 epimeric alcohols, e.g.: Fructose is reduced to Mannitol and Sorbitol. CHM 203 ❑ Important members of Sugar Alcohols: a- Glycerol: It is the alcohol of glyceraldehyde or dihydroxyacetone. It is a component of triacylglycerol and most phospholipids. b- Ribitol: It is the alcohol of ribose. It is a component of riboflavin (vitamin B2) c- Myo-inositol: It is found in animal tissues. It is one of the isomers of inositol which is hydroxylated cyclohexane. It is a component of a phospholipid termed lipositol (phosphatidyl inositol). CHM 203  Myo-inositol: ✓ In plants, it is found as its hexaphosphate derivative known as phytic acid. ✓ Excess Phytic acid inhibits the absorption of Ca2+, Mg 2+, Cu 2+ and Fe2+ from the intestine due to the formation of insoluble salts. CHM 203 (C) Deoxysugars: - These are sugars in which the hydroxyl group is replaced by a hydrogen atom. The most important examples are: a- 2-deoxy Ribose: It is present in the structure of DNA. b- L-Fucose (6-deoxy-L-galactose): It is present in some cell membrane glycoproteins and blood group antigens. CHM 203 (D) Amino-Sugars: These are sugars in which an amino group (NH2) replaces the hydroxyl group on the second carbon e.g. Glucosamine (GluN), Galactosamine (GlaN) and Mannosamine (ManN) Amino-sugars are important constituents of glycosaminoglycans (GAGs) and some types of glycolipids and glycoproteins. Several antibiotics contain amino-sugars which are important for their antibiotic activity. CHM 203 CHM 203  (E) Amino-sugar Acids: These are formed by the addition of acids to amino-sugars. Addition of pyruvic acid to mannosamine gives neuraminic acid. ✓ The N-acetyl derivatives of the amino-sugar acids are called Sialic Acids e.g. N-acetyl neuraminic acid (NANA). ✓ NANA enters in the structure of glycolipids and glycoproteins. CHM 203 CHM 203  (F) Ester Formation: - The hydroxyl groups of monosaccharides can form esters with acids. a- Phosphate esters: e.g. Glucose 1-P and Glucose 6-P. b- Sulfate esters. CHM 203  (G) Glycosides: They are products of condensation of the anomeric carbon of the sugar (in Cyclic Form) with: 1) Another sugar (Glycon): e.g. formation of disaccharides and polysaccharides Or 2) Non-Carbohydrate compound (Aglycon): such as alcohols, phenols or nitrogenous bases. CHM 203  Any glycosidic linkage is named according to the anomeric carbon to which it is attached (α or β) and according to the type of parent sugar, e.g. glucosidic, galactosidic or fructosidic linkage. The condensation of monosaccharide units in the form of oligosaccharides and polysaccharides are in the form of glycosides, where the second sugar reacts as an alcohol and condenses with the anomeric carbon of the first sugar by removal of H2O (O-linked glycosides).  Types of glycosidic linkages: a- O-glycosidic bond: They are formed by the reaction of the hydroxyl group of anomeric carbon of a monosaccharide with the hydroxy group of the alcohol or phenol. b- N-glycosidic bond: The anomeric carbon atom of a sugar is linked to the nitrogen atom of nitrogenous bases (e.g. Nucleosides) CHM 203 CHM 203 CHM 203  DISACCHARIDES Disaccharides consist of two monosaccharide units connected by glycosidic linkage. They are classified according to their reducing property into: 1. Reducing disaccharides: Formed of 2 sugars (a Parent sugar and a Second sugar). They have a free anomeric carbon in the second sugar unit, so they exist in both α and β forms. Examples: Maltose, Isomaltose, Cellobiose and Lactose. 2. Non-reducing disaccharides: Sucrose. CHM 203  If the glycosidic linkage involves the carbonyl group of both sugars (e.g. Sucrose) the resulting disaccharide is non-reducing. On the other hand, if the glycosidic linkage involves the carbonyl group of one of its two sugars (e.g. Lactose and Maltose), the resulting disaccharide is reducing. CHM 203 CHM 203  A- Reducing Disaccharides : 1- Maltose (malt sugar): ✓ It is formed of two glucose units, connected by α 1-4 glucosidic bond. ✓ It is formed by the action of amylase enzyme on starch. During fermentation by yeast in the production of beer, starch is hydrolyzed by the yeast to produce maltose and other products. ✓ It is hydrolyzed by the action of maltase enzyme into two molecules of glucose. CHM 203 CHM 203  2- Lactose (milk sugar): ✓ It is formed of one galactose and one glucose units, connected by β 1-4 galactosidic bond. ✓ It is hydrolyzed by lactase enzyme into galactose and glucose units. CHM 203 CHM 203  B- Non –Reducing Disaccharides 1- Sucrose (Cane sugar) (Table sugar): ✓ It is present in sugar cane, beets and some fruits. ✓ It is formed by condensation of a molecule of α-D-glucose with a molecule of β-D-fructose through α-1,2-glucosidic or β-2,1- fructosidic linkage. ✓ Both carbonyl groups (aldehyde group of glucose and ketone group of fructose) are involved in the linkage, so it is a nonreducing sugar. CHM 203 CHM 203  OLIGOSACCHARIDES ✓ These are polymers of 3-10 monosaccharide units. ✓ They are not an important source of carbohydrates in diet for humans as most of them are nondigestible. ✓ They are present as a constituent of many types of glycolipids and glycoproteins e.g. Oligosaccharides which are constituent of ABO blood group substance, immunoglobulins, and glycolipids and glycoproteins of cell membranes. CHM 203 THANK YOU