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CARBOHYDRATE CHEMISTRY DR SHAMIM SHAIKH MOHIUDDIN DEPT OF BIOCHEMISTRY UNIVERSITY OF DAMMAM KSA OVERVIEW -Carbohydrate primarily composed of Carbon, Hydrogen and Oxygen -Carbohydrates may be defined as polyhydroxyaldehyde or polyhydroxyketones or compounds which produce them on hydrolysis CLASSIFICATION OF CARBOHYDRATE -Mainly classified into three major groups depends on number of sugar units -Monosaccharides -Oligosaccharides -Polysaccharides Monosaccharides -Monosaccharides are the simplest group of carbohydrate and cannot be further hydrolyzed. -Monosaccharides are divided into different categories, based on the functional group and number of C atoms. Aldose: When functional group in monosaccharides is an aldehyde group(-CHO) they are known as Aldose example- glucose Ketose: When functional group in monosaccharide is an ketone group(-CO) they are known as Ketose example-fructose Oligosaccharides -Oligosaccharides contain 2-10 monosaccharide molecule which are liberated on hydrolysis -Based on the number of monosaccharide units they are Further classified into disaccharides, trisaccharides etc Polysaccharides -Polysaccharides are polymers of monosaccharides unit With high molecular weight -Polysaccharides are of two types -Homopolysaccharides -Heteropolysaccharides STRUCTURAL ASPECT OF MONOSACCHARIDE =Stereoisomer= Stereo isomers refers to a compound that have the same structural formula but differs in their spatial configuration. -Stereoisomerism is a very important character of monosaccharides =Asymmetric carbon atom= A carbon atom is said to be asymmetric when it is attached to four different atoms or group. D and L isomers -D and L isomers are mirror images of each other -The spatial orientation of –H and –OH groups on the carbon atom that is adjacent to the primary alcohol carbons determines whether. the sugar is D- or L- form. -OH in right side : D-series sugar -OH in left side : L-series sugar Optical activity of sugars -Optical activity is a characteristic feature of compounds with asymmetric carbon -A beam of polarized light when passed through a solution of an optical isomer, it will be rotated either to the right or left. Rotation of polarized light to right- Dextrorotatory(+) Rotation of polarized light to left- Levorotatory(-) Racemic mixture -If D and L isomers are present in equal concentration is known as Racemic mixture -Racemic mixture does not show any optical activity since the dextro and levorotatory activities cancel each other Epimers -If two monosaccarides differ from each other in their configuration around a single specific carbon(other than anomeric) atom, they are referred to as epimers to each other. -Example: Glucose and Galactose are epimers with regards to carbon no 4 Enantiomers Enantiomers are a special type of stereoisomerism that are mirror images of each other. -The two members are designated as D and L sugars -Majority of the sugars in the higher animals(including man) are of D type Diastereomers -This represents the stereo isomers that are not mirror image of one another. Anomer -The α and β cyclic forms of D-glucose are known as anomer. -They differ from each other in configuration only around C1 which is known as anomeric carbon atom -In α anomer, the –OH group held by anomeric carbon is on the opposite side of the group –CH2OH of sugar ring -In β anomer, the –OH group held by anomeric carbon is on the same side of the group –CH2OH of sugar ring Mutarotation -Mutarotation is defined as the change in the specific optical rotation representing the interconversion of α and β forms of D glucose to an equilibrium mixture α –D-glucose ↔ Equilibrium mixture ↔ β-D-glucose + 112.2º + 52.7º + 18.7º -The Equilibrium mixture contains β anomer – 63℅ α anomer – 36℅ open chain form - 1℅ -β form is more predominant due to its stable conformation DISACCHARIDES Disaccharides consists of two monosaccharides units held together by a glycosidic bond. -They are crystalline, water soluble and sweet to taste. -Disaccharides are of two types -Reducing disaccharides with free aldehyde or keto gr e.g. Maltose, Lactose -Non reducing disaccharides with no free aldehyde or keto gr. e.g. Sucrose Maltose -Composed of two α-D-glucose units held together by Glycosidic bond. -Maltose can be hydrolyzed by dilute acid or enzyme maltase to liberate two molecules of α-D-glucose Lactose -Lactose is more commonly known as milk sugar. -It is composed of β-D-galactose and β-D-glucose held together by glycosidic bond. -It is hydrolyzed by intestinal enzyme Lactase to glucose and galactose. Sucrose -Sucrose is made up of α-D-glucose and β-D-fructose -Two monosaccharides are held together by a glycosidic bond, between C1 of α-glucose and C2 of β-fructose. -Sucrose is a non-reducing sugar because the reducing group of glucose and fructose are involved in glycosidic bond. -Sucrose is most abundant among the naturally occurring sugars and important sources of dietary carbohydrate -It is sweeter than most other common sugar except fructose. -the intestinal enzyme, sucrase, hydrolyzed sucrose to glucose and fructose. POLYSACCHARIDES -It consist of repeat units of monosaccharides or their derivatives, held together by glycosidic bonds. -Polysaccharides are linear as well as branched polymer -Two types 1. Homopolysaccharides -Homopolysaccharides on hydrolysis yield only a single type of monosaccharides. -They are named based on the nature of the monosaccharide units. e.g. -Glucans are polymers of glucose -Fructosans are polymers of fructose 2. Heteropolysaccharides -Heteropolysaccharides on hydrolysis yield a mixtureof few monosaccharides or their derivatives Starch -Also known as glucosan or glucan -It is a homopolysaccharide composed by D-glucose units which is held by α(1→4)-glycosidic bonds -Starch consists of two polysaccharide components a. Water soluble amylose(15-20%) b. Water insoluble amylopectin(80-85%) -Chemically amylose is a long unbranched chain with 200-1000 D-glucose units held by α-(1→4) glycosidic linkage H H H 5 4 5 1 3 D-Glucose H O 1 4 2 α(1→4)Glycosidic bond 3 2 D-Glucose O n -Amylopectin containing few thousand glucose units looks like a branched tree(20-30 glucose units per branch).It is a branched chain with α(1→6) glycosidic bonds at the branching points and α(1→4) glycosidic bond everywhere Else. H 5 4 H5 1 3 O 4 2 1 2 3 O 6 α(1→6)glycosidic bond CH2OH 6 CH2 H 1 4 O H H H 5 H 3 2 5 4 O 1 3 α(1→4)glycosidic bond 2 Glycogen -Glycogen is the carbohydrate reserve in animals. -Present in Liver, muscle and brain -Structure of glycogen is similar to that of amylopectin with more number of branches. -Glucose is the repeating unit in glycogen joined together by α(1→4)glycosidic bond andα(1→6)glycosidic bonds at branching points. Cellulose -Cellulose occurs exclusively in plants and totally absent in animal body. -It is composed of β-D-glucose units linked by β(1→4)glycosidic bonds -Cellulose cannot be digested in human, due to lack of the enzyme that cleaves β-glycosidic bond 5 H 1 4 5 H 4 1 O 3 2 3 2 β(1→4)glycosidic bond β-D-Glucose β-D-Glucose n