Chemistry of Carbohydrates Lecture Notes PDF

CHEMISTRY OF CARBOHYDRATES LECTURE NOTES PREPARED AND COMPILED BY DR. A. B. AKAWA DEPARTMENT OF MEDICAL BIOCHEMISTRY  Introduction Carbohydrates are widely distributed both in animal and plant tissues. Chemically, they contain the elements,...

CHEMISTRY OF CARBOHYDRATES LECTURE NOTES PREPARED AND COMPILED BY DR. A. B. AKAWA DEPARTMENT OF MEDICAL BIOCHEMISTRY  Introduction Carbohydrates are widely distributed both in animal and plant tissues. Chemically, they contain the elements, Carbon, Hydrogen and Oxygen The empirical formula of many simple carbohydrates is [CH2O]n, where n = 3, hence the name “carbohydrate” i.e hydrated carbon. They are also called “saccharides” in greek meaning sugar. Some carbohydrates also contain nitrogen, phosphorus or sulfur By definition, carbohydrates are polyhydroxyl aldehydes or ketones or compounds that yield these on hydrolysis.  Functions of carbohydrates  Main sources of energy in the body e.g brain cells and red blood cells are almost wholly dependent on carbohydrate as the energy source. Energy production from carbohydrate will be 4 kcal/g.  Storage form of energy e.g. glycogen in animal tissue and starch in plants  Serve as structural component e.g glycosaminoglycans in humans, cellulose in plants and chitin in insects  Non digestible carbohydrates like cellulose, serve as dietary fibres  They are constituents of nucleic acids, RNA and DNA e.g Ribose and Deoxyribose sugar  They play a role in lubrication, cellular inter communication and immunity  They are also involved in detoxification e.g. glucuronic acid  Classification of carbohydrates  Carbohydrates are classified into three groups;  Monosaccharides  Disaccharides  Polysaccharides Monosaccharides  They are also called simple sugars.  They consist of a single sugar group.  They consist of a single polyhydroxy aldehyde or ketone unit and thus cannot be hydrolyzed to a simpler form e.g trioses, pentoses, hexoses e.t.c depending on the number of carbon atoms they possess  The most abundant monosaccharide in nature is the six carbon sugar called D- glucose. S/N Monosaccharides (Empirical Aldose ketose formular) 1 Trioses (C3H6O3) Glyceraldehyde Dihydroxyacetone 2 Tetroses (C4H804) Erythrose Erythrulose 3 Pentoses (C5H10O5) Ribose, Xylose Ribulose, Xylulose 4 Hexoses (C6H12O6) Glucose, Galactose, Fructose Mannose 5 Heptoses (C7H14O7) Glucoheptose Sedoheptulose Glyceraldehyde Erythrose (Aldotetrose) Erythrulose (Ketotetrose) Sedoheptulose  SUMMARY OF CLASSIFICATION 1.Simple sugars (Saccharides) (i) Monosacharides  Trioses e.g glyceraldehyde  Tetroses e.g erythrose  Pentoses e.g Ribose  Hexoses e. g glucose and fructose (a) Aldoses e.g glucose (b) Ketoses e.g Fructose  Heptoses e.g Glucoheptose (ii) Disaccharides e.g Sucrose, Maltose and Lactose (iii) Trisaccharides e.g Raffinose (iv) Tetrasaccharides e.g Starchyose 2. Polysaccharides e.g Starch, Cellulose  Structure of Glucose Biomedically, glucose is the most important monosaccharide. The structure of glucose can be represented in the following ways:-  Straight chain structural formula or Fisher projection  Cyclic formula or ring formula or Haworth projection  Monosaccharide in solution is mainly present in ring form In solution, aldehyde (CHO) or ketone (C=O) group of monosaccharide react with a hydroxyl (OH) group of the same molecule forming a bond hemi-acetal or hemi-ketal respectively Disaccharides  When two molecules are combined together by glycosidic bond linkage, a disaccharide is formed.  The important disaccharides are:- o Maltose (Reducing disaccharide) o Lactose (Reducing disaccharide) o Sucrose (Non-reducing disaccharide)  Maltose  This contains two glucose residues or units joined by glycosidic linkage between C-1 (anomeric carbon) of one glucose residue and C-4 of the other leaving one free anomeric carbon of the second glucose residue which can act as a reducing agent. Thus, maltose is a reducing disaccharide  Maltose is produced as an intermediate product in the digestion of starch and glycogen by the action of the enzyme α-amylase  Maltose = glucose + glucose Lactose (milk sugar)  It is present in milk.  Lactose contains one unit of β-galactose and one unit of glucose that are linked by a β(1-4) glycosidic linkage.  The anomeric carbon of the glucose unit is available for oxidation and thus lactose is a reducing disaccharide.  Lactose is hydrolysed to glucose and galactose by lactase enzyme in human beings.  Lactose = glucose + galactose Sucrose This is a disaccharide of glucose and fructose In contrast to maltose and lactose, sucrose contains no free anomeric carbon atom. Sucrose is a non reducing sugar because the linkage involves the first carbon of glucose and second carbon of fructose and free reducing groups are not available. The anomeric carbon of both glucose and fructose are involved in the glycosidic bond, sucrose is therefore a non-reducing sugar. Sucrose is hydrolysed to fructose and glucose by an enzyme called sucrase also known as invertase A sugar solution which is originally non-reducing, but becomes reducing after hydrolysis is inferred as sucrose (specific sucrose test)  Polysaccharides (Glycans)  Carbohydrates composed of ten or more monosaccharide units or their derivatives (such as amino sugars and uronic acids) are generally classified as polysaccharides.  In polysaccharides, monosaccharide units are joined together by glycosidic linkages.  Polysaccharides are subclassified into two groups (1) Homopolysaccharides (Homoglycan) (2) Heteropolysaccharides (Heteroglycan)  When a polysaccharide is made up of several units of one and the same type of monosaccharide unit only, it is called homopolysaccharide e.g. starch, glycogen and cellulose  Heteroplysaccharides on the other hand contain two or more different types of monosaccharide units or their derivatives e.g hyaluronic acid and chondroitin sulphate  Homopolysaccharide Examples of homopolysaccharides are starch, glycogen and cellulose o Starch It is the storage form of glucose in plants. It is composed of two constituents which are amylose and amylopectin. Amylose is a linear polymer of D-glucose units joined by a α(1-4) glycosidic linkages. Amylopectin is structurally identical to those of amylose α(1-4) glycosidic linkages with side chains joining them by α(1-6) linkages. Thus amylopectin is a branched polymer having both α(1-4) and α(1-6) linkages. The branched points in amylopectin are created by α-1-6 bonds and occur at intervals of 20 to 30 units of glucose. Structure of amylose Structure of amylopectin o Glycogen (Animal starch)  Glycogen is the major storage form of carbohydrate (glucose) in animals found mostly in liver and muscle.  The function of glycogen is to act as a readily available source of glucose for energy within the muscle itself.  The structure of glycogen is similar to that of amylopectin except that it is more highly branched, having α(1-6) linkages at intervals of about 8 to 10 glucose units.  Liver glycogen is concerned with the storage and maintenance of the blood glucose.  Diagrammatic representation of glycogen molecule o Cellulose  It is the chief constituents of the cell wall of plants.  It constitutes 99% of cotton, 50% of wood.  It is the most abundant organic material in nature.  It is an unbranched polymer of glucose and it consist of long straight chains linked by β(1-4) glycosidic linkages and not α(1-4) as in amylose.  β(1-4) glycosidic linkages are hydrolysed by the enzyme cellobiase but this enzyme is absent in animal and human digestive system and hence cellulose cannot be digested  Herbivorous animals have large caecum, which harbour bacteria. These bacteria can hydrolyze cellulose and the glucose product is utilized by the animal.  White ants (termites) also digest cellulose with the help of intestinal bacteria. Structure of cellulose  Other examples of homopolysaccharides are Dextrin, Inulin and Chitin  Dextrins are produced by partial hydrolysis of starch by acids or α-amylase enzymes.  Inulin is a long chain homoglycan composed of D-fructose (fructosans) linked by β (1-2) glycosidic linkage. It is the reserve carbohydrate present in onion bulbs and tubers. Inulin is not hydrolysed by α-amylase but is hydrolysed by inulinase which is not present in the humans and so it is not utilized as food. It is clinically used to find renal clearance value and glomerular filteration rate. Note that inulin and insulin are not the same.  Chitin is present in exoskeletons of crustaceans and insects. It is composed of units of N-acetyl-glucosamine with β (1-4) glycosidic linkages.  Heteropolysaccharides (Heteroglycans) They are also known as mucopolysaccharides or glucosaminoglycans (GAGs) containing uronic acid and amino sugars. They also contain acetylated amino groups, sulfate and carboxyl groups because of the presence of these charged groups, they attract water molecules and so they produce viscous solutions. Mucopolysaccharides in combination with proteins form mucoproteins A GAGs is an unbranched heteropolysaccharide made up of repeating disaccharides one component of which is always an amino sugar, hence the name glycosaminoglycans either D-glucosamine or D-galactosamine Examples of glycosaminoglycans are Hyaluronic acid, chondroitin sulfate, keratan sulfate, Dermatin sulfate, Heparin, Hepatin sulfate.  ISOMERISM IN CARBOHYDRATES Compounds possessing identical molecular formula but different structures are referred to as isomers. The phenomenon of existence of isomers is called isomerism Types of Isomerism exhibited by sugars  Ketose – aldose Isomerism  D and L Isomerism  Optical Isomerism  Epimerism  Anomerism Ketose – aldose Isomerism Ketose – aldose Isomerism exist between glucose and fructose in which they are both isomers of each other having the same molecular (chemical) formular (C6H1206) but they differ in structural formula with respect to their functional groups. There is a keto group in C2 of fructose and an aldehyde group in C1 of glucose D and L Isomerism D and L isomerism depends on the orientation of the H and OH groups around the asymmetric carbon. Asymmetric carbon means that four different groups are attached to the same carbon C5 in glucose determines whether the sugar belongs to D or L-isomer When OH group on this carbon atom is on the right, it belongs to the D- series, when it is on the left, it is the member of the L-series. The structures of D and L glucose is based on the reference monosaccharide called, D and L glyceraldehyde, a three carbon sugar which has a single asymmetric carbon atom. D and L isomers (enantiomeric pairs) of glyceraldehyde and glucose  All monosaccharides can be considered as molecules derived from glyceraldehyde by successive addition of carbon atoms. Therefore, penultimate carbon atom is the reference carbon atom for naming the mirror images. It may be noted that in D and L varieties, the groups in 2nd 3rd, 4th and 5th carbon atoms are totally reversed, so as to produce the mirror images. These two forms are stereo-isomers. D sugars are naturally ocuring sugars and body can metabolise only D- sugars. Optical Isomerism The presence of asymmetric carbon atoms exhibits optical activity on the compound. Optical activity is the capacity of a substance to rotate the plane polarized light passing through it. When a beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated either to the right (dextrorotatory (d) or (+) or to the left (laevorotatory (l) or (-). When equal amount of D and L isomers are present, the resulting mixture has no optical activity. Since the activity of each isomer cancel one another, such a mixture is said to be a racemic mixture. Epimerism When two monosaccharides differ from each other in their configuration around a single asymmetric carbon (other than anomeric carbon) atom, they are refered to as epimers of each other. Galactose and Mannose are two epimers of glucose. They differ from glucose in the configuration of groups (H and OH) around C-4 and C-2 respectively. Galactose and mannose are not epimers of each other as they differ in configuration at two asymmetric carbon atoms around C-2 and C-4 Epimers of glucose Anomerism D glucose has two anomers which are the α and β varieties. These anomers are produced by the spatial configuration with reference to the first carbon atom. Hence, these carbon atoms are known as anomeric carbon atoms. The differences between α and β anomeric forms are dependent on the C-1 atom only and so, α and β forms are anomers. The Ist carbon, aldehyde group is condensed with the hydroxyl group of the 5th carbon to form a ring. Mutarotation is defined as the change in the specific optical rotation by the interconversion of α and β forms of D-glucose to an equilibrium mixture. Non reducing sugars cannot show mutatrotation due to the absence of the free anomeric OH group.  Actions of strong acids and alkalies on sugars  Action of strong acids – furural formation  Action of Alkalies – Enolization  Oxidation – Sugar acid formation  Reduction – Sugar alcohol formation  Action of phenylhydrazine – osazone formation Action of strong acids such as H2SO4 or HCl on heating with sugar will yield furfural derivatives with the elimination of water. These derivatives may further condense with α-naphthol, thymol or resorcinol to produce coloured complexes  This is the basis of the :-  Molisch’s test  Seliwanoff’s test  Bial’s test  Action of dillute alkalies such as cuprous oxide or sodium hydroxide on sugars will change the aldoses and ketoses to enediols. Enediol is the enol form of sugar because two OH groups are attached to the double bonded carbon. Enediols are good reducing agents and form basis of the Benedicts test and Fehlings test. Thus alkali enolizes the sugar and thereby causes them to be strong reducing agents.  Assignment Write notes on the following test for sugar  Molisch’s test  Seliwanoff’s test  Bial’s test  Benedict’s test  Fehling’s test