BCH 201 Carbohydrates - General Biochemistry I PDF

BCH 201 (General Biochemistry I) CHEMISTRY OF CARBOHYDRATES  CARBOHYDRATES Carbohydrates are the most abundant organic molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally...

BCH 201 (General Biochemistry I) CHEMISTRY OF CARBOHYDRATES  CARBOHYDRATES Carbohydrates are the most abundant organic molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally means ‘hydrates of carbon’. Some of the carbohydrates possess the empirical formula (CH2O)n , satisfying that these carbohydrates are in fact carbon hydrates. However, there are several non-carbohydrate compounds (e.g. acetic acid, C2H4O2; lactic acid, C3H6O3) which also appear as hydrates of carbon. Further, some of the genuine carbohydrates (e.g. rhamnohexose, C6H12O5; deoxyribose, C5H10O4) do not satisfy the general formula. Hence carbohydrates cannot be always considered as hydrates of carbon. Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. The term ‘sugar’ is applied to carbohydrates soluble in water and sweet to taste. Glyceraldehyde (an aldehyde) Dihydroxyacetone (a ketone)  A carbon double-bonded to an oxygen is called a carbonyl group. Compounds in which the carbon of a carbonyl group is bonded to two other carbons are called ketones. Carbonyl functional group: C=O When one of the groups attached to the carbonyl group is a hydrogen instead of a carbon, the compound is called an aldehyde  Functions of carbohydrates 1. They are the most abundant dietary source of energy (4 Cal/g) for all organisms. 2. Carbohydrates are precursors for many organic compounds (fats, amino acids). 3. Carbohydrates (as glycoproteins and glycolipids) participate in the structure of cell membrane and cellular functions such as cell growth, adhesion and fertilization. 4. They are structural components of many organisms. These include the fiber (cellulose) of plants, exoskeleton of some insects and the cell wall of microorganisms. 5. Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body.  CLASSIFICATION OF CARBOHYDRATES Carbohydrates are classified into three major classes: ✓Monosaccharides ✓Oligosaccharides ✓ polysaccharides Carbohydrates Monosaccharides Oligosaccharides Polysaccharides Aldoses Ketoses Disaccharides Homopolysaccharides Heteropolysaccharides Fig 1.0: Major classes of carbohydrates with examples Monosaccharides Monosaccharides (Greek : mono-one) are the simplest group of carbohydrates and are often referred to as simple sugars. They have the general formula Cn(H2O)n, and they cannot be further hydrolysed. The monosaccharides are divided into different categories, based on the functional group and the number of carbon atoms. Oligosaccharides Oligosaccharides (Greek: oligo-few) contain 2-10 monosaccharide molecules which are liberated on hydrolysis. Based on the number of monosaccharide units present, the oligosaccharides are further subdivided to disaccharides, trisaccharides etc. Polysaccharides Polysaccharides (Greek: poly-many) are polymers of monosaccharide units with high molecular weight (up to a million). They are usually tasteless (non-sugars) and form colloids with water. The polysaccharides are of two types – homopolysaccharides and heteropolysaccharides Stereoisomerism Stereoisomerism is an important character of monosaccharides. Stereoisomers are the compounds that have the same structural formulae but differ in their spatial configuration. A carbon is said to be asymmetric when it is attached to four different atoms or groups. The number of asymmetric carbon atoms (n) determines the possible isomers of a given compound which is equal to 2n. Glucose contains 4 asymmetric carbons, and thus has 16 isomers (allose altrose, galactose, gulose, glucose, idose, mannose and tallose). Glyceraldehyde (triose) is the simplest monosaccharide with one asymmetric carbon atom. It exists as two stereoisomers and has been chosen as the reference carbohydrate to represent the structure of all other carbohydrates. D- and L-isomers The D and L isomers are mirror images of each other. The spatial orientation of H and OH groups on the carbon atom (C5 for glucose) that is adjacent to the terminal primary alcohol carbon determines whether the sugar is D- or L-isomer. If the OH group is on the right side, the sugar is of D-series, and if on the left side, it belongs to L-series. The structures of D- and L-glucose based on the reference monosaccharide, D- and L-glyceraldehyde. It may be noted that the naturally occurring monosaccharides in the mammalian tissues are mostly of D-configuration. The enzyme machinery of cells is specific to metabolise D-series of monosaccharides. Optical activity of sugars Optical activity is a characteristic feature of compounds with asymmetric carbon atom. When a beam of polarized light is passed through a solution of an optical isomer, it will be rotated either to the right or left. The term dextrorotatory (d+) and levorotatory (l–) are used to compounds that respectively rotate the plane of polarized light to the right or to the left. Racemic mixture : If d- and l-isomers are present in equal concentration, it is known as racemic mixture or dl mixture. Racemic mixture does not exhibit any optical activity, since the dextro- and levorotatory activities cancel each other. Epimers If two monosaccharides 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 (Fig.2.4). For instance, glucose and galactose are epimers with regard to carbon 4 (C4-epimers). That is, they differ in the arrangement of OH group at C4. Glucose and mannose are epimers with regard to carbon 2 (C2-epimers). The interconversion of epimers (e.g. glucose to galactose and vice versa) is known as epimerization, and a group of enzymes— namely—epimerases catalyse this reaction. Enantiomers Enantiomers are a special type of stereoisomers that are mirror images of each other. The two members are designated as D- and L-sugars. Enantiomers of glucose are depicted in Fig.2.5. The term diastereomers is used to represent the stereoisomers that are not mirror images of one another. Anomers- The α and β cyclic forms of D-glucose are known as anomers. They differ from each other in the configuration only around C1 known as anomeric carbon (hemiacetal carbon). In case of α anomer, the OH group held by anomeric carbon is on the opposite side of the group CH2OH of sugar ring. The reverse is true for β-anomer. The anomers differ in certain physical and chemical properties. 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. Mutarotation depicted in Fig. 2.6 Mutarotation of fructose : Fructose also exhibits mutarotation. In case of fructose, the pyranose ring (six-membered) is converted to furanose (five-membered) ring DISACCHARIDES Among the oligosaccharides, disaccharides are the most common (Fig.2.12). As is evident from the name, a disaccharide consists of two monosaccharide units (similar or dissimilar) held together by a glycosidic bond. They are crystalline, water-soluble and sweet to taste. e.g. maltose, lactose. sucrose, trehalose. Maltose Maltose is composed of two α-D-glucose units held together by α (1 - 4) glycosidic bond. Sucrose Sucrose (cane sugar) is the sugar of commerce, mostly produced by sugar cane and sugar beets. Sucrose is made up of α-D-glucose and β-D-fructose. The two monosaccharides are held together by a glycosidic bond (α1 - β2), between C1 of α-glucose and C2 of β-fructose Lactose Lactose is more commonly known as milk sugar since it is the disaccharide found in milk. Lactose is composed of β-D-galactose and β-D glucose held together by β (1 - 4) glycosidic bond. POLYSACCHARIDES Polysaccharides (or simply glycans) consist of repeat units of monosaccharides or their derivatives, held together by glycosidic bonds. They are primarily concerned with two important functions-structural, and storage of energy. Polysaccharides are linear as well as branched polymers. This is in contrast to structure of proteins and nucleic acids which are only linear polymers. The occurrence of branches in polysaccharides is due to the fact that glycosidic linkages can be formed at any one of the hydroxyl groups of a monosaccharide. Polysaccharides are of two types They are divided into two: Homopolysaccharides and Heteropolysaccharides. Homopolysaccharides (homoglycans): Contains same type of monosaccharides or their derivatives, e.g. Glucans such as Starch, Glycogen, Dextran, Chitin and Cellulose are polymers of glucose whereas fructosans (i. e Inulin) are polymers of fructose. Heteropolysaccharides (heteroglycans); contains more than one type of sugar residues or their derivatives. Examples include; Glycosaminoglycans, Peptidoglycan, Agar and Glycoconjugates. HOMOPOLYSACCHARIDES STARCH is the major form of stored carbohydrate in plants. It is composed of a mixture of two substances: amylose and amylopectin. Amylose, the unbranched type of starch, consists of glucose residues in α-1, 4 glycosidic linkages Amylopectin is highly branched, it consists of glucose residues in α-1, 6 glycosidic linkages at the branching points and α- 1, 4 glycosidic bond on the linear chain Glycogen Glycogen is the carbohydrate reserve in animals, hence often referred to as animal starch. It is present in high concentration in liver, followed by muscle, brain etc. The 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 bonds, and α (1 - 6) glycosidic bonds at branching points Cellulose Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom. It is a predominant constituent of plant cell wall. Cellulose is totally absent in animal body. Cellulose is composed of β -D-glucose units linked by β (1 - 4) glycosidic bonds. Cellulose cannot be digested by mammals— including man—due to lack of the enzyme that cleaves β -glycosidic bonds (α amylase breaks α bonds only). Certain ruminants and herbivorous animals contain microorganisms in the gut which produce enzymes that can cleave β -glycosidic bonds. Inulin Inulin is a polymer of fructose linked by β-1, 2 glycosidic bonds and also contains a terminal glucose. i.e., fructosan. It occurs in dahlia bulbs, garlic, onion etc. It is a low molecular weight (around 5,000) polysaccharide easily soluble in water. Inulin is not utilized by the body. It is used for assessing kidney function through measurement of glomerular filtration rate (GFR) Chitin Chitin is composed of N-acetyl D glucosamine units held together by β(1 - 4) glycosidic bonds. It is a structural polysaccharide found in the exoskeleton of some invertebrates e.g. insects, crustaceans. HETEROPOLYSACCHARIDES When the polysaccharides are composed of different types of sugars or their derivatives, they are referred to as heteropolysaccharides or heteroglycans. Examples includes: Glycosaminoglycan, Peptidoglycan, Agar, Glycoconjugate etc. 1. MUCOPOLYSACCHARIDES Mucopolysaccharides are heteroglycans made up of repeating units of sugar derivatives, namely amino sugars and uronic acids. These are more commonly known as glycosaminoglycans (GAG). They are linear polymers of repeating disaccharide units containing a derivative of an amino sugar, either glucosamine or galactosamine. At least one of the sugars in the repeating unit tends to be modified with either carboxylate group or sulphated group. Examples are: hyaluronic acid, chondroitin sulphate, heparin, keratan sulphate and dermatan sulphate. Some of the mucopolysaccharides are found in combination with proteins to form mucoproteins or mucoids or proteoglycans. Mucoproteins may contain up to 95% carbohydrate and 5% protein. Mucopolysaccharides are heteroglycans present on the cell surface and in the extracellular matrix of animals. GLYCOPROTEINS Several proteins are covalently bound to carbohydrates which are referred to as glycoproteins. The carbohydrate content of glycoprotein varies from 1% to 90% by weight. Sometimes the term mucoprotein is used for glycoprotein with carbohydrate concentration more than 4%. Glycoproteins are very widely distributed in the cells and perform variety of functions. These include their role as enzymes, hormones, transport proteins, structural proteins and receptors. A selected list of glycoproteins and their major functions is given in Table 2.4. The carbohydrates found in glycoproteins include mannose, galactose, N-acetyl-glucosamine, N-acetylgalactosamine, xylose, L-fucose and N-acetylneuraminic acid (NANA). NANA is an important sialic acid 2. AGAR AND PECTINS Agar, is an heteroglycan consisting of D-galactose and an L-galactose derivative repeats, ether-linked between C-3 and C-6. Mostly found in sea weeds, is a polymer of galactose sulfate and glucose. Since agar is not digested, it serves as a dietary fiber. Agarose (with galactose and anhydrogalactose) is useful in the laboratory as a major component of microbial culture media, and in electrophoresis. Pectins, found in apples and citrus fruits, contain galactouronate and rhamnose. Pectins, being non-digestible, are useful as dietary fiber. They are also employed in the preparation of jellies. 3. PEPTIDOGLYCAN is the rigid compound of bacterial cell wall. It is an heteroglycan composed of repeating units of N-acetylmuramic (NAM) acid and N-acetylglucosamine (NAG) in β-1,4 glycosidic linkage. This polysaccharide provides mechanical strength. Lysozyme, the enzyme present in tears, kills bacterial by hydrolyzing the β-1, 4 glycosidic bonds between NAM and NAG. N-acetylmuramic acid is made of acetylglucosamine modified by lactic acid at carbon three.