Carbohydrates[1].pptx

CARBOHYDRATES  Learning Define Objectives carbohydrates in chemical terms Classify carbohydrates into four major groups with examples of each group Define Asymmetric (chiral carbon) and Differentiate between D- Series and L-Series Describe the biomedical importa...

CARBOHYDRATES  Learning Define Objectives carbohydrates in chemical terms Classify carbohydrates into four major groups with examples of each group Define Asymmetric (chiral carbon) and Differentiate between D- Series and L-Series Describe the biomedical importance of carbohydrates. Definition 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’. Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. They have General formula Cn(H2O)n. Hence, also called hydrates of carbon. Polyhydroxy compounds (poly-alcohols)  CLASSIFICA TION are divided into Carbohydrates four major groups— Monosaccharides Disaccharides Oligosaccharides and Polysaccharides General Properties in Reference to Glucose Asymmetric carbon A carbon atom to which four different atoms or groups of atoms are attached is said to be asymmetric. Van’t Hoff’s rule of ‘n’  The number of possible isomers of any given compound depends upon the number of asymmetric carbon atoms the molecule possesses. According to Van’t Hoff’s rule of ‘n’; 2n equals the possible isomers of that compound, where, n = represents the number of asymmetric carbon atoms in a compound. STEREOISOMERISM  The presence of asymmetric carbon atoms in a compound gives rise to the formation of isomers of that compound. The compounds which are identical in composition and differs only in spatial configuration are called stereoisomers.  Two such isomers of glucose—D-Glucose and L-Glucose are mirror image of each other. D-Series and L-Series  The orientation of the H and OH groups around the carbon atom just adjacent to the terminal primary alcohol carbon, e.g. C-atom 5 in glucose determines the series. When the – OH group on this carbon is on the right, it belongs to D-series, when the – OH group is on the left, it is a member of L-series.  Most of the monosaccharides occurring in mammals are D-sugars, and the enzymes responsible for their metabolism are specific for this configuration. Optical Activity  The ability of a substance to rotate the plane of polarization of a beam of light that is passed through it. (In plane-polarized light, the vibrations of the electric field are confined to a single plane.)  Presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarised light is passed through a solution exhibiting optical activity, it will be rotated to the right or left in accordance with the type of compound, i.e. the optical isomers or enantiomorphs; when it is rotated to right, the compound is called Dextrorotatory (D or + sign), when rotated to left, the compound is called Laevorotatory (I or – sign). Racemic When equal amounts of dextrorotatory and laevorotatory isomers are present, the resulting mixture has no optical activity, since the activities of each isomer cancels each other. Such a mixture is said to be Racemic. Resolution The separation of optically active isomers from a racemic mixture is called resolution.  CYCLIC If theSTRUCTURES open-chain form of D- Glucose, which may be called as Aldehydo-D-Glucose is taken, and condense the aldehyde group on carbon-1, with the alcoholic-OH group on carbon-5, two different forms of glucose are formed.  Anomers and anomeric carbon When the OH group extends to right, it is α- D-Glucose and it extends to left, it is β-D- Glucose  Carbon-1, after cyclization has four different groups attached to it and thus it becomes now asymmetric.  The two cyclic compounds, α and β have different optical rotations, but they will not be same because the compounds as a whole are not mirror-images of each other.  Compounds related in this way are called anomers and carbon-1, after cyclisation  MUTAROTAT When IONaldohexose an is first dissolved in water and the solution is put in optical path so that plane polarized light is passed, the initial optical rotation shown by the sugar gradually changes until a constant fixed rotation characteristic of the sugar is reached. This phenomenon of change of rotation is called as mutarotation. Explanation Ordinary crystalline glucose happens to be in the α-form. The above change in optical rotation represents a conversion from α-Glucose to an equilibrium mixture of α and β- forms. The mechanism of mutarotation probably involves opening of the hemiacetal ring to form traces of the aldehyde form, and then recondensation to the cyclic forms. The aldehyde form is extremely  Epimers and Twoepimerisation sugars which differ fromone another only in configuration around a single carbon atom are termed Epimers. Examples 1. Glucose and galactose are examples of an epimeric pairs which differ only with respect of C4. 2. Similarly, mannose and glucose are epimers in respect of C2. ENANTIOMERS  Enantiomers are a pair of molecules that exist in two forms that are mirror images of one another but cannot be superimposed one upon the other.  Enantiomers are in every other respect chemically identical.  A pair of enantiomers is distinguished by the direction in which when dissolved in solution they rotate polarized light, either dextro (d or +) or levo (l or -) rotatory.  The structural basis of Biomedical Importance of Carbohydrates  Chief source of energy.  Constituents of compound lipids and conjugated proteins.  Degradation products act as “promoters” or ‘catalysts’.  Certain carbohydrate derivatives are used as drugs like cardiac glycosides/antibiotics.  Lactose principal sugar of milk—in lactating mammary gland.  Degradation products utilized for synthesis of other substances such as fatty acids, cholesterol, amino acid, etc.  Constituents of mucopolysaccharides which form the ground substance of mesenchymal tissues.  Deficiency of certain enzymes in metabolic pathways of different carbohydrates can cause diseases, e.g. galactosemia, glycogen storage diseases (GSDs), lactose intolerance, etc.  Uncontrolled glucose metabolism is seen in diabetes mellitus. MONOSACCHARIDES OF BIOLOGICAL IMPORTANCE ALDEHYDE OR KETONE GROUP: i. Aldomonosaccharides (Aldoses). ii. Ketomonosaccharides (Ketoses). CARBON CHAIN LENGTH. a) Trioses. b) Tetroses. c) Pentoses. d) Hexoses. e) Heptoses. (a) Trioses: Both D-glyceraldehyde and dihydroxyacetone occur in the form of phosphate esters, as intermediates in glycolysis. They are also the precursors of glycerol, which the organism synthesizes and incorporates into various types of lipids. (b) Tetroses: Erythrose-4-P occurs as an intermediate in hexosemonophosphate shunt which is an alternative pathway for glucose oxidation. (c) Pentoses  D-ribose is a constituent of nucleic acid RNA; also as a constituent of certain coenzymes, e.g. FAD, NAD, coenzyme A.  D-2-deoxyribose is a constituent of DNA. (d) Hexoses 1. D-Glucose (Synonyms: Dextrose, Grape Sugar) It is the chief physiological sugar present in normal blood continually and at constant level, i.e. about 0.1 per cent. All tissues utilize glucose for energy. Erythrocytes and Brain cells utilize glucose solely for energy purposes. Occurs as a constituent of disaccharide and polysaccharides. Stored as glycogen in liver and muscles mainly. Shows mutarotation. 2. D-galactose Seldom found free in nature. In combination it occurs both in plants and animals. Occurs as a constituent of milk sugar lactose and in tissues as a constituent of galactolipid and glycoproteins. It is an epimer of glucose and differs in orientation of H and OH on carbon-4. It is less sweet than glucose and less soluble in water. It is dextrorotatory and shows mutarotation. 3. D-fructose  It is a ketohexose and commonly called as fruit sugar, as it occurs free in fruits.  It is very sweet sugar, much sweeter than sucrose and more reactive than glucose. It occurs as a constituent of sucrose and also of the polysaccharide inulin. It is laevorotatory and hence is also called laevulose.  Exhibits mutarotation. 4. D-mannose  It does not occur free in nature but is widely distributed in combination as the polysaccharide mannan, e.g. in ivory nut.  In the body, it is found as a constituent of glycoproteins.. DISACCHAR IDES  The disaccharides are formed by the union of two constituent monosaccharides with the elimination of one molecule of water. The points of linkage, the glycosidic linkage varies, and the properties of the disaccharides depend to a great extent on the type of the linkage. If both of the two potential aldehyde/or ketone groups are involved in the linkage the sugar will not exhibit reducing properties and will not be able to form osazones, e.g. sucrose.(Non-reducing) But if one of them is not bound in this way, it will permit reduction and osazone formation by the sugars, e.g. Lactose and  PROPERTIES OF 1. DISACCHARIDES Maltose  Maltose or malt sugar is an intermediary in acid hydrolysis of starch and can also be obtained by enzyme hydrolysis of starch.  In the body, dietary starch digestion by Amylase in gut yields maltose, which requires a specific enzyme maltase to form glucose.  It is a rather sweet sugar and is very soluble in water.  It has reducing properties.  As anomeric carbon of one glucose is free, can form α and β forms and exhibit mutarotation.  On hydrolysis Maltose yields two molecules of glucose. Maltos e  2. Lactose Lactose is milk sugar found in appreciable quantities in milk to the extent of about 5 percent and occurs at body temperature as an equilibrium mixture of the α and β forms It is not very soluble and is not so sweet. It is dextrorotatory. Specific enzyme which hydrolyses lactase present in intestinal juice. On hydrolysis it yields one molecule of D- Glucose and one molecule of D-Galactose. It has reducing properties. As anomeric carbon of glucose is free, can form α and β forms and exhibits mutarotation. Lacto se  3. Sucrose Ordinary table sugar is sucrose. It is also called as ‘Cane sugar’, as it can be obtained from sugarcane. Also obtained from sugar beet, and sugar maple. Also occurs free in most fruits and vegetables, e.g. pineapples, and carrots. It is very soluble and very sweet and on hydrolysis yields one molecule of D-Glucose and one molecule of D-Fructose. The specific enzyme which hydrolyses sucrose is sucrase present in intestinal juice. As both aldehyde and ketone groups are linked together (α 1 → 2), it does not have reducing properties. As both anomeric carbons are involved in Sucro se  Invert Sugars and ‘Inversion’ Sucrose is dextrorotatory (+62.5°) but its hydrolytic products are levorotatory because fructose has a greater specific levorotation than the dextrorotation of glucose. As the hydrolytic products invert the rotation, the resulting mixtures of glucose and fructose (hydrolytic products) is called as Invert Sugar and the process is called as Inversion. Honey is largely ‘invert sugar’ and the presence of fructose accounts for the greater sweetness of honey.  POLYSACCHARID ES Polysaccharides are more complex substances. These are polymerized products of many monosaccharide units. They may be: 1.HomoPolysaccharide (Homoglycans) are composed of single kind of monosaccharides, e.g. starch, glycogen and cellulose. 2. HeteroPolysaccharide (heteroglycans) are composed of two or more different monosaccharides, e.g. hyaluronic acid, chondroitin sulfate.  HOMOPOLYSACCHARIDE S (HOMOGLYCANS) 1. Starch is a polymer of glucose, and occurs in many plants as storage foods. It may be found in the leaves, and stem, as well as in roots, fruits and seeds Composition of starch granule: It consists of two polymeric units of glucose called (i)Amylose ((15–20%))and (ii)Amylopectin (80–85%), but they differ in molecular architecture and in certain properties.  Solubility: Starch granules are insoluble in cold water, but when their suspension is heated, water is taken up and swelling occurs, viscosity increases and starch gels or pastes are formed. Reaction with I2: Both the granules and the colloidal solutions react with Iodine to give a blue colour. This is chiefly due to amylose, which forms a deep-blue complex, which dissociates on heating. Amylopectin solutions are coloured blue- violet or purple. Ester Formation: Starches are capable 2. Glycogen It is reserve carbohydrates In animals also called as animal starch. Formation of glycogen from glucose is called as Glycogenesis and breakdown of glycogen to form glucose is called as glycogenolysis. Molecular weight: The molecular weight varies from 1,000,000 to 4,000,000. Solubility: Glycogen is not readily soluble in water and it forms an opalascent solution. It can be precipitated from opalascent solution by ethyl alcohol, and in drying, it forms a pure white powder. Action of alkali: Glycogen is not destroyed by a hot strong KOH or NaOH solution. Action with iodine: Glycogen gives a deep-red  Structure: Glycogens have a complex structure of highly branched chains. It is a polymer of D-Glucose units and resemble amylopectin. Glucose units in main stem are joined by α1 → 4 glucosidic linkages and branching occurs at branch points by α1 → 6 glucosidic linkage. A branch point occurs for every  3- It is a Inulin polymer of D-fructose and has a low molecular weight (MW = 5000) It occurs in the bulbs of onion and garlic. It is levorotatory and gives no colour with iodine. Acids hydrolyse it to D-fructose; similarly it is also hydrolysed by the enzyme inulinase which is absent in human. It is used in physiological investigation for determination of  4. Cellulose is aCellulos polymer of glucose. It is a fibrous, tough, water insoluble substance, e wall of plants. found in the cell Herbivorous animals, with the help of bacteria, can utilise a considerable proportion of the cellulose ingested, but in human beings no cellulose splitting enzyme is secreted by GI mucosa, hence it is not of any nutritional value.  5. When starch isDextrin partially hydrolysed by the s or enzymes, it is broken action of acids down into a number of products of lower molecular weight known as dextrins Dextrin solutions are often used as mucilages (mucilages on the back of the postage stamp)  6. Dextran It is a polymer of D-Glucose. They are made up of units of a number of D- s having α1 → 6, α1 → 4 Glucose molecules, Dextran solution, having molecular wt approx. 75,000 have been used as Plasma Expander. When given IV, in cases of blood loss (haemorrhage), it increases the blood volume. 7. Agar It is a homopolysaccharide. Made up of repeated units of galactose which is sulphated. Present in seaweed.  HETEROPOLYSACCHARIDES (HETEROGLYCANS)— MUCOPOLYSACCHARIDES (MPS) They are essential components of tissues, where they are generally present either in free form or in combination with proteins. Carbohydrate content varies. When carbohydrate content is > 4 percent, they are called Mucoproteins and when < 4 percent they are called as Glycoproteins.  I. Acidic Sulphate free MPS Hyaluronic Acid A sulphate free mucopolysaccharide. It was first isolated from vitreous humour of eye. It occurs both free and salt-like combination with proteins. It is composed of repeating units of N-acetyl glucosamine and D- Glucuronic acid. Chondroitin Another sulphate free acid mucopolysaccharide and found in cornea. It differs from hyaluronic acid only in that it contains N-acetyl galactosamine II.Sulphate Containing Acid MPS 1 Keratan Sulphate (Kerato Sulphate) A sulphate containing acid MPS and found in cornea, isolated from bovine cornea. Composition: It is composed of repeating disaccharide unit consisting of N-acetyl glucosamine and galactose. 2 Chondroitin Sulphates These are found in mammalian tissues and cartilage. They occur in combination with proteins and are called as Chondroproteins.  3. Heparin It is also called α-Heparin. It is an anticoagulant present in liver and it is produced mainly by mast cells of liver. Structure: It is a polymer of repeating disaccharide units of D-Glucosamine (Glc N) and either of the two uronic acids-D-Glucuronic acid (Glc UA) and L- Iduronic acid (IDUA) Molecular weight of Heparin varies from 17,000 to 20,000. It occurs in combination with proteins as proteoglycans..  III. Neutral MPS Many of the neutral nitrogenous polysaccharides of various types are found in pneumococci capsule. Type specificity of pneumococci resides on specific polysaccharides present on capsule.