Lecture Note on Carbohydrates-1 PDF

Dr. Manas Denre Course title Fundamentals of Plant Biochemistry and Biotechnology Topic “Carbohydrate: Importance and classification, structure of Monosaccharides, Reducing and oxidizing properties of Monosaccharides,...

Dr. Manas Denre Course title Fundamentals of Plant Biochemistry and Biotechnology Topic “Carbohydrate: Importance and classification, structure of Monosaccharides, Reducing and oxidizing properties of Monosaccharides, Mutarotation, structure of Disaccharides and Polysaccharides.” Dr. Manas Denre Assistant Professor-cum-Junior Scientist Department of Soil Science & Agricultural Chemistry Agriculture College Garhwa Birsa Agricultural University Ranchi-834006, Jharkhand  Carbohydrates Carbohydrates or saccharides, the word “saccharide” is derived from the Greek sakcharon meaning “sugar”  Carbohydrates are the most abundant bio-molecules on Earth. Each year, photosynthesis converts more than 100 billion metric tons of CO2 and H2O into cellulose and other plant products.  Certain carbohydrates (starch and glycogen) are a dietary staple in most parts of the world and the oxidation of carbohydrates is the central energy yielding pathway in most non-photosynthetic cells. These provide about 55-65 % of energy.  Insoluble carbohydrate polymers serve as structural and protective elements in the cell walls of bacteria and plants and in the connective tissues of animals.  Other carbohydrate polymers lubricate skeletal joints and participate in recognition and adhesion between cells.  More complex carbohydrate polymers covalently attached to proteins or lipids act as signals that determine the intracellular location or metabolic fate of molecules. Definition: Carbohydrates are polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.  carbohydrates have the empirical formula (CH2O)n (n is an number of carbon ranging from 3 to 7)  They contain carbon, hydrogen and oxygen  Ratio of hydrogen and oxygen is 2:1 so they are called hydrates of carbon  Molecules having the general formula [(CH2O)n] of carbohydrates but are not carbohydrates, such as Formaldehyde [(HCHO)-CH2O], Acetic acid [(CH3COOH)-C2H4O2] and Lactic acid [(CH3CHOHCOOH)-C3H6O3]  Molecules not having the general formula [(CH2O)n] of carbohydrates but are carbohydrates, such as Deoxyribose (C6H12O5), Rhamnose (C6H12O5) and Digitoxose (C6H12O4) The word “saccharide” is derived from the Greek sakcharon meaning “sugar” Classification of carbohydrates: There are generally three types of carbohydrates basis on their number of sugar residues as given bellows: A. Monosaccharides (1 sugar unit) B. Disaccharides (2 sugar units) C. Oligosaccharides (2-20 sugar units) D. Polysaccharides (>20 sugar units) Detail Classification of Carbohydrates Name of carbohydrates Number of Name sugar unit Monosaccharides 1 Oligosaccharides 2 Disaccharides (2-20 sugar units) 3 Trisaccharides 4 Tetrasaccharides 5 Pentasaccharides 6 Hexasaccharides Polysaccharides >20 A. On the basis of nature Homopolysaccharides of sugar unit (Two Heteropolysaccharides types) B. On the basis of nature Storage polysaccharides of function (Three Structural polysaccharides types) Mucopolysaccharides A. Monosaccharides Simplest sugar, consist of a single polyhydroxy aldehyde or ketone unit and cannot hydrolyzed further Characteristics  Colourless, crystalline solids and sweet taste.  Soluble in water and insoluble in non polar solvents.  Contain at least three carbon atoms.  Consist of more than five carbons tend to have cyclic structure.  Back bone consist of unbranched carbon chain and ranging from 3-7 carbon atoms.  Consist of either aldehydes or ketones with two or more hydroxyl groups.  The most abundant monosaccharide in nature is the six carbon sugar D-glucose.  All the carbon atoms are linked by single bonds but only one carbon atom consist a double bond with oxygen to form carbonyl group. (Six carbon sugar) Classification of Monosaccharides (According to position of carbonyl group)  Two types (Two families) a. Aldose: Carbonyl group is at an end of the carbon chain b. Ketose: Carbonyl group is at any other position (without end) (Carbonyl group) Nomenclature of monosaccharides (According to number of carbon atoms) Number of carbon Name Aldose Ketose 3 Trioses aldotrioses ketotrioses 4 Tetroses aldotetroses ketotetroses 5 Pentroses aldopentroses ketopentroses 6 Hexoses aldohexoses ketohexoses 7 Heptoses aldoheptoses ketoheptoses  Monosaccarides Have Asymmetric centers All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral or middle carbon atom) carbon atoms and thus occur in optically active isomeric forms. The two different stereoisomeric forms of a monosaccharide is called enantiomers. One of these two forms is designated the D isomer and other the L isomer. These are able to rotate polarized light to either Right side (D/d or dextrorotatory) Left side (L/l or levorotatory) When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D isomer; when on the left, it is the L isomer. The absolute configurations of sugars are known from x-ray crystallography. To represent three- dimensional sugar structures on paper, we often use Fischer projection formulas. General formula for calculation of stereoisomeric form of monosaccharide Common formula is 2n Here n = number of chiral centre No. of No. of chiral Formula No. of stereoisomeric Carbon atom centre forms 3 1 21 = 2 2 4 2 22 = 2 × 2 4 5 3 23 = 2 × 2 × 2 8 6 4 24 = 2 × 2 × 2 × 2 16 Stereoisomers have the same chemical formula but differ in the position of the hydroxyl groups (OH) on one or more of their asymmetric carbons. Enantiomers are stereoisomer that are mirror images of each other. Epimers are stereoisomers that differ in the position of the hydroxyl group at only one asymmetric carbon. Example: D-glucose and D-galactose are epimers that differ at carbon 4; D- glucose and D mannose are epimers that differ at carbon 2. Family of Aldoses and Ketoses The Common Monosaccarides have cyclic structures Monosaccharide with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals. An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule, the bond produced is a glycosidic bond  Cyclic form of aldohexoses (D-glucose) In which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers. These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran. The systematic names for the two ring forms of D-glucose are α-D-glucopyranose and β-D- glucopyranose. Conformation: Two conformations of a molecule are in inter convertible without the breakage of covalent bond. Configurations: Two configurations can be inter-converted only by breaking a covalent bond. Cyclic form of ketohexoses (D-fructose) In which the free hydroxyl group at C-5 has reacted with the keto group at C-2, rendering the latter carbon asymmetric and producing two stereoisomers. These five-membered ring compounds are called furanose because they resemble the five membered ring compound furan. The systematic names for the two ring forms of D- fructose are α-D-fructofuranose and β-D- fructofuranose.  Some Important Monosaccarides Glucose: Main respiratory substrate for all cells Dextrorotatory, so called as dextrose Present in grapes so also called as grape sugar Also called blood sugar/ corn sugar Fructose: Sweetest natural occurring sugar Levorotatory so also called as laevulose Present in human semen secreted by seminal vesicle Present in fruits so also called as fruit sugar Galactose: Also known as brain sugar/cerebrose Occurs as part of milk sugar lactose Found as a component of glycolipids (eg. cerebrosides) Synthesized in mammary glands to make lactose of milk Mannose: Not found in free stage Occurs in albumin of egg and in wood as a component of hemicellulose Erythrose: Erythrose-4 phosphate is an intermediate in the pentose phosphate pathway and the calvin cycle A chewing gum formation to fight cavities comprises sufficient erythose to give the chewing gum anti-caries properties Ribose: Important component of RNA, ATP, NAD and NADP Deoxyribose: Part of DNA Xylose: It is derived from hemicellulose It is found in the embryos of the most plants D-xylose is a sugar widely used as diabetic sweetner in food and beverage Arabinose: It gets its name from gum arabic from which it was first isolated It is inhibitor of sucrase enzyme that break down sucrose into glucose and fructose in the small intestine L-arabinose operon also known as the araBAD operon has been the subject of much biomolecular research Derived monosaccharides Deoxyribose sugar: Part of DNA Amino sugar: Hydroxyl group of sugar is replaced by amino or acetylamino group Eg. Glucosamine (Chitin) and galactosamine (Chondroitin sulphate) Sugar acid: These are produced by oxidation of aldehydic carbon, terminal hydroxyl carbon or both Eg. Glucuronic acid, Galacturonic acid and Ascorbic acid (Vitamin C) Sugar alcohol: Formed by replacement of -CHO group by –OH Eg. Mannitol from mannose and sorbitol from glucose Monosaccharides are Reducing Agents Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe3+) or cupric (Cu2+) ion (Fig. A). The carbonyl carbon is oxidized to a carboxyl group. Glucose and other sugars capable of reducing ferric or cupric ion are called reducing sugars. This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar. By measuring the amount of oxidizing agent reduced by a solution of a sugar, it is also possible to estimate the concentration of that sugar. For many years this test was used to detect and measure elevated glucose levels in blood and urine in the diagnosis of diabetes mellitus. Now, more sensitive methods for measuring blood glucose employ an enzyme, glucose oxidase (Fig. B) (a) Oxidation of the anomeric carbon of glucose and other sugars is the basis for Fehling’s reaction. The cuprous ion (Cu) produced under alkaline conditions forms a red cuprous oxide precipitate. In the hemiacetal (ring) form, C-1 of glucose cannot be oxidized by Cu2. However, the open- chain form is in equilibrium with the ring form, and eventually the oxidation reaction goes to completion. The reaction with Cu2 is not as simple as the equation here implies; in addition to D-gluconate, a number of shorter- chain acids are produced by the fragmentation of glucose. (b) Blood glucose concentration is commonly determined by measuring the amount of H2O2 produced in the reaction catalyzed by glucose oxidase. In the reaction mixture, a second enzyme, peroxidase, catalyzes reaction of the H2O2 with a colorless compound to produce a colored compound, the amount of which is then measured spectrophotometrically.  Oligosaccharides Oligosaccharide: Oligo meaning Few and Saccharide meaning sugar  These are such carbohydrate chains which formed by condensation of few (2-20 sugar units) monosaccharides. Nomenclature of oligosaccharides Number of Name Examples sugar unit 2 Disaccharides Sucrose, Maltose, Lactose, Trehalose and Cellulobiose 3 Trisaccharides Raffinose (Glucose + Fructose+ Galactose) 4 Tetrasaccharides Stachyose(Glucose + Fructose+ 2Galactose) 5 Pentasaccharides Barbanose (2Glucose + Fructose + 2Galactose) 6 Hexasaccharides - Disaccharides Smallest and commonest oligosaccharides Contain only two units of monosaccharide, therefore also called double sugar Two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other sugar. Example: Sucrose (Cane sugar), Maltose (Germination seed), Lactose (Milk sugar), Trehalose (Insect heamolymph) and Cellulobiose (Break down of cellulose) Sucrose: Derived from cane sugar or sugar beet It is also called as stable sugar or cane sugar It is non reducing sugar because it doesn’t have any free functional groups [Aldehyde or Ketone] It is dextrorotatory, bur after hydrolysis it give the dextrorotatory glucose (D- glucose) and levorotatory fructose (L-fructose). Therefore, it hydrolysis brings about a change in rotation from dextrorotation and levorotation (Invert sugar). The Sucrase enzyme available in intestine that help to breakdown of sucrose in glucose and fructose units. Lactose: It is derived from milk, therefore it also called milk sugar Least sweet among naturally occurring sugar Human milk has high amount of lactose Souring of milk is due to conversion of lactose into lactic acid. Intestinal juice contain lactose enzyme that help to breakdown of lactose into glucose and galactose (Brain sugar) Some human body does not digested the milk due to their absent or low release of lactose enzyme in the intestine. Maltose: It is also called malt sugar Present in germination of seed Intermediate compound of starch digestion Trehalose:  Trehalose (from Turkish 'trehala' – a sugar derived from insect cocoons + -ose) is a sugar consisting of two molecules of glucose. It is also known as mycose or tremalose. Some bacteria, fungi, plants and invertebrate animals synthesize it as a source of energy, and to survive freezing and lack of water. Present in heamolymph of insect Cellobiose:  Obtained from partial digestion of cellulose  A glycosylglucose consisting of two glucose units linked via a β(1→4) bond.  Cellobiose is a disaccharide with the formula (C6H7(OH)4O)2O.  It is classified as a reducing sugar.  In terms of its chemical structure, it is derived from the condensation of a pair β- glucosemolecules forging a β(1→4) bond.  It can be hydrolyzed to glucose enzymatically or with acid.  Cellobiose has eight free alcohol (OH) groups, one acetal linkage and one hemiacetal linkage, which give rise to strong inter- and intramolecular hydrogen bonds.  It can be obtained by enzymatic or acidic hydrolysis of cellulose and cellulose-rich materials such as cotton, jute, or paper.  Cellobiose can be used as an indicator carbohydrate for Crohn's disease and malabsorption syndrome.  It is soluble in nonpolar organic solvents.  Polysaccharides Polysaccharide: Poly meaning many and saccharide meaning sugar These are such carbohydrate chains which formed by condensation of many (more than 20 sugar units) monosaccharides. Most commonly uncounted carbohydrates in nature Contain a large number of monosaccharide units jointed together of glycosidic linkages Right end called the reducing end and the left end is called the non-reducing end A. Type of monosaccharides (Two types) (On the basis of nature of sugar unit) 1. Homo-polysaccharides: One type of monosaccharide units 2. Hetero-polysaccharides: more than one type of monosaccharide units B. Type of monosaccharides (Three types) (On the basis of nature of function) 1. Storage polysaccharides (Starch, glycogen and Inulin) 2. Structural polysaccharides (Chitin, Cellulose, Agar) 3. Mucopolysaccharides (Hyaluronic acid, Heparin, Chondroitin sulphate, Keratin suphate, Heparan sulphate and Dermatan sulphate) Starch (Amylum): Main storage polysaccharide of plant Most important dietary source for human beings High content of starch found in cereal crops, tuber crops, root crops, leafy vegetables Form helical secondary structure Can hold iodine molecules in the helical portion Starch in iodin is blue in colour Water soluble Contain about 15-20 % of starch Starch contains two types of glucose polymer i. Amylose ii. Amylopectin Amylose consists of long, unbranched chains of D-glucose residues connected by (1-4) linkages. Such chains vary in molecular weight from a few thousand to more than a million Amylopectin also has a high molecular weight (up to 100 million) but unlike amylose is highly branched. The glycosidic linkages joining successive glucose residues in amylopectin chains are (1-4); the branch points (occurring every 24 to 30 residues) are (1-6) linkages. Water insoluble Contain about 80-85 % of Starch Give red colour with Iodin Glycogen It is the main storage polysaccharide of animal cells. Storage mainly in liver, muscle and kidney. Like amylopectin, glycogen is a polymer of (1-4)-linked subunits of glucose, with (1-6)-linked branches. Glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. Glycogen is especially abundant in the liver. each branch in glycogen ends with a non-reducing sugar unit, a glycogen molecule has as many non-reducing ends as it has branches, but only one reducing end. Therefore, when glycogen is used as an energy source, glucose units are removed one at a time from the non-reducing ends. Degradative enzymes that act only at non-reducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides. Inulin: Polymer of fructose Found in root of dahlia, artichoke, sweet potato, onion, garlic Water soluble Not found in human body and is readily filtered through kidney, so used in testing of kidney function. Cellulose Cellulose is a homopolysaccharide consisting of a linear chain of several hundred to many thousands of β (1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Cellulose is the most abundant organic polymer on earth. The cellulose content in cotton fiber is 90% The cellulose content of weed is 40-50% The cellulose content of dried hemp is ~ 57% Chitin A linear homopolysaccharide composed of N-acetylglucosamine residues in linkage. The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods—insects, lobsters, and crabs, for example— and is probably the second most abundant polysaccharide, next to cellulose, in nature. Agar: A natural component of seaweeds Made up of sulphated galactose units In microbiology it is used as cell culture media Obtained from gracilaria and gelidium Dextran:  Polymers of glucose produced by micro-organsms  Used as plasma volume expanders in transfusion  Dextran is an α-D-1,6-glucose-linked glucan with side-chains 1-3 linked to the backbone units of the Dextran biopolymer.  The degree of branching is approximately 5% in native (un-fractionated) dextran and decreases with decreasing molecular weight. The branches are mostly 1-2 glucose units long.  Dextran can be obtained from fermentation of sucrose-containing media by Leuconostoc mesenteroides B512F. Dextrin:  Intermediate substances join the digestion of starch and glycogen  Glucose and maltose are formed by hydrolysis of dextrin  Also occurs as storage food in yeast and bacteria  Dextrins are a group of low-molecular-weight carbohydrates.  produced by the hydrolysis of starch or glyycogen.  Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds.  Dextrins can be produced from starch using enzymes like amylases, as during digestion in the human body and during malting and mashing or by applying dry heat under acidic conditions (pyrolysis or roasting).  Dextrins produced by heat are also known as pyrodextrins.  The starch hydrolyses during roasting under acidic conditions, and short-chained starch parts partially rebranch with α-(1,6) bonds to the degraded starch molecule. Mucopolysaccharides: Their building blocks are amino sugar and chemically modified sugar like glucosamine (n-acetyl galactosamine) These are commonly known as glycosaminoglycans (GAG) Present in Bhindi and Isabgol Hyaluronic acid is found in vitreous humour umbilical card, joint and connective tissue and also as a binding material in animal cell coat (Animal cement) Heparin is an anticoagulant in blood Chondroitin sulphate is a compound of cartilage tendons and bones Keratin sulphate is found in costal cartilage and cornea Dermatan sulphate is found in wall of arteries. Heteropolysaccharides: contain two or more different kinds (Peptidoglycan) (a) The repeating unit of peptidoglycan is an N- acetylglucosamine–N-acetylmuramic acid disaccharide whose lactyl side chain forms an amide bond with a tetrapeptide. The tetrapeptide of S. aureus is shown. The isoglutamyl residue is so designated because it forms a peptide link via its -carboxyl group. (b) The S. aureus bacterial cell wall peptidoglycan, showing its pentaglycine connecting bridges (purple). Conclusions 1. Monosaccharides, the simplest carbohydrates, are classified as aldoses or ketoses. 2. The cyclic hemiacetal and hemiketal forms of monosaccharides have either the or configuration at their anomeric carbon but are conformationally variable. 3. Monosaccharide derivatives include aldonic acids, uronic acids, alditols, deoxy sugars, amino sugars, and α - and β–glycosides. 4. Polysaccharides consist of monosaccharides linked by glycosidic bonds. 5. Cellulose and chitin are polysaccharides whose β(1 - 4) linkages cause them to adopt rigid and extended structures. 6. The storage polysaccharides starch and glycogen consist of - glycosidically linked glucose residues. 7. Glycosaminoglycans are unbranched polysaccharides containing uronic acid and amino sugars that are often sulfated. 8. Proteoglycans are enormous molecules consisting ofhyaluronate with attached core proteins that bear numerous glycosaminoglycans and oligosaccharides. 9. Bacterial cell walls are made of peptidoglycan, a network of polysaccharide and polypeptide chains. 10. Glycosylated proteins may contain N-linked oligosaccharides (attached to Asn) or O-linked oligosaccharides (attached to Ser or Thr) or both. Different molecules of a glycoprotein may contain different sequences and locations of oligosaccharides. 11. Oligosaccharides play important roles in determining protein structure and in cell-surface recognition phenomena. Thanks