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Structure and properties of Carbohydrates PTT 103 (Biochemistry) Department of Chemical Engineering Technology, UniMAP [email protected] LECTURE 3 –STRUCTURE AND PROPERTIES OF CARBOHYDRATES Structure, properties and function of carbohydrates and their derivatives • Classification of carbohydrates Course outcome: • Ability to differentiate basic structure, properties, functions and classification of important biomolecules. • LECTURE 3 -CARBOHYDRATES Section Section Section Section TOPICS: 1. Role & Significance of Carbohydrates 2. Monosaccharide 3. Oligosaccharides 4. Polysaccharides SIGNIFICANCE OF CARBOHYDRATES Carbohydrates are the most abundant biomolecules in nature, having a direct link between solar energy and the chemical bond energy in living organisms. Source of rapid energy production Structural building blocks of cells Components of several metabolic pathways Recognition of cellular phenomena, such as cell recognition and binding (e.g., by other cells, hormones, and viruses) CARBOHYDRATES Carbohydrate : compounds contains H, C & O with the comp : (CH2O)n (Hydrate of carbon) Carbohydrates : Consist of sugar (saccharum) Sugars : compound that contains alcohol & carbonyl functional group Carbonyl func. group : >C=O Adehyde aldose Ketone ketose Examples: Classification Carbohydrate Mono saccharide Glucose, fructose Ribose (aldopentose) Deoxy ribose Oligo saccharide disaccharides Glycoproteins (bacterial cell walls Poly saccharide cellulose, chitin, starch, glycogen, glucoaminoglycans Glycoconjugates glycoproteins and proteoglycans Monosaccharides Simple sugars, polyhydroxy aldehydes or ketones Aldehyde functional group: aldose Ketone functional group: ketose Simplest aldose: glyceraldehyde Simplest ketose: dihydroxyacetone Classified according to no. of carbon. 3 carbon (trioses), 4 carbon (tetroses, 5 carbon (pentose), 6 carbon (hexoses) Glucose: six carbon aldose: aldohexose Monosacharides : Exp. aldoses & ketoses Aldotriose Ketotriose Aldotetrose Ketotetrose Aldopentoses Ketopentose Aldohexose Ketohexose 2.2. MONOSACCHARIDES STEREOISOMERS Isomers: same chemical formulas, different structures Total no of possible isomers can be determined by Van Hoff’s rule: compound with n chiral C atoms has a max of 2n possible stereoisomers. Chiral: asymmetric carbons, i.e carbon atom with four different substituents Eg: n = 4, there are 16 stereoisomers (8-L stereoisomers, 8-D stereoisomers). In optical isomers- the ref C is the asymmetric C that is most remote from the C=O carbon. In D-aldose family sugars, the OH group is to the right on the chiral C atom farthest from the most oxidized C (aldehyde group) in the molecule. D- and L- enantiomers Stereoisomers that are not enantiomers (mirror-image) are called diastereoisomers. Eg: aldopentoses, D-ribose and L-ribose are enantiomers. The D-ribose and D-arabinose are diastereomers because they are isomers but not mirror image. Diastereomers that differ in the configuration at a single asymmetric C atom are called epimers. Eg: D-glucose and D-galactose are epimers because they differ only in the configuration of the OH group at C-4. D-mannose and D-galactose are not epimers- differ more than 1 C. MONOSACCHARIDES STEREOISOMERS The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and has two different optical isomers, or enantiomers the projection in which the carbohydrate backbone is drawn vertically with the carbonyl shown on the top. Cyclic structure of monosacharides • in aqueous solution, monosaccharides 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. • Sir Norman Haworth showed that the linear form of glucose (and other aldohexose) could undergo intramolecular reaction to form a cyclic hemiacetal. • the analogous intramolecular reaction of ketose sugar yields a cyclic hemiketal. The new chiral center in cyclic (c1) is called anomeric carbon In aldose sugar… α-sugar β-sugar • The OH of newly formed hemiacetal occurs on C1 is at ‘down’ position • In Fischer projection, α-anomeric OH occurs on the right • The OH of newly formed hemiacetal occurs on C1 is at ‘up’ position • In Fischer projection, β-anomeric OH occurs on the left Pyranoses& Furanoses Pyranoses: six-membered ring compounds ( resemble pyran ) Furanoses : fivemembered rings, (resemble furan) The structure systematic names glucose & fructose become HAWORTH STRUCTURES An English chemist W.N. Haworth gave a more accurate picture of carbohydrate structure. Haworth Structures To convert from traditional Fischer formula of a D-pentose or D-hexose to a Haworth formula, the following steps should be followed: Draw a 5 or 6-membered ring with the O placed as shown below: Starting with anomeric carbon to the right of the ring O, place OH group either above or below the plane of the ring. Group that pointing to the left in Fischer projection should go above (β-) the plane of the ring, and those pointing right should go below the ring (α-) In D-sugars, the last C position (eg: C-6 glucose) is always up FISHER AND HAWORTH FORMS OF SUGAR SUMMARY OF SUGAR STRUCTURES ISOMERS- compounds that have the same chemical formula e.g. fructose, glucose, mannose, and galactose are isomers of each other having formula C6H12O6. EPIMERS- refer to sugars whose configuration differ around one specific carbon atom e.g. glucose and galactose are C-4 epimers and glucose and mannose are C-2 epimers. ENANTIOMERS- a special type of isomerism found in pairs of structures that are mirror images of each other. The mirror images are termed as enantiomers and the two members are designated as D- and L- sugar. The vast majority of sugars in humans are D-sugars. CYCLIZATION OF SUGARS- most monosaccharides with 5 or more carbon atoms are predominately found in a ring form, where the aldehyde or ketone group has reacted with an alcoholic group on the same sugar group to form a hemiacetal or hemiketal ring. Pyranose ring- if the ring has 5 carbons and 1 oxygen. Furanose ring- if the ring is 5-membered (4 carbons and 1 oxygen Exercise 1 Draw cyclic Haworth structure for: D-fructose Name the anomers that result from the cyclization of these molecules. IMPORTANT REACTIONS IN MONOSACCHARIDES Monosaccharides undergo the following reactions : 1. 2. 3. 4. 5. 6. Mutarotation Oxidation Reduction Isomerization Esterification Glycoside formation IMPORTANT REACTIONS IN MONOSACCHARIDES Details 1. Mutarotation – alfa and beta forms of sugars are readily interconverted when dissolved in water. Mutarotation produces an equilibrium mixture of α and β- forms in both furanose and pyranose ring structures. 2 Oxidation and reduction in presence of oxidising agents, metal ions (Cu2+) and enzymes, monosacchs undergo several oxidation reactions e.g. Oxidation of aldehyde group (R-CHO) yields aldonic acid; of terminal CH2OH (alcohol) yields uronic acid; and of both the aldehyde and CH2OH gives aldaric acid. The carbonyl groups in both aldonic and uronic can react with an OH group in the same molecule to form a cyclic ester known as a lactone. sugars that can be oxidized by weak oxidizing agent ie. Benedict’s reagent, called reducing sugars. Because the reaction occurs only with sugars that can revert to open chain form, all monosaccharides are reducing sugars. REDUCING SUGARS All monosacchs are reducing sugars. They can be oxidised by weak oxidising agent such as Benedict’s reagent Benedict's reagent is a solution of copper sulfate, sodium hydroxide, and tartaric acid. Aqueous glucose is mixed with Benedict's reagent and heated. The reaction reduces the blue copper (II) ion to form a brick red precipitate of copper (I) oxide. Because of this, glucose is classified as a reducing sugar. 3. REDUCTION reduction of the aldehyde and ketone groups of monosacchs yield sugar alcohols (alditols) Sugar alcohols e.g.sorbitol, are used commercially in processing foods and pharmaceuticals. sorbitol- improves the shelf-life of candyit helps prevent moisture loss. IMPORTANT REACTIONS (Cont) 4. ISOMERIZATION Monosaccharides undergo several types of isomerization e.g. D-glucose in alkaline solution for several hours contain D-mannose and D-fructose. Both isomerization involves an intramolecular shift of a H atom and a relocation of double bond. The conversion of glucose to mannose is termed s epimerization. Important reactions… (cont) 5 ESTERIFICATION Free OH groups of carbohydrates react with acids to form esters. This reaction an change the physical and chemical propteries of sugar. 6. GLYCOSIDE FORMATIONHemiacetals and hemiketals reaction with alcohols to form the corressponding aceta or ketal. On the contrary when a cyclic hemiacetal or hemiketal form of monosaccharide reacts with alcohol, the new linkage is called glycosidic linkage and the compound glycoside. IMPORTANT MONOSACCHARIDES GLUCOSE FRUCTOSE GALACTOSE D-Glucose: D-glucose (dextrose) is the primary fuel in living cells especially in brain cells that have few or no mitochondria. Cells such as eyeballs have limited oxygen supply and use large amount of glucose to generate energy Dietary sources include plant starch, and the disaccharides lactose, maltose, and sucrose Important monosaccharides. Cont FRUCTOSE ◦ D-fructose (levulose) is often referred as fruit sugar and is found in some vegetables and honey ◦ This molecule is an important member of ketose member of sugars ◦ It is twice as sweet as sucrose (per gram basis) and is used as sweeting agent in processed food products ◦ It is present in large amounts in male reproductive tract and is synthesised in the seminal vesicles. Important monosaccharides. Cont.... GALACTOSE ◦ is necessary to synthesize a variety of biomolecules (lactose-in mammalary glands, glycolipids, certain phospholipids, proteoglycans, and glycoproteins) ◦ Galactose and glucose are epimers at carbon 4 and interconversion is catalysed by enzyme epimerase. ◦ Medical problems – galactosemia (genetic disorder) where enzyme to metabolize galactose is missing; accumulation of galactose in the body can cause liver damage, cataracts, and severe mental retardation MONOSACCHARIDE DERVATIVES URONIC ACIDS – formed when terminal CH2OH group of a mono sugar is oxidised ◦ Important acids in animals – D-glucuronic acid and its epimer L-iduronic acid ◦ In liver cells glucuronic acid combines with steroids, certain drugs, and bilirubin to improve water solubility therby helping the removal of waste products from the body ◦ These acids are abundant in the connective tissue carbohydrate components. Mono sugar derivatives AMINO SUGARS – ◦ Sugars in which a hydroxyl group (common on carbon 2) is replaced by an amino group e.g. Dglucosamine and D-galactosamine ◦ common constituents of complex carbohydrate molecule found attached to cellular proteins and lipids ◦ Amino acids are often acetylated e.g. N-acetylglucosamine. Mono sugar derivatives DEOXYSUGARS ◦ monosaccharides in which an - H has replaced an – OH group ◦ Important sugars: L-fucose (formed from D-mannose by reduction reactions) and 2-deoxy-D-ribose ◦ L-fucose – found among carbohydrate components of glycoproteins, such as those of the ABO blood group determinates on the surface of red blood cells ◦ 2-deoxyribose is the pentose sugar component of DNA. GLYCOSIDIC BONDS • • • • Monosaccharides can be linked by glycosidic bonds (joining of 2 hydroxyl groups of sugars by splitting out water molecule) to create larger structures. Disaccharides contain 2 monosaccharides e.g. lactose (galactose+glucose); maltose (glucose+glucose); sucrose (glucose+fructose) Oligosaccharides – 3 to 12 monosaccharides units e.g. glycoproteins Polysaccharides – more than 12 monosaccharides units e.g. glycogen (homopolysaccharide) having hundreds of sugar units; glycosaminoglycans (heteropolysaccharides) containing a number of different monosaccharides species. Disaccharides and oligosaccharides Monosaccharides are linked together by glycosidic bonds to form a variety of molecules that perform diverse biological functions. Disaccharides are glycosides composed of 2 monosaccharides. While oligosaccharide is often used for relatively small sugar polymers that consist of 2-10 or more monosaccharide units. If one monosaccharide molecule is linked thru its anomeric carbon to the oh group on carbon 4 of another monosaccharide, the glycosidic linkage is designated as 1,4. Anomeric OH group may potentially be in either α (down-position) or βconfiguration (up position). Important sugars of Disaccharides LACTOSE (milk sugar) disaccharide found in milk; composed of one molecule of galactose (OH group in C-1) and glucose (OH group at C-4) linked through beta(1,4) glycosidic linkage (anomeric C of galactose is in βconfigurations); because of the hemiacetal group of the glucose component, lactose is a reducing sugar Lactose intolerance Digestion of disaccharides and other carbohydrates is mediated by enzymes synthesized by cells lining the small intestines. Deficiency in any of these enzymes can cause an unpleasant symptoms. Carbohydrates are absorbed as monosaccharides- any undigested disaccharide molecules would pass into large intestine, where osmotic pressure draws water from surrounding tissue causing diarrhea. Bacteria in the colon digest disaccharides thru fermentation, thus producing gas- cramp and bloating. Most common- lactose intolerance- deficiency in enzyme lactase. • MALTOSE ( malt sugar) An intermediate product of starch hydrolysis; it is a disaccharide with an alfa(1,4) glycosidic linkage between two D-glucose molecules; in solution the free anomeric carbon undergoes mutarotation resulting in an equilibrium mixture of alfa and beta – maltoses; it does not occur freely in nature SUCROSE common table sugar: cane sugar or beet sugar produced in the leaves and stems of plants; it is a disaccharide containing both alfa-glucose and beta-fructose residues linked by alfa,beta(1,2)glycosidic bond. CELLOBIOSE degradation product of cellulose containing two molecules of glucose linked by a beta (1,4) glycosidic bond; it does not occur freely in nature OLIGOSACCHARIDE SUGARS Oligosaccharides are small polymers often found attached to polypeptides in glycoproteins and some glycolipids. They are attached to membrane and secretory proteins found in endoplasmic reticulum and Golgi complex of various cells Two classes: N-linked and O-linked