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Medical Biochemistry and Molecular Biology Medical Biochemistry and Molecular Biology CARBOHYDRATE CHEMISTRY Disaccharides By Ayman Elsamanoudy 2 Objectives (ILOs) I. to understand the concept and importances of glycosidic bond. II. to study the definition, classification of disaccharides III. to know the biological importance of different disaccharides. IV.To know the natural sources of disaccharides. V. to understand how any disorder in disaccharides digestion or absorption leads to disease. 3 Glycosides . A glycoside is a compound formed by binding of a sugar and the hydroxyl group of a second compound which, may be sugar (glycon ) or may be not a sugar (aglycon). Disaccharides → Sugar + Sugar. Methyl glycoside → Sugar + Methyl alcohol. Cardiac glycoside → Galactose + Steroid. Sugars are joined to alcohols and amines by glycosidic bonds. 4 Types of glycosidic bond . 1- O-glycosidic bonds : . Monosaccharides can link to each other by O-glycosidic bonds to form disaccharides, oligosaccharides, and polysaccharides. 2-N- glycosidic bond. The anomeric carbon atom of a sugar can be linked to the nitrogen atom of an amine by an N- glycosidic bond. 5 . Naming of glycosidic bonds: . Glycosidic bonds between sugars are named according : to the numbers of the connected carbons & the position of the anomeric hydroxyl group of the sugar. So, If this anomeric hydroxyl group is in the α configuration, the linkage is an α-bond. If it is in the β-configuration, the linkage is a β- bond 6 .Examples of glycosides: . Disaccharides as maltose, lactose and sucrose. Polysaccharides . Glycolipids. Glycoproteins. Nucleotides as ATP, CTP, GTP where the aglycone is purine and pyrimidine bases (N- glycosides). Cardiac glycosides as digitalis which is used in treatment of heart failure where the aglycone is a steroid7 . Disaccharides . Disaccharides consist of two sugars joined by an O-glycosidic bond. The most abundant disaccharides are sucrose, lactose and maltose. Other disaccharides include isomaltose, cellobiose and trehalose. The disaccharides can be classified into : Homodisaccharides Heterodisaccharides. 8 Other classification of disaccharides (1)Reducing disaccharide (2) Non-Reducing disaccharide . Princible : presence of free aldehyde or ketone group (a free reducing carbonyl group) is the cause of reducing property. REDUCING SUGARS •When Benedicts or Fehling test is performed with the disaccharides maltose and sucrose, the following result is obtained: Sucrose is a Maltose is a non-reducing sugar reducing sugar SUCROSE MALTOSE RESULT RESULT I-Hommodisaccharides Maltose Isomaltose Celebiose (malt sugar ) structure 2α-glucose 2 α-glucose 2β-D-glucose Type of α-1-4 glucosidic α 1-6 glucosidic β1 4 glucosidic bond bond bond bond. 11 Free aldehyde group 12 C1 C6 13 Free active carbonyl group Free active carbonyl group CH OH CH2OH 2 O O OH H H H H 1 O 4 H OH H OH H OH H H OH H OH -Glucose -Glucose Cellobiose B 1-4 glucosidic bond 14 Maltose Isomaltose Celebiose (malt sugar ) Anomeric free free free Carbon Reducing Reducing Reducing Reducing property Produced by It is produced by the hydrolysis by the acid from starch by of some hydrolysis of the action of polysaccharides cellulose amylase such as dextran enzymes 15 Maltose (malt Isomaltose Celebiose sugar ) Composition 2α-glucose units 2 α-glucose 2β-D-glucose Type of bond α-1-4 glycosidic α 1-6 glycosidic bond β1 4 bond glycosidic bond. Anomeric C free free free Reducing Reducing Reducing Reducing property Produced by It is produced by the hydrolysis of by the acid from starch by some polysaccharides hydrolysis of the action of such as dextran cellulose 16 amylase enzymes II- Heterodisaccharides: are formed of 2 different monosaccharide units Sucrose Lactose Composition α-D-glucose+ β–D-fructose β-D-galactose and β-D- glucose Type of bond α-1-β-2 glucosidic bond OR a β (14) galactosidic bond β 2- α-1 fructosidic bond 17 Sucrose Lactose Anomeric C no free aldehyde or free ketone group Reducing is not a reducing Reducing property sugar 18 19 Free active carbonyl group CH OH CH2OH 2 O O OH ..... H OH H H H 1 O 4 H OH H OH H ..... OH H H H OH H OH -Galactose Glucose and Lactose B 1-4 galactosidic bond 20 21 CH2OH O H H -Glucose H 1 OH H OH No free active H OH carbonyl groups CH OH 2 O O -Fructose 2 H OH H CH2OH OH H Sucrose 22 Sucrose Lactose Effect of The hydrolysis of sucrose to glucose and hydrolysed by the hydrolysis fructose is catalysed by sucrase (also called intestinal lactase enzyme invertase), into galactose and glucose Present in Milk sugar Table sugar It may appear in urine in late pregnancy and Cane sugar, during lactation beet sugar 23 Sucrose Lactose Composition α-D-glucose + β–D-fructose β-D-galactose and β-D-glucose Type of bond α-1-β-2 glucosidic bond OR a β (14) galactosidic bond β 2- α-1 fructosidic bond Anomeric C no free aldehyde or ketone group free Reducing is not a reducing sugar Reducing property Effect of The hydrolysis of sucrose to hydrolysed by the intestinal lactase glucose and fructose is catalysed hydrolysis by sucrase (also called invertase), enzyme into galactose and glucose Present in Table sugar Milk sugar Cane sugar, beet sugar It may appear in urine in late pregnancy and during lactation 24 Comparison between sucrose and invert sugar Sucrose Invert sugar Composition α-D-glucose + β–D-fructose Mix of α-D-glucopyranose & β–D-fructofuranose Type of bond α-1-β-2 glycosidic bond No bond Anomeric C no free aldehyde or ketone free group Reducing property is not a reducing sugar Reducing Specific rotation Sucrose has a specific rotation (glucose = +52.5° , fructose = -91°). So, mixture of its component monosaccharides has of + 66.5° i.e. dextrorotatory a net negative rotation Optical activity Dextrorotatory Levorotatory Present in Cane sugar, beet sugar bee honey 25 Digestion of lactose •The intestinal villi secrete an enzyme called lactase (β-D- galactosidase) to digest lactose, and produce glucose and galactose, which can be absorbed. Lactose intolerance . More than half of the world’s adults are lactose intolerance. Lactose intolerance is the inability to metabolize lactose, because the lactase is absent in the intestinal system or its availability is lowered. Lactose intolerance . In the absence of lactase, lactose remains uncleaved and passes intact into the colon. The intestinal bacteria quickly switch over to lactose metabolism, and produces large amounts of gases (a mixture of hydrogen, carbon dioxide, and methane). This, leads to appearance of abdominal symptoms, including stomach cramps(colic), bloating, and flatulence. Treatment for this disorder is simple to remove lactose from diet. Lactose intolerance Most adult mammals have low levels of lactase and so much of the lactose they might ingest moves through their digestive tract to the colon, where bacterial fermentation generates large amounts of CO2, H2 and irritating organic acids. These products cause painful digestive upsets known as lactose intolerance. 28 Which of the following carbohydrates have at least 2 different monosaccharid A) Sucrose B) Fructose C) Glucose D) Starch The monosaccharide units are linked byα1 →4 glycosidic linkage in (A) Maltose (B) Sucrose (C) Cellulose (D) Cellobiose 29 A dissaccharide formed by 1,B2-glycosidic linkage between their monosaccharide units is (A) Lactose (B) Maltose (C) Trehalose (D) Sucrose What monosaccharides make up a sucrose (table sugar) molecule? Galactose and fructose(A Galactose and maltose(A Lactose and fructose(A Glucose and fructose (A 30 A carbohydrate found only in milk is (A) Glucose (B) Galactose (C) Lactose (D) Maltose A carbohydrate, when hydrolyzed known commonly as invert sugar, is (A) Fructose (B) Sucrose (C) Glucose (D) Lactose 31 Short Questions 1. Glycosides(def,types,examples). 2. Enumerate homodissacharides and differentiate between them 3. Enumerate heterodissacharides and differentiate between them. 4. Enumerate reducing disacharides and mention the type of bond in each of them. 5. Enumerate disacharides that contain α glycosidic bond and draw the structural formula of each of them. 6. Enumerate disacharides that contain β glycosidic bond and draw the structural formula of each of them 32 7.Compare between maltose and cellibiose 8.Compare between maltose and isomaltose. 9.Compare between sucrose and lactose 10.Discuss in short;lactose intolerance. 11.Compare between sucrose and invert sugar. 12. Compare between sucrose and maltose. 13. Enumerate disacharides that contain α-D glucose. 14. Enumerate disacharides that contain β-D glucose. 14. Enumerate disacharides that contain glucosidic bond. 33 . 15. Mention disacharide that contain galactosidic bond and draw the structural formula of it. 16. Mention disacharide that contain fructosidic bondbond and draw the structural formula of it. 17. Explain the role of free active carbonyl group in determing the property of disacharides ,give examples. 34 GREAT THANKS Ayman Elsamanoudy 10/21/2014 Ahmed A.Albadry 35.
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