Di- and Oligosaccharides PDF

Carbohydrates Chemistry Di- and Oligosaccharides  Prof / Ayman El Baz Professor of Medical Biochemistry & Molecular Biology Dr/ Sara El Derbaly Assistant Professor of Medical Biochemistry& Molecular Biology  Learning outcomes At the end of the lectu...

Carbohydrates Chemistry Di- and Oligosaccharides  Prof / Ayman El Baz Professor of Medical Biochemistry & Molecular Biology Dr/ Sara El Derbaly Assistant Professor of Medical Biochemistry& Molecular Biology  Learning outcomes At the end of the lecture, the students should be able to: Define disaccharides. Classify disaccharides. Recognize the structure of different types of disaccharides and their significance. Define oligosaccharides. Recognize the existence of oligosaccharides and their significance. Correlate their knowledge to a clinical situation.  Case scenario (Clinical correlate) Hamed, a twenty-six years old patient had been experiencing occasional discomfort after meals especially an hour after eating cheese and milk. He had abdominal cramps and diarrhea. His doctor advised him not to eat diary products. Disaccharides Definition: Disaccharides consist of 2 monosaccharide units joined by an O- glycosidic bond. Disaccharides Classification: The disaccharides can be classified into: – Homodisaccharides: formed of the same monosaccharide e.g. maltose, isomaltose and cellobiose. – Heterodisaccharides: formed of different monosaccharide units e.g. sucrose and lactose. The most abundant disaccharides are sucrose, lactose and maltose. Disaccharides can be also classified into: Non-reducing disaccharides: The two aldehyde/or ketone groups are involved in the linkage (anomeric C not free) e.g. sucrose. Reducing disaccharides: If one of aldehyde/or ketone groups is free (free anomeric C) e.g. Lactose and Maltose. Homodisaccharides 1. Maltose (malt sugar, maltose syrup): It is formed of two α D-glucose units linked together by α 1-4 glycosidic bond. It is produced by the action of amylase enzymes on starch. It is a reducing sugar. It is hydrolyzed by maltase enzyme to produce 2 glucose molecules. Homodisaccharides 2. Isomaltose: It is formed of two α-glucose but linked together by α 1-6 glycosidic bonds. It is a reducing sugar. It is obtained by the hydrolysis of some polysaccharides such as dextran. Homodisaccharides 3. Cellobiose: It is formed of two units of β-glucose linked together by β1- 4 glycosidic bond. It is obtained by the acid hydrolysis of cellulose.  Homodisaccharides Disaccharide Maltose (malt sugar) Isomaltose Cellobiose Structure Two α D-glucose units Two αD-glucose units Two units β D- glucose units Bond α 1-4 glycosidic α 1-6 glycosidic β 1-4 glycosidic Reduction Reducing sugar Reducing sugar Reducing sugar Source From starch by the Hydrolysis of some Acid hydrolysis of action of amylase polysaccharides such cellulose enzymes as dextran. Heterodisaccharides 1. Sucrose (cane sugar, beet sugar, table sugar). Formed of α D-glucose and β D-fructose linked by an α-1-β-2 glycosidic bond (involving the anomeric active carbon atoms). There is no free aldehyde or ketone group in sucrose. So, it is non reducing sugar. Heterodisaccharides 1. Sucrose (cont.): The hydrolysis of sucrose to glucose and fructose is catalyzed by sucrase (also called invertase). Sucrose is dextrorotatory. An equilibrium mixture of glucose and fructose is levorotatory. This equilibrium mixture is called invert sugar (in bee honey) and has free carbonyl groups (reducing sugar). Heterodisaccharides 2. Lactose (milk sugar): It is the principal carbohydrate in milk and is of critical nutritional importance to mammals in the early stages of their lives. It is formed from β-galactose and β-glucose via a β (1-4) link with free anomeric C. It is a reducing sugar. Heterodisaccharides 2. Lactose (Cont.): It may appear in urine in late pregnancy and during lactation. It is hydrolyzed by the intestinal lactase enzyme into galactose and glucose. Clinical Correlates: – Low levels of lactase enzyme leads to undigested lactose that undergoes bacterial fermentation in the colon with the generation of large amounts of CO2, H2 and irritating organic acids. – These products cause painful digestive upsets known as lactose intolerance. Oligosaccharides Definition: Oligosaccharides consist of 3-10 monosaccharide units joined together by glycoside bonds. e.g. Maltotriose 3 X glucosidic or glyosidic (purely glucose) Trisaccharide 3 monosaccharides  Oligosaccharides Existence and Significance: Oligosaccharides are important as constituents of the glycolipids present in cell membrane. Oligosaccharides are also important as constituents of the glycoproteins. Oligosaccharides Existence and Significance (Cont.): Integral membrane proteins contain covalently attached oligosaccharides on their extracellular face. Many secreted proteins, such as antibodies and coagulation factors also contain oligosaccharide units. These oligosaccharide are attached to either the side- chain O2 atom of Serine or Threonine amino acids by O- glycosidic linkages or to the side chain nitrogen of Asparagine amino acid by N-glycosidic linkages.  Case Report and Clinical Correlates The case represents symptoms of lactose intolerance (abdominal discomfort after meals containing dairy products, abdominal cramps and diarrhea.). Low levels of lactase leads to bacterial fermentation of the undigested lactose in the colon with the generation of large amounts of CO2, H2 and irritating organic acids.  Hana is a 2 years old infant presented with gastric distention and diarrhea. These symptoms started when her mother starts to add table sugar to her food. All her laboratory profiles were within normal. Her doctor suspect the presence of congenital sucrose intolerence. Explain the biochemical basis of the disease? Reference: https://www.sucraid.net/symptoms/  Vasudevan’s Textbook of Biochemistry For Medical Students, 6th Edition. Chatterjea’s Textbook of Medical Biochemistry, 8th edition. BRS Biochemistry, Molecular Biology, and Genetics, 5th Edition.