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L20. Carbohydrates: Structure and functions of Di and Polysaccharides Prof. Nelofar Sami Khan www.gmu.ac.ae COLLEGE OF MEDICINE Learning Objectives: At the end of the session, you should be able to •Name the common disaccharides and discuss their physiologic importance. •Classify polysaccharide...
L20. Carbohydrates: Structure and functions of Di and Polysaccharides Prof. Nelofar Sami Khan www.gmu.ac.ae COLLEGE OF MEDICINE Learning Objectives: At the end of the session, you should be able to •Name the common disaccharides and discuss their physiologic importance. •Classify polysaccharides based on composition, with examples. •Describe the structure of Starch, Cellulose and Glycogen and explain their physiologic importance. •Reading: Satyanarayana U and Chakrapani U. Biochemistry; Elsevier; 5th Edition; 2020. ISBN- 978-8131262535. Chapter 2, pages 19 - 22. Disaccharides Disaccharides are formed when two monosaccharides are linked together by a glycosidic bond. Maltose • It is composed of 2 glucose monomers in an α-(1,4) glycosidic bond. • Found in Germinating cereals. • Produced in the body by digestion of starch. Maltose is glucose-(α-1,4)-glucose. Lactose • It is found exclusively in the milk of mammals. • Consists of galactose and glucose in a β 1→ 4 glycosidic bond. • Exclusive carbohydrate for infants. • Malabsorption (Lactose diarrhea, flatulence. intolerance) leads to Sucrose • Prevalent in sugar cane and sugar beets. • It is composed of glucose and fructose linked through an α 1→2 glycosidic bond. • Important dietary carbohydrate. Polysaccharides • Contain more than 10 monosaccharides units. • Composed of repeating units of monosaccharides (or their derivatives) held together by glycosidic bonds. • Have structural as well as energy storage functions in the body. • Can be homopolysaccharides or heteropolysaccharides 6 8 Homopolysaccharides of Glucose • Starch Found in cereals, potatoes, Major dietary carbohydrate, Source of glucose for the body. •Glycogen Carbohydrate reserve of animals •Cellulose Plant product, not digested by humans, Major dietary fiber. 10 Starch ∙ It is a homopolysaccharide formed from glucose. ∙ It is composed of two types of polysaccharide chains: ✔ Amylose (unbranched) ✔ Amylopectin (branched) ∙ The enzyme Amylase acts on starch and breaks it down to maltose [Glu-Glu α (1→4)] and isomaltose [Glu-Glu α(1→6)]. 13 Glycogen ∙ ∙ ∙ ∙ It is the storage form of carbohydrate in animals. It is found mainly in the liver and the muscles. Similar to Amylopectin, more branched. It is a highly branched homopolymer of glucose in α (1→4) linkages with α(1→6) at branch points. Branches occur every 8-10 residues. ∙ Glycogen is a very compact structure. ∙ Can be stored in a small volume, with little effect on cellular osmolarity. ∙ Contains up to 25,000 glucose units. Glycogen 15 Cellulose ∙ It is a homopolymer of β-D-glucose connected in β (1 → 4) linkages. ∙ It is found in woody portions of plants. ∙ It cannot be digested by the human body due to the absence of enzymes that digest β1→4 linkages. ∙ It is an important part of fiber in diet. ∙ Certain ruminants and herbivorous animals contain microorganisms in the gut which produce enzymes that can cleave β-glycosidic bonds DIGESTION OF DIETARY CARBOHYDRATES A. Begins in the mouth • Salivary α-amylase acts briefly on dietary starch and glycogen, hydrolysing random α(1→4) bonds. B. Small intestine • Pancreatic α-amylase continues the process of starch digestion. C. Final digestion by disaccharidases Disaccharidases synthesized by the intestinal mucosal cells and remain associated with the luminal side of the brush border membranes. 17 DIGESTION OF DIETARY CARBOHYDRATES Examples of Disaccharidases: • Isomaltase cleaves the α(1→6) bond in isomaltose into Glucose • Maltase cleaves maltose and maltotriose into Glucose • Sucrase cleaves sucrose into Glucose and Fructose • Lactase (β-galactosidase) cleaves lactose into Glucose and Galactose • Trehalase cleaves Trehalose [α(1→1) disaccharide of glucose found in mushrooms and other fungi] 18 Abnormal degradation of disaccharides • Any defect in a specific disaccharidase activity of the intestinal mucosa causes the passage of undigested carbohydrate into the large intestine. • Presence of osmotically active material, water is drawn from the mucosa into the large intestine, causing osmotic diarrhoea. • Also, Bacterial fermentation of the remaining carbohydrate to two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO2 and H2 gas, cause abdominal cramps, diarrhoea, and flatulence. 19 LACTOSE MALABSORPTION • Primary lactose malabsorption: The most common cause is lactase enzyme non-persistence. Less common causes include congenital lactase deficiency. • Lactase non-persistence: Genetically regulated reduction of lactase enzyme activity. • It is determined by racial or ethnic factors and is the underlying mechanism of lactose malabsorption in healthy individuals. Approx. 90% of adults of African or Asian descent are lactase-deficient. • The majority of the world's populations develop low intestinal lactase levels during mid-childhood (approximately at age three to five years). This characteristic is most frequent in Asian and African populations. • In contrast, the majority of Caucasians, particularly of northern European background, maintain elevated lactase activity into adulthood. 20 Lactose intolerance: • The age-dependent loss of lactase activity represents a reduction in the amount of enzyme rather than a modified inactive enzyme. • Treatment: Reduce consumption of milk while eating yogurts and cheeses, as well as green vegetables such as broccoli, to ensure adequate calcium intake; to use lactase-treated products; or to take lactase in pill form prior to eating. Diagnosis: Identification of a specific enzyme deficiency can be obtained by performing oral tolerance tests with the individual disaccharides. Measurement of hydrogen gas in the breath is a reliable test for determining the amount of ingested carbohydrate not absorbed by the body, but which is metabolized instead by the intestinal flora. Sucrase-isomaltase complex deficiency: • This deficiency results in an intolerance of ingested sucrose. • The disorder is found in about 10% of the Inuit people of Greenland and Canada, whereas 2% of North Americans are heterozygous for the deficiency. • Treatment includes the dietary restriction of sucrose, and enzyme replacement therapy. 23 Table 14–4 Disaccharides of Physiologic Importance Sugar Sucrose Composition D-Glucopyranosyl-(1-2) D-Fructofuranoside Source Clinical Significance Cane and beet sugar, sorghum Sucrose intolerance (rare, and some fruits and vegetables lack of sucrase)—diarrhea and flatulence Lactose D-Galactopyranosyl-(1-4) D-Glucopyranose Milk (and many pharmaceutical Lack of lactase (alactasia) preparations as a filler) leads to lactose intolerance—diarrhea and flatulence Maltose D-Glucopyranosyl-(1-4) D-Glucopyranose Enzymic hydrolysis of starch (amylase); germinating cereals and malt Isomaltose D-Glucopyranosyl-(1-6) D-Glucopyranose Enzymic hydrolysis of starch (the branch points in amylopectin) Lactulose D-Galactopyranosyl-(1-4) D-Fructofuranose Heated milk (small amounts), mainly synthetic Not hydrolyzed by intestinal enzymes, used Thank You
