Oligosaccharides and polysaccharides

Oligosaccharides and polysaccharides

1. Lecture 4 Disaccharides Polysaccharides

2. Diasaccharides Disaccharides consists of two monosaccharide units (similar or dissimilar) held together by a glycosidic bond They are crystalline, water-soluble and sweet to taste The diasaccharides are of two types Reducing diasacchrides with free aldehyde or keto group e.g . Maltose, lactose Non reducing disaccharides with no free aldehyde or keto group e.g. sucrose, trehalose

3. Maltose Composed of two α -D-glucose units held together by α (1-4) glycosidic bond. Maltose can be hydrolysed by enzyme maltase to liberate two molecules of α -D-glucose

4. Isomaltose The glucose units are held together by α (1-6) glycosidic linkage

5. Cellubiose Is a disaccharide identical in s tructure with maltose except that the former has β (1-4) glycosidic linkage. It is formed during the hydrolysis of cellulose

6. Sucrose ( Cane sugar) mostly produced by sugar cane and sugar beets. It is made up of α -D-glucose and β -D-fructose. The two monosaccharides are held together by glycosidic bond ( α 1- β 2) It is an important source of dietary carbohydrate. It is sweeter than most other common sugars like fructose glucose and maltose.

7. It is employed as a sweetening agent in food industry The intestinal enzyme- sucrase - hydolyses sucrose to glucose and fructose which are absorbed.

8. Lactose It is more commonly known as milk sugar It is composed of β -D- galactose and β -D-glucose( β (1-4) glycosidic bond)

9. Polysaccharides Polysaccharides or simply glycans consists of repeat units of monosaccharides or their derivatives held together by glycosidic bonds They are primarily concerned with two important functions- structural and storage of energy They are linear as well as branched polymers

10. The occurrence of branches in polysaccharide is due to the fact that the glycosidic linkages can be formed at any one of the hydroxyl groups of monosacchraides .

11. H omopolysaccharides On hydrolysis yield only a single type of monosacchraide e. g.Glucans are polymers of glucose Fructosans are polymers of fructose

12. Heteropolysaccharides On hydrolysis yield a mixture of few monosaccharides or their derivatives

13. H omopolysaccharides Starch : It is the carbohydrate reserve of the plants which is the most important dietary source for higher animals High content is found in cereals, roots, tubers, vegetables It is a homopolymer composed of D-glucose units held together by α - glycosidic bonds ( glucans or glucosans )

14. It consists of two poysaccharides components Water soluble amylose (15-20%) Water insoluble amylopectin (80-85%) Chemically amylose is a long unbranched chain with 200-1000 glucose units held by α (1-4) glycosidic linkages Amylopectin is a branched chain with α ( 1-6) glycosidic bonds at the branching points and α (1-4) linkages everywhere else.

15. Starches are hydrolyzed by amylase to liberate dextrins and finally maltose and glucose units Amylase acts specifically on α (1-4) glycosidic bonds Dextrins The breakdown products of starch by the enzyme amylase or dilute acids Starch is sequentially hydrolyzed through different dextrins and finally to maltose and glucose

16. Dextrans They are polymers of glucose produced by micro-organisms. They are used as plasma volume expanders in transfusion Inulin Is a polymer of fructose i.e. fructosan Inulin is not utilized by the body Used for accessing kidney functions through measurement of glomerular filtration rate (GFR)

17. Occurs in a garlic, union Easily soluble in water Glycogen The carbohydrate reserve in animals (animals starch) Present in high concentration in liver, followed by muscle, brain etc. Also found in plants that do not possess chlorophyll (yeast, fungi)

18. Structurally, glycogen is similar to that of amylopectin with more number of branches Glucose is the repeating unit held together by α (1-4) glycosidic bonds and α ( 1-6) glycosidic bonds at branching points Number of glucose units ( upto 25000) vary in glycogen depending on the source from which glycogin is obtained

19. Cellulose Occurs exclusively in plants Predominant constituent of plant cell-wall Totally absent in animal body Composed of β -D-Glucose units linked by β (1-4) glycosidic bonds Cannot be digested by mammals due to lack of enzymes Certain ruminants and herbivores animals contain micro-organisms in the gut which enzymes which can cleave the bond

20. Hydrolysis of cellulose yields a disaccharide cellobiose , followed by β -D-Glucose Cellulose is the major constituent of fiber, the non- digestable carbohydrate The functions of dietary fibres include: Decreasing the absorption of glucose and cholesterol from the intestines besides increasing the bulk of feces.

21. Chitin Composed of N-acetyl D-glucosamine units held together by β (1-4) glycosidic bond It is a structural polysaccharide found in the exoskeleton of some invertebrates, for example, insects, crustaceans