Carbohydrates: Polysaccharides - PDF

POLYSACCHARIDES Most of the carbohydrates found in nature occur in the form of high molecular weight polymers called polysaccharides. When polysaccharides are composed of a single monosaccharide building block, they are termed homopolysaccharides. Polysaccharides composed of more than one type of mo...

POLYSACCHARIDES Most of the carbohydrates found in nature occur in the form of high molecular weight polymers called polysaccharides. When polysaccharides are composed of a single monosaccharide building block, they are termed homopolysaccharides. Polysaccharides composed of more than one type of monosaccharide are termed heteropolysaccharides.  Homopolysaccharides Examples of homopolysaccharies are polymers of glucose (which are called glucans) e.g. starch, dextrins, glycogen, cellulose. 1. Starch Starch (a polymer of glucose) is the storage form of carbohydrate in plants. It consists of two types of molecules: the unbranchedamylose and the branched amylopectin. Both forms of starch are polymers of α-D-Glucose units, linked by α-glucosidic linkages. The linkage is α-1,4glucosidic linkage in amylose and amylopectin.. Branches of about 30 α-D-glucose units are attached with α- 1,6glucosidic linkage at the branching point. Amylose Amylopectin 2. Glycogen Glycogen is the major form of storage form of carbohydratein animals. It is a homopolymer of glucose inα-1,4glucosidic linkage; it is also highly branched, with α-(1,6)branch linkages occurring every 8-10 residues. 3. Cellulose Cellulose is present in the plant fibers and cell wall of plant cells. It is a linear polymer of glucose units linked by β-1,4glucosidic linkages. Although cellulose is indigestible (cellulose enzyme is absent in humans and amylase enzyme cannot hydrolyze the β-1,4glucosidic linkage), its presence in diet is important. This is because cellulose increases the bulk of food which thus stimulates intestinal contractions and prevents constipation.  Heteropolysaccharides These are mixed polysaccharides formedof more than one type of simple sugar units. The most important members of this group are the glycosaminoglycans (GAGs). Glycosaminoglycans (GAGs) GAGs are long, unbranched, heteropolysaccharide chains, generally composed of a repeating disaccharide unit made up of: Most of the GAGs are covalently linked to a core protein, forming a proteoglycan monomer (formed of 95% carbohydrate and 5% protein). Proteoglycans are important constituents of extracellular matrix. Members of GAGs 1. Hyaluronic acid - The only GAG that is not linked to a protein core. - The only GAG that is non-sulfated. - Found in synovial fluid of joints and vitreous humor of the eye. - Composition:D-glucuronic acid + N-acetyl-D-glucosamine. 2. Chondroitin sulfate - Most prevalent GAG in the body being widely spread in cartilage, tendons, and ligaments. - Composition:D-glucuronic acid + N-acetyl-D- galactosamine. 3. Dermatan sulfate: - Found in skin. - Composition:L-iduronic acid + N-acetyl-D- galactosamine. 4. Keratan sulfate: - Found in the cornea of the eye and plays a role in maintaining its transparency. - Composition:D-galactose + N-acetyl-D-glucosamine-sulfate. 5. Heparin - Natural anticoagulant. - Composition: L-iduronic acid, or D-glucuronic acid-sulfate +D-glucosamine- bisulfate. 6. Heparan sulfate - Associated with the plasma membrane of cells and play an important role in cell membrane receptors and in cell-cell interactions. -Composition: similar to heparin but with more D-glucuronicacidand less sulfate residues.