Polysaccharides PDF

Polysaccharides Afira Waqar, PhD Department Of Biosciences, SHU Introduction  Glycans  Most carbohydrates found in nature occur as polysaccharides  polymers of medium to high molecular weight (>20,000)  Polysaccharides differ from each other in the identity of ...

Polysaccharides Afira Waqar, PhD Department Of Biosciences, SHU Introduction  Glycans  Most carbohydrates found in nature occur as polysaccharides  polymers of medium to high molecular weight (>20,000)  Polysaccharides differ from each other in the identity of  recurring monosaccharide units  in the length of their chains  In the types of bonds linking the units  in the degree of branching Types  Homopolysaccharides  Heteropolysaccharides Homopolysaccharides  Homopolysaccharides contain only a single monomeric species  Some homopolysaccharides serve as storage forms of monosaccharides that are used as fuels;  starch  glycogen  Other homopolysaccharides serve as structural elements in plant cell walls and animal exoskeletons.  cellulose  chitin Heteropolysaccharides  heteropolysaccharides contain two or more different kinds of monomeric species  provide extracellular support for organisms of all kingdoms Heteropolysaccharides  Extracellular support  the rigid layer of the bacterial cell envelope (the peptidoglycan)  built from two alternating monosaccharide units  In animal tissues, the extracellular space is occupied by several types of heteropolysaccharides  which form a matrix that holds individual cells together  provides cells, tissues, and organs.  protection,  shape  support vary in length  polysaccharides generally do not have defining lengths /molecular weights.  This difference is a consequence of the mechanisms of assembly of the two types of polymer.  For polysaccharide synthesis  there is no template  the program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of the monomeric units,  there is no specific stopping point in the synthetic process  the products thus vary in length. storage polysaccharides  The most important storage polysaccharides  starch in plant cells  glycogen in animal cells. Both polysaccharides occur intracellularly as large clusters or granules.  Starch and glycogen molecules are heavily hydrated, because  they have many exposed hydroxyl groups available to hydrogen-bond with water.  Most plant cells have the ability to form starch and starch storage is especially abundant in tubers  such as potatoes  in seeds Starch  Starch contains two types of glucose polymer, amylose and amylopectin.  Amylose consists of  Long  unbranched chains of D-glucose residues  connected by ( 1→ 4) linkages (as in maltose).  Such chains vary in molecular weight from a few thousand to more than a million.  Amylopectin also has a high molecular weight (up to 200 million)  is highly branched  The glycosidic linkages joining glucose residues in amylopectin chains are 1→ 4  the branch points (occurring every 24 to 30 residues are 1→ 6 linkages. Glycogen  Glycogen is the main storage polysaccharide of animal cells.  Like amylopectin, glycogen is a polymer of ( 1→ 4) linked subunits of glucose  with 1→ 6-linked branches  BUT glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. Glycogen  Glycogen is especially abundant in the liver  where it may constitute as much as 7% of the wet weight  it is also present n skeletal muscle Clusters  In hepatocytes glycogen is found in large granules  clusters of smaller granules composed of single, highly branched glycogen molecules  with an average molecular weight of several million.  Such glycogen granules also contain the enzymes  responsible for the synthesis  degradation of glycogen Glycogen Granules Have Many Tiers of Branched Chains of ?-Glucose Dextrans  Dextrans are bacterial and yeast polysaccharides  made up of 1→ 6-linked poly-D-glucose  all have 1→ 3 branches  some also have 1→ 2 or 1→ 4 branches.  Dental plaque  formed by bacteria growing on the surface of teeth, is rich in dextrans  which are adhesive and allow the bacteria to stick to teeth and to each other. Dextrans  Dextrans also provide a source of glucose for metabolism.  Synthetic dextrans are used in several commercial products  for example, Sephadex  that serve in the fractionation of proteins by size-exclusion chromatography.  The dextrans in these products are chemically cross-linked to form insoluble materials of various sizes. Size exclusion Chromatography Structural Polysaccharides Cellulose  Structural role  a fibrous, tough, water-insoluble substance, is found in the cell walls of plants, particularly in  Stalks  stems  Trunks  all the woody portions of the plant body.  Cellulose constitutes much of the mass of wood  cotton is almost pure cellulose. Cellulose  Like amylose, the cellulose molecule is a homopolysaccharide  Linear  unbranched  consisting of 10,000 to 15,000 D-glucose units.  But there is a very important difference:  in cellulose the glucose residues have the β configuration  very different macroscopic structures and physical properties Chitin  is a linear homopolysaccharide composed  of N-acetylglucosamine residues in (β1→4) linkage.  The only chemical difference from cellulose is  the replacement of the hydroxyl group at C-2 with an acetylated amino group. Chitin  Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates.  Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods—insects, lobsters, and crabs, for example— and is probably  the second most abundant polysaccharide, next to cellulose, in nature;  an estimated 1 billion tons of chitin are produced each year in the biosphere. Heteropolysaccharides glycosaminoglycans  The ECM is composed of an interlocking meshwork of heteropolysaccharides  glycosaminoglycans, are a  family of linear polymers composed of repeating disaccharide units  They are unique to animals and bacteria  not found in plants  One of the two monosaccharides is always either  N-acetylglucosamine  or N-acetylgalactosamine  the other is in most cases auronic acid, usually  D-glucuronic or  L-iduronic acid  Hyaluronan  contains alternating residues of -glucuronic acid and N- D acetylglucosamine  Highly viscous solutions that serve as  lubricants in the synovial fluid of joints  give the vitreous humor of the vertebrate eye its jellylike consistency  Chondroitin sulfate (Greek chondros, “cartilage”) contributes to the tensile strength of cartilage, tendons, ligaments  Keratan sulfates present in cornea, cartilage, bone, and a variety of horny structures formed of dead cells: horn, hair, hoofs, nails, and claws.  Heparan sulfate  a therapeutic agent used to inhibit coagulation Thank You