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Polysaccharides - Biochemistry for Msc Students

This note is based on polysaccharides and glycoprotein which is useful for MSc zoology students. All the points including the structure is being added.

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Polysaccharides - Biochemistry for Msc Students

  1. 1. POLYSACCHARIDES Notes prepared by Keven Liam William 209701 1st Msc Zoology St. Albert’s College(Autonomous), Ernakulam1
  2. 2. Introduction • They are repeating units of monosaccharides or their derivatives held together by glycosidic bonds. • They are simply known as glycans. • They can be straight chain of monosaccharides known as linear polysaccharides or it can be branched known as branched polysaccharide. • They are not sweet in taste • They do not form crystals. • They are carbohydrates with high molecular weight. 2
  3. 3. Classification of polysaccharides Polysaccharides Homopolysaccharides Heteropolysaccharides 3
  4. 4. 4
  5. 5. Homopolysaccharides • Polysaccharides that contains the same type of monosaccharide units are known as homopolysaccharides. • Glucans are polymers of glucose and fructosans are polymers of fructose. • Examples : – Glucosan – Starch, Glycogen, cellulose – Fructosan – Insulin – Galactosan - Agar 5
  6. 6. GLUCOSANS/GLUCAN 1. STARCH • It is a storage polysaccharide and the most common polysaccharide seen in plants • It is a homopolymer composed of D – glucose units held by alpha – glycosidic linkage. • Composed of 10 – 30% Amylose and 70 – 90% Amylopectin depending on the source. • They can be hydrolysed into simpler carbohydrates by acids, various enzymes or a combination of the two. The resulting fragments are known as dextrin. 6
  7. 7. a) AMYLOSE • It is a straight long unbranched chain polymer composed of 250 – 300 D – glucose units held by α (1-4) glycosidic linkage. • It is less soluble in water • Gives a dark blue/black colour when iodine solution is added 7
  8. 8. b) AMYLOPECTIN 8
  9. 9. • Amylopectin is a branched chain polymer of D – glucose units. • It is more soluble in water • Gives a reddish brown colour when iodine solution is added. • Branched chain with α (1-6) glycosidic bonds at the branching points and α (1-4) glycosidic bonds in the straight chain. • It contains few thousand glucose units looks like a branched tree (20-30) glucose units per branch. 9
  10. 10. 2. GLYCOGEN • It is also known as animal starch. • It is stored in the muscles and liver. • It is present in plants with no chlorophyll (eg: yeast, fungi) • Structure of glycogen is similar to that of amylopectin with more number of branches. • Glucose is the repeating unit in glycogen joined together with by α (1-4) glycosidic bonds and α (1-6) glycosidic bonds at branching points • Present in cells as granules with high molecular weight. • Complete hydrolysis yields glucose 10
  11. 11. • Glycogen serves as a buffer to maintain the blood glucose level. • The concentration of glycogen is higher in the liver than in muscle, but more glycogen is stored in the skeletal muscle overall because of its much greater mass. 11
  12. 12. 3. CELLULOSE • Polymer of β – D – Glucose linked by β (1-4) linkages. • Complete hydrolysis yields glucose and partial hydrolysis yields cellobiose. • Cellobiose is made up of two molecules of D – glucose linked by β – glucosidic linkages between C1 and C4 of adjacent glucose units. • It is the most abundant of all carbohydrates. • Gives no colour with iodine. • It is tasteless, odourless and insoluble in water and most organic solvents. 12
  13. 13. • Herbivorous animals utilise cellulose with the help of bacteria. • Human beings lack any enzyme that hydrolyzes the β (1-4) bonds and so cannot digest cellulose. It is an important source of “bulk” in the diet and the major component of dietary fiber stimulating peristalsis and elimination of indigestible food residues. 13
  14. 14. 4. CHITIN • It is a linear homopolysaccharide composed of N – acetyl glucosamine in β – linkage. • Only difference from cellulose is the replacement of the hydroxyl group at C – 2 with an acetylated amino group. • It is the principal component of hard exoskeleton of arthopods and present in the cell wall of fungi. • Second most abundant in nature 14
  15. 15. Structure of Chitin 15
  16. 16. DEXTRINS • It is produced by the partial hydrolysis of starch along with maltose and glucose. • They are often referred to as either amylodextrins, erythrodextrins or achrodextrins. • They are used as mucilages (glues) • They are used in infant formulas. • Indigestible dextrin are developed as soluble fiber supplements for food products. • It is also used as thickening agents in food processing. 16
  17. 17. DEXTRANS • They are polymers of D – glucose • They are synthesised by the action of Leuconostoc mesenteroides. • They are exocellular enzyme produced by the organisms which bring about polymerisation of glucose moiety of sucrose molecule DEXTRANS. • They differ from dextrins in structure. • Contains α (1-4), α (1-6) and α (1-3) linkages. • They are used as plasma expanders. • They are also used as molecular sieves to separate proteins and other large molecules 17
  18. 18. FRUCTOSANS 1. INULIN • It is a heterogeneous polymer of D – fructose • It has a low molecular weight than starch. • It has got a linear chain with no branching. • Occurs in the tubers of the Dehlia, in the roots of Jerusalem artichoke, dandelion and in the bulbs of onion and garlic. • β(1-2) linked fructo furanoses • Complete hydrolysis yields fructose • Used for the evaluation of glomeular filtration rate. • Used as a low glycemic index sweetner. 18
  19. 19. GALACTOSANS AGAR • It is a polymer of galactose units. • It is obtained from the cell walls of some species of red algae (Sphaerococcus Euchema) and species of Gelidium. • When agar is dissolved in hot water and cooled, it becomes gelatinous. • Used in microbiology, to make salt bridges and gel plugs for use in electrochemistry. • Used as a laxative, a thickener for soups, jellies, ice cream. • Used as a clarifying agent in brewing and for sizing fabrics. 19
  20. 20. Heteropolysaccharides • They are high molecular weight carbohydrate polymers containing more than one kind of monosaccharide. • Chemically, they are formed mostly of repeated disaccharide units that contains amino sugar and uronic acid. • Some contain amino sugar and monosaccharide units without the presence of uronic acid. • Amino group is generally acetylated. • Carbohydrate content more than 4% - mucoproteins. • Carbohydrate content less than 4% - glycoproteins. 20
  21. 21. Classification of heteropolysaccharides Mucopolysaccharides Acidic Sulphate free Sulphate containing Neutral 21
  22. 22. Acidic Sulphate free mucopolysaccharides • Acidic polysaccharides are polysaccharides that contain carboxyl groups, phosphate groups and/or sulphuric ester groups. • Example for acidic sulphate free mucopolysaccharides are – Hyaluronic acid – Chondroitin 22
  23. 23. HYALURONIC ACID • Composed of N – acetyl glucosamine and D – Glucuronic acid. • On hydrolysis yields equimolecular quantities of D – Glucosamine, D – Glucoronic acid and acetic acid. • Occurrence – synovial fluid, ECM of loose connective tissue. Serves as a lubricant and shock absorber. • Hyaluronidase – an enzyme that catalyses the de polymerisation of hyaluronic acid and by reducing its viscosity facilitates diffusion of materials into tissue spaces. • Clinically the enzyme is used to increase the efficiency of absorption of solutions administered by clysis. 23
  24. 24. Structure of Hyaluronic acid 24
  25. 25. CHONDROITIN • It is another sulphate free acidic mucopolysaccharide. • It is found in cornea and also in cranial cartilages. • Composed of N – acetyl galactosamine and D – Glucoronic acid. 25
  26. 26. Acidic sulphate containing mucopolysaccharides They are mainly of four types: 1) Chondroitin sulphate 2) Keratan sulphate 3) Heparin 4) Heparitin sulphate 26
  27. 27. CHONDROITIN SULPHATE • It is a sulphated glycosaminoglycan composed of a chain of alternating sugars (N –acetylgalactosamine and glucuronic acid). It is usually found attached to protein as part of proteoglycan. • It is a principle mucopolysaccharide in ground substance of mammalian tissues and cartilages. • Four chondroitin sulphate are isolated which are chondroitin sulphate A, B, C and D. • Chondroitin sulphate A  Consists of repeating units of N – acetyl – D – galactosamine and D – Glucuronic acid. N – acetylgalactosamine is esterified with sulphate in position 4 of galactosamine.  It is present in cartilages, bone and cornea. 27
  28. 28. • Chondroitin sulphate B  It is present in skin, cardiac valve and tendon  It has L – iduronic acid in place of glucuronic acid which is found in other chondroitin sulphate  L – iduronic acid is an epimer of D – Glucuronic acid  It consists of repeating units of L – iduronic acid and N – acetyl galactosamine at C4 sulphate moiety is present.  It has weak anticoagulant property  Sometimes it is found in the skin and hence it is known as dermatan sulphate. 28
  29. 29. • Chondroitin sulphate C  It is found in cartilage and tendon.  Structure is similar to chondroitin sulphate A except that sulphate group is present at position 6 of the galactosamine molecule instead of position 4 • Chondroitin Sulphate D  It is isolated from the cartilage of shark  It resembles in structure to chondroitin sulphate C, except that it has second sulphate attached to carbon 2 or 3 of uronic acid. 29
  30. 30. 30
  31. 31. KERATAN SULPHATE • It is a sulphate containing acid mucopolysaccharide • It is found in coastal cartilage, cornea, aorta, nucleous pulposus. • It consists of repeating disaccharide units of N – acetyl – D – glucosamine 6 – sulphate and galactose. • They are no uronic acids in this molecule. 31
  32. 32. 32
  33. 33. HEPARIN • It is an anticoagulant present in liver which is produced by the mast cells present in liver. • It is found in lungs, thymus, spleen, walls of large arteries, skin, blood • It is a polymer of repeating disaccharide unit of D – Glucosamine and either of the two uronic acid (D – Glucuronic acid and L – iduronic acid) • In fully formed heparin molecule 90% or more of uronic acid residues are L – iduronic acid 33
  34. 34. 34
  35. 35. NEUTRAL MUCOPOLYSACCHARIDES • It is found in Pneumococci capsule. • It acts as blood group substances. Four monosaccharides: Galactose, Fucose, Galactosamine (acetylated) and glucosamine (acetylated) are present in all types of blood group substances. • It is also found in egg protein known as ovalbumin. 35
  36. 36. GLYCOPROTEINS • Glycoproteins are proteins that contain oligosaccharide chains or glycans which are covalently attached to polypeptide side chains. • Almost all the plasma proteins of humans with the exception of albumin are glycoproteins. • Glycosylation (enzymic attachment of sugars) is the most frequent post – translational modification of proteins. • Non – enzymic attachment of sugars to proteins can also occur and it is referred to as glycation. 36
  37. 37. Difference between glycoproteins and proteoglycans Features Glycoproteins Proteoglycans Composition Carbohydrates less than protein (1 to 70%) Carbohydrates more than protein (95%) Carbohydrate chain length Smaller (2 – 10 sugar residues) Very long Serial disaccharide repeats No (Very heterogeneous) Yes Branching of carbohydrate chain Yes No 37
  38. 38. • Eight sugars are commonly found in the oligosaccharide chains of glycoproteins which include: – Galactose (Gal) – Glucose (Glc) – Xylose (Xyl) – Mannose (Man) – Fucose (Fuc) – N – acetylglucosamine (GlcNAc) – N – acetylgalactosamine (GalNAc) – N – acetyl neuraminic acid (NeuAc) 38
  39. 39. 39
  40. 40. • The percentage of carbohydrate in glycoproteins is highly variable – Some glycoproteins such as IgG contains low amounts (4%) of carbohydrate by weight, while glycophorin, the human red cell membrance glycoprotein contains 60% of carbohydrate. • The carbohydrate can be distributed fairly evenly along the polypeptide chain or concentrated in defined regions. Functions of oligosaccharide chains of glycoproteins  Stabilisation of protein structure  Prevent degradation of the protein by proteinases  Increase in the polarity and solubility of a protein  Control of protein half life in blood  Important determinant in receptor – ligand binding  It may affect the sites of metastasis selected by cancer cells. 40
  41. 41. Functions of glycoproteins 1. Structural  They are found throughout matrices and act as receptors on cell surfaces that bring other cells and proteins (collagen) together giving strength and support to a matrix  In certain bacteria, a slime layer that surrounds the outermost components of cell walls are made up of glycoproteins of high molecular weight.  In nerve tissue, glycoproteins are abundant in grey matter and appear to be associated with synaptosomes, axons and microsomes. 2. Enzymes  Glycoprotein enzymes are of three types which include oxidoreductases, transferases and hydrolases. 41
  42. 42. 3. Hormones  There are many glycoproteins that functions as hormones such as human chorionic gonadotropin (HCG) which is present in human pregnancy urine, thyroid stimulating hormone (TSH).  Another example is erythroprotein which regulates erythrocyte production 4. Adhesion  Glycoprtoeins serve to adhere cells to cells and cells to substratum. Cell – cell adhesion is the basis for the development of functional tissues in the body.  In different domains of the body, the different glycoproteins act to unite cells, for example nerve cells recognize and bind to one another via the glycoprotein N –CAM (nerve cell adhesion molecule).  N – CAM is also found on muscle cells indicating a role in the formation of myoneural junctions. 42
  43. 43. 5. Reproduction  Glycoproteins found on the surface of spermatozoa appear to increase a sperm cell’s attraction for the egg by altering the electrophoretic mobility of the plasma membrane.  Hen ovalbumin is a glycoprotein found in egg white that serves as a food storage unit for the embryo.  The zona pellucida is an envelope made of glycoprotein that surrounds the egg and prevents polyspermy from occuring after the first sperm cell has penetrated the egg’s plasma membrane. 6. Protection  Human sweat glands secrete glycoproteins which protect the skin from other excretory products that could harm the skin.  Mucins are also found on the outer body surfaces of fish to protect the skin. Not only does mucin serve the function of protection, but it also acts as a lubricant.  Mucins form a highly viscous gel that protects epithelium from chemical, physical and microbial disturbances. Examples of mucin sites are the human digestive tract, urinary tract and respiratory tracts. 43
  44. 44. Types of glycoproteins  Based on the nature of the linkage between their polypeptide chains and their oligosaccharide chains, glycoproteins can be divided into three major classes:  O – linked  N – linked  GPI – anchored 1. N – linked glycans  They are found in the ovalbumin and the immunoglobulins.  Another use of N – linked oligosaccharides is in intracellular targeting in eukaryotic organisms.  Amide nitrogen of aspargine and N – acetylglucosamine (GlcNAcAsn)  Anomeric carbon of NAG – attached to amide nitrogen of an Asn (Aspargine)  It is 5 times more abundant than O – linked 44
  45. 45. 2. O – linked glycans  Hydroxyl side chain of serine or threonine and a sugar such as N – acetylgalactosamine (GalNAc – Ser[Thr])  Anomeric carbon of NAG – attached to O of serine or threonine.  Mucins which are found extensively in salivary secretions, contain many short O – linked glycans.  Increase the viscosity of the fluids in which they are dissolved. 3. GPI – anchored or GPI – linked (Glycosylphosphatidylinositol – anchored)  Carboxyl terminal amino acid of a protein via a phosphoryl – ethanolamine moiety joined to an oligosaccharide (glycan), which in turn is linked via glucosamine to phosphatidylinositol.  The GPI anchor may allow greatly enhanced mobility of a protein in the plasma membrane.  Some GPI anchors may connect with signal transduction pathways.  Some examples include Acetylcholinesterase, Alkaline phosphatase 45
  46. 46. 46