Biochemistry of Carbohydrates

god bless me,,,,its all yours

1. DEPARTMENT OF VETERINARY BIOCHEMISTRY CARBOHYDRATES SUBMITTED BY ;- BHAGRAJ GODARA M.V.Sc 1 st year RAJASTHAN UNIVERSITY OF VETERINARY AND ANIMAL SCIENCES,BIKANER

2. Carbohydrates The most abundant organic molecules in nature Provide a significant fraction of the energy in the diet of most organisms Important source of energy for cells Can act as a storage form of energy Can be structural components of many organisms Can be cell-membrane components mediating intercellular communication Can be cell-surface antigens Can be part of the body’s extracellular ground substance Can be associated with proteins and lipids Part of RNA, DNA, and several coenzymes (NAD + , NADP + , FAD, CoA)

3. CARBOHYDRATES Polyhydroxy aldehydes or ketones or substances that yield these compounds on hydrolysis CHO | H- C - OH | CH 2 OH C H 2 OH | C = O | CH 2 OH Glyceraldehyde Dihydroxyacetone Aldehyde group Keto group Carbohydrate with an aldehyde group: Aldose Carbohydrate with a ketone group : Ketose Empirical formula of many simpler carbohydrates: (CH2O)n (hence the name hydrate of carbon)

4. Monosaccharides Polyhydroxy aldehydes or ketones that can’t easily be further hydrolyzed “Simple sugars” Number of carbons Name Example 3 Trioses Glyceraldehyde 4 Tetroses Erythrose 5 Pentoses Ribose 6 Hexoses Glucose, Fructose 7 Heptoses Sedoheptulose 9 Nonoses Neuraminic acid

5. Oligosaccharides Hydrolyzable polymers of 2-6 monosaccharides Disaccharides composed of 2 monosaccharides Examples: Sucrose, Lactose Polysaccharides Hydrolyzable polymers of > 6 monosaccharides Homopolysaccharides: polymer of a single type of monosaccharides Examples: Glycogen, Cellulose Heteropolysaccharides: polymer of at least 2 types of monosaccharide Example: Glucosaminoglycans

6. Monosaccharides Number of carbons Name Example 3 Trioses Glyceraldehyde 4 Tetroses Erythrose 5 Pentoses Ribose 6 Hexoses Glucose, Fructose 7 Heptoses Sedoheptulose 9 Nonoses Neuraminic acid

7. Common Monosaccharides

8. Aldose sugars

9. Ketose sugars

10. Conformational formulas The structure of Glucose can be represented in three ways: Straight Chain Form Haworth Projection Chair Form

11. D & L Stereoisomers Those monosaccharides that are of physiological significance exist in the D-configuration, where the hydroxyl group is on the right side . The mirror-image, called enantiomers, are in the L-configuration. They are related to glyceraldehyde which exists in two isomers D and L Monosaccharides can exist in either of two configurations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl group. e,g D & L form of glucose is determine by carbon 5.

12. L and D Enantiomers It also called mirror images or Optical Isomers :

13. L and D Enantiomers

14. Epimers Two monosaccharide that differ from each other by position of OH group on one carbon. ( eg. carbon 2 and 4 of glucose). Mannose Glucose Galactose

15. Formation of Hemiacetals and Hemiketals An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule, the bond produced is a glycosidic bond (p. 245).

16. Pyranose and Furanose ring Monosaccharide with 5 or 6 carbon atoms tend to cyclyze in solution. This terminology indicate that the ring structure of monosaccharide is similar to either pyran or furan

17. α - and β anomers Isomers that differ on the position of OH group around anomeric carbon (which was carbonyl carbon) when the ring is formed.

18. Cyclization Formation of the two cyclic forms of D-glucose. Reaction between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the and anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of and anomers is called mutarotation .

19. Glycosidic bonds the Iinkages between the carbon atoms and the status of the anomeric carbon ( α or β ). For instance, lactose-which is formed by a bond between C1 of β -D- galactopyranosyl -(1→4)- D-glucopyranose Lactose: β -D- galactopyranosyl -(1→4)- D-glucopyranose Maltose: α -D-glucopyranosyl-(1→4)- D-glucopyranose Isomaltose: α -D-glucopyranosyl-(1→6)- D-glucopyranose Sucrose: α -D-glucopyranosyl-(1→2)- D- fructofuranose

20. Physiologieally important glycosides 1. Glucovanillin (vanillin-D-glucoside)is a natural substance that imparts vanilla flavour . 2. Cardiac glycosides( steroidal glycosides:) Digoxin and digitoxin contain the aglycone steroid and they stimulate muscle contraction. 3. Streptomycin, an antibiotic used in the Treatment of tuberculosisi s a glycoside. 4. Ouabain inhibits Na+ - K+ ATPase and blocks the active transport of Na+.

21. REACTIONS OF MONOSACCHARIDES Tautomerization or enolization Reducing Properties Oxidation Reduction Dehydration Osazone formation

22. Tautomerization or enolization The process of shifting a hydrogen atom from one carbon atom to another to produce enediols is known as tautomerization. Sugars possessing anomeric carbon atom undergo tautomerization in alkaline solutions. When glucose is kept in alkaline solution for several hours,it undergoes isomerization to form D-fructose and D-mannose. This reaction known as the Lobry de Bruyn -von Ekenstein transformation -results in the formation of a common intermediate-namely enediol -for all the three sugars,

23. Reducing Properties The sugars are classified as reducing or nonreducing. The reducing property is attributed to the free aldehyde or keto group of anomeric carbon. ln the laboratory, many tests are employed to identify the reducing action of sugars. These include Benedict's test, Fehling's test, Barfoed’s tesf etc. The reduction is much more efficient in the alkaline medium than in the acid medium. The enediol forms or sugars reduce cupric ions (Cu2+) of copper sulphate to cuprous ions (Cu+), which form a yellow precipitate of cuprous hydroxide or a red precipitate of cuprous oxide as shown next

24. Oxidation Depending on the oxidizing agent used, the terminal aldehyde (or keto) or the terminal alcohol or both the groups may be oxidized. For instance, consider glucose : 1. Oxidation of aldehyde group (CHO ------> COOH) resultsi n the formation of gluconic acid. 2. Oxidation of terminal alcohol group (CH2OH ------+C OOH) leads to the production of glucuronic acid.

25. Reduction When treated with reducing agents such as sodium amalgam, the aldehyde or keto group of monosaccharide is reduced to corresponding alcohol, as indicated by the general formula : The important monosaccharides and their correspondinga lcoholsa re given below. D-Glucose------- D- Sorbitol D-Galactose ------ D-Dulcitol D-Mannose ------ D-Mannitol D-Fructose --------D-Mannitol + D-Sorbitol D-Ribose ---------- D- Ribitol

26. Dehydration When treated with concentrated sulfuric acid, monosaccharides undergo dehydration with an elimination of 3 water molecules. Thus hexoses give hydroxy methyl furfural while pentoses give furfural on dehydration. These furfurals can condense with phenolic compounds (a-naphthol) to form coloured products. This is the chemical basis of the popular Molisch test. In case of oligo- and polysaccharides they are first hydrolysed to monosaccharide by acid, and this is followed by dehydration.

27. Osazone formation Phenylhydrazine in acetic acid, when boiled with reducing sugars, forms osazones.the first two carbons (C1 and C2) are involved in osazone formation. The sugars that differ in their configuration on these two carbons give the same type of osazones , since the difference is masked by binding with phenylhydrazine Thus glucose, fructose and mannose give the same type (needle-shaped) osazones.Reducing disaccharides also give osazones maltose sunflower-shaped, and lactose powderpuff shaped.

28. DERIVATIVES OF MONOSACCHARIDES Sugar acids Sugar Alcohols Alditols Amino sugars Deoxy sugars L-Ascorbic acids(vitamin C

29. Sugar acids Oxidation of aldehyde or primary alcohol group in monosaccharide results in sugar acids.Gluconic acid is produced from glucose by oxidation of aldehyde (C1 group) whereas glucuronic acid is formed when primary alcohol group (C6) is oxidized. They are produced by reduction of aldoses or ketoses For instance, D-Glucose------- D- Sorbitol D-Galactose ------ D-Dulcitol D-Mannose ------ D-Mannitol D-Fructose --------D-Mannitol + D-Sorbitol D-Ribose ---------- D- Ribitol Sugar alcohols (polyols) :

30. Alditols The monosaccharides, on reduction, yield polyhydroxy alcohols, known as alditols. Ribitol is a constituent of flavin coenzymes; glycerol and myo-inositol are components of lipids. Xylitol is a sweetener used in sugar lesgsums and candies This is a water-soluble vitamin, the structure of which closely resemblesth at of a monosaccharide. L-Ascorbic acid (vitamin C)

31. Amino sugars When one or more hydroxyl groups of the monosaccharides are replaced by amino groups, the products formed are amino sugars e.g. D-glucosamine, D-galactosamine. They are present as constituents of heteropolysaccharides The amino groups of amino sugars are sometimes acetylated e.g. N-acetyl D-glucosamine. N-Acetylneuraminic acid (NANA) is a derivative of N-acetyl mannose and pyruvic acid. It is an important constituent of glycoproteins and glycolipids. The term sialic acid is used to include NANA and its other derivatives. Certain antibiotics contain amino sugars which may be involved in the antibiotic activity e.g. erythromycin

32. Deoxysugars These are the sugars that contain one oxygen less than that present in the parent molecule. The groups -CHOH and -CH2OH become -CH2 and -CH3 due to the absence of oxygen. D-2-Deoxyriboseis the most important deoxy sugar since it is a structural constituent of DNA (in contrast to D-ribose in RNA).

33. Disaccharides Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other. They are crystalline,water soluble and sweet to taste TheDisaccharides are of two types 1. Reducing disaccharides with free aldehyde or keto group e.g. maltose, lactose. 2. Non-reducing disaccharides with no free aldehyde or keto group e.g. sucrose, trehalose

34. Maltose Maltose is composed of two α -D-glucose units held together by α (1-4) glycosidic bond. The free aldehyde group present on C1 of second Glucose answers the reducing reactions ,besides the osazone formations (sunflower-shaped). Maltose can be hydrolysed by dilute acid or the enzyme maltase to liberate two molecules of α -D-glucose. ln isomaltose, the glucose units are held together by α (1 - 6) glycosidic linkage. Cellobiose is another disaccharide, identical in structure with maltose, except that the former has β (1-4) glycosidic linkage. Cellobiose is formed during the hydrolysis of cellulose

35. LACTOSE Lactose is more commonly known as milk sugar since it is the disaccharide found in milk. Lactose is composed of β -D-galactose and β –D-glucose held together by β (1-4) glycosidic bond. The anomeric carbon of C1 glucose is free, hence lactose exhibits reducing properties and Forms osazones (powder-puff or hedgehog shape). Lactose of milk is the most important carbohydrate in the nutrition of young mammals. It is hydrolysed by the intestinal enzyme lactase to glucose and galactose.

36. SUCROSE Sucrose(cane sugar) is the sugar of commerce, mostly produced by sugar cane and sugar beets. Sucrose is made up of α -D-glucose and β - D-fructose. The two monosaccharides Are held together by a glycosidic bond ( α 1- β 2),between C1 of D-glucose and C2 of β -D -fructose. The reducing groups of glucose and fructose are involved in glycosidic bond, hence sucrose is a non-reducing sugar, and it cannot form osazone , It is the major carbohydrate produced in photosynthesis.

37. Polysaccharides Polysaccharide(or simply glycans) consist of repeat units of monosaccharides or their derivatives, held together by glycosidic bonds. They are primarily concerned with two important functions-structural an d storage of energy polymers of medium to high molecular weight. Polysaccharides, also called glycans, differ from each other in the identity of their recurring monosaccharide units, in the length of their chains, in the types of bonds linking the units, and in the degree of branching. Homopolysaccharides contain only a single type of monomer; heteropolysaccharides contain two or more different kinds Some homopolysaccharides serve as storage forms of monosaccharides that are used as fuels(starch and glycogen),Other homopolysaccharides (cellulose and chitin) serve as structural elements in plant cellwalls and animal exoskeletons.

38. Homo- and heteropolysaccharides. Polysaccharides may be composed of one, two, or several different monosaccharides, in straight or branched chains of varying length.

39. STARCH Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals, including man. High content of starch is found in cereals, roots, tubers, vegetables etc. Starch is a homopolymer composed of D-glucose units held by a glycosidic bonds. lt is known as glucosan or glucan. Starch contains two types of glucose polymer, amylose and amylopectin.The former consists of long, unbranched chains of D-glucose residues connected by ( α 1-4) linkages. Such chains vary in molecularweight from a few thousand to more than a million. Amylopectin also has a high molecular weight ( upto 100 million) but unlike amylose is highly branched.The glycosidic linkages joining successive glucose residues in amylopectin chains are ( α 1-4); the branch points (occurring every 24 to 30 residues) are ( α 1-6) linkages.

40. Dextrans Dextrans breakdown product of starch (amylase) are bacterial and yeast polysaccharidesmade up of ( α 1-6)-linked poly-D-glucose; all have ( α 1-3) branches, and some also have ( α 1-2) or ( α 1-4) branches. Dental plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans . Synthetic dextrans are used in several commercial products (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 porosities, admitting macromolecules of various sizes.

41. INULIN Inulin is a polymer of fructose i.e., fructosan . It occurs in dahlia bulbs, garlic, onion etc. lt is a low molecular weight (around 5,000) polysaccharide easily soluble in water. Inulin is not utilized by the body. lt is used for assessing kidney function through measurement of glomerular filtration rate (GFR).

42. Glycogen Glycogen is the main storage polysaccharide of animal cells.referred as animal starch, Like amylopectin, glycogen is a polymer of ( α 1-4)-linked subunits of glucose, with ( α 1 -6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. Glycogen is especially abundant in the liver where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle. In hepatocytes glycogen is found in large granules, which are themselves clusters of smaller granules composed of single, highly branched glycogen molecules with an average molecular weight of several million. Such glycogen granules also contain, in tightly bound form, the enzymes responsible for the synthesis and degradation of glycogen. en iespecially abundant in the liver

43. Cellulose Cellulose is composed of β -D-glucose units linked by β (1- 4) glycosidic bonds, Cellulose cannot be digested by mammals including man-due to lack of the enzyme that cleaves β – glycosidec bonds ( α amylase break α bonds only). Certain ruminants and herbivorous animals contain microorganismis in the gut which produce enzymes that can cleave β - glycosidic bonds. Hydrolysis of cellulose yields a disaccharide cellobiose, followed by β –D-glucose. Cellulose, though not digested, has great importance in human nutrition. lt is a major constituento l fiber , the non- digestable carbohydrate. The functions of dietary fiber include decreasing the absorption of glucose and Cholesterol from the intestine, besides increasing the bulk of feces .

44. Cellulose Structure The structure of cellulose. (a) Two units of a cellulose chain; the D-glucose residues are in (1n4) linkage. The rigid chair structures can rotate relative to one another. (b) Scale drawing of segments of two parallel cellulose chains, showing the conformation of the D-glucose residues and the hydrogen-bond cross-links. In the hexose unit at the lower left, all hydrogen atoms are shown; in the other three hexose units, the hydrogens attached to carbon have been omitted for clarity as they do not participate in hydrogen bonding.

45. Chitin Chitin is composed of N-acetyl D-glucosamine units held together by β (1-4) glycosidic bonds. lt is a structural polysaccharide found in the exoskeleton of some invertebrates e.g. inscts , crustaceans and and is probably the second most abundant polysaccharide, next to cellulose, in nature. A short segment of chitin,a homopolymer of N -acetyl- D -glucosamine units in β (1-4) linkage.

46. Heteropolysaccrides heteropolysaccharides contain two or more different kinds of monosaccharides MUCOPOLYSACCHARIDES are heteroglycans madeup of repeating units of sugar derivatives namely amino sugars and uronic acids. These are more commonly known as glycosaminoglycans (GAG). Acetylated amino groups, besides sulfate and carboxyl groups are generally present in GAG structure. The presence of sulfate and carboxyl groups contributes to acidity of the molecules, making them acid mucopolysaccharides. Some of the mucopolysaccharides are found in combination with proteins to form mucoproteins or mucoids or proteoglycans Mucoproteins(mucoid or proteoglycans) may contain up to 95% carbohydrate and 5% protein.

47. MUCOPOLYSACCARIDES HYALURONIC ACID CHODROTIN 4-SULFATE HEPARIN DERMATIN SULFATE KERATIN SULFATE

48. Glycoconjugates the informational carbohydrate is covalently joined to a protein or a lipid to form a glycoconjugate, which is the biologically active molecule. Specific carbohydratecontaining molecules act in cell-cell recognition and adhesion, cell migration during development, blood clotting, the immune response, and wound healing, to name but a few of their many roles Glycolipids are membrane lipids in which the hydrophilic head groups are oligosaccharides, which, as in glycoproteins, act as specific sites for recognition by carbohydrate- binding proteins

49. Proteoglycans Proteoglycans are macromolecules of the cell surface or extracellular matrix in which one or more glycosaminoglycan chains are joined covalently to a membrane protein or a secreted protein. The glycosaminoglycan moiety commonly forms the greater fraction (by mass) of the proteoglycan molecule, dominates the structure, and is often the main site of biological activity. In many cases the biological activity is the provision of multiple binding sites, rich in opportunities for hydrogen bonding and electrostatic interactions with other proteins of the cell surface or the extracellular matrix. Proteoglycans are major components of connective tissue such as cartilage, in which their many noncovalent interactions with other proteoglycans, proteins, and glycosaminoglycans provide strength and resilience.

50. Glycoproteins Glycoproteins have one or several oligosaccharides of varying complexity joined covalently to a protein. They are found on the outer face of the plasma membrane, in the extracellular matrix, and in the blood. Inside cells they are found in specific organelles such as Golgi complexes, secretory granules, and lysosomes. The oligosaccharide portions of glycoproteins are less monotonous than the glycosaminoglycan chains of proteoglycans; they are rich in information, forming highly specific sites for recognition and high affinity binding by other proteins.

51. Antifreeze glycoproteins The Antarctic fish live below -2*C, a temperature at which the blood would Freeze. lt is now known that these fish contain antifreeze glycogtratein which lower the freezing point of water and interfere with the crystal formation of ice. Antifreeze giycoproteins consist of 50 repeating units of the tripeptide, alanine-alanine-threonine. Each threonine residue is bound to β -galactosyl (1-3) α ( N- acetygl alactosamine

52. The Sugar Code Carbohydrates as Informational Molecules: Monosaccharides can be assembled into an almost limitless variety of oligosaccharides, which differ in the stereochemistry and position of glycosidic bonds, the type and orientation of substituent groups, and the number and type of branches. Oligosaccharides are far more information-dense than nucleic acids or proteins. Lectins , proteins with highly specific carbohydrate-binding domains, are commonlyfound on the outer surface of cells, wherethey initiate interaction with other cells. Invertebrates, oligosaccharide tags “read” by lectinsgovern the rate of degradation of certain peptidehormones , circulating proteins, and blood cells

53. The adhesion of bacterial and viral pathogens to their animal-cell targets occurs through binding of lectins in the pathogens to oligosaccharides in the target cell surface. Lectins are also present inside cells, where they mediate intracellular protein targeting. X-ray crystallography of lectin-sugar complexes shows the detailed complementarity between the two molecules, which accounts for the strength and specificity of their interactions with carbohydrates. Selectins are plasma membrane lectins that bind carbohydrate chains in the extracellular matrix or on the surfaces of other cells, thereby mediating the flow of information between cell and matrix or between cells.