Chemistry of Carbohydrates

Chemistry of carbohydrates

1. Carbohydrates are broadly defined as polyhydroxy aldehydes and their derivatives or as substances that yields one of these compounds. Composed of carbon, hydrogen and oxygen. Functional groups present include hydroxyl groups. -ose indicates sugar. CARBOHYDRATES

2. Major source of energy for the cell. Major structural component of plant cell. Immediate energy in the form of GLUCOSE. Reserve or stored energy in the form of GLYCOGEN. FUNCTIONS OF CARBOHYDRATES

3. Carbohydrates are classified according to the number of subunits that make them up 3 types of carbohydrates: Monosaccharide Oligosaccharides polysaccharides CLASSIFICATION OF CARBOHYDRATES

4. Monosaccharide are also called simple sugars or the compounds which possess a free aldehyde or ketone group or two or more hydroxyl groups. They are the simplest sugars and cannot be hydrolyzed further into smaller units. Monosaccharide contain a single carbon chain and are classified on the basis of number of carbon atoms they possess, and as aldoses and ketoses depending upon their groups. MONOSACCHARIDES

5. Oligosaccharides contain 2-10 monosaccharide molecules which are liberated on hydrolysis. Based on the number of monosaccharide units present, the oligosaccharides are further subdivided to disaccharides, trisaccharides . OLIGOSACCHARIDES

6. Polysaccharides are polymers consisting of hundreds or thousands of monosaccharide units. They are also called glycans or complex carbohydrates. They may be either linear or branched in structure. Polysaccharides have high molecular weight and are not sweetish and donot exhibit any of the properties of aldehydes and ketone group. POLYSACCHARIDES

7. Polysaccharides are of two types: HOMOPOLYSACCHARIDES ( homoglycans ) HETROPOLYSACCHARIDES ( hetroglycans )

8. When a polysaccharide is made up of several units of one and the same type of monosaccharide unit, it is called homopolysaccharide . The most common homoglycans are starch, cellulose, dextrins , glycogen, inulin . Some homopolysaccharides serve as storage form of monosaccharide used as fuel while others serve as structural elements in plants. HOMOPOLYSACCHARIDES

9. They contain two or more different types of monosaccharide units or their derivatives. Hetropolysaccharides present in human beings is glycosaminoglycans ( mucopolysaccharides ) eg . Heparin Blood groups polysaccharides Keratan sulfate Dermatan sulfate Hyaluronic acid HETROPOLYSACCHARIDES

10. The compounds possessing identical molecular formula but different structures are referred to as isomers. The phenomenon of existence of isomers is called isomerism. The types of isomerism exhibited by sugars are ketose-aldose isomerism, D and L isomerism, optical isomerism, epimerism and anomerism . ISOMERISM

11. Glucose and fructose are isomers of each other having the same chemical formula but they differ in structural formula with respect to their functional groups. There is a keto group in position two of fructose and an aldehyde group in position one of glucose. This type of isomerism is called as ketose-aldose isomerism. KETOSE-ALDOSE ISOMERISM

12. D and L isomers depends on the orientation of the H and OH groups around the asymmetric carbon atom adjacent to the terminal primary alcohol carbon. Eg . Carbon atom number 5 in glucose determines whether the sugar belongs to D or L isomer. D and L- ISOMERISM

13. When OH group on this carbon atom is on the right, it belongs to D- series, when it is on the left, it belongs to L-series. D and L isomers are mirror images of each other, these two forms are called enatiomers .

14. When two monosaccharide differ from each other in their configuration around a single asymmetric carbon atom, they are referred to epimer of each other. Galactose and mannose are two epimer of glucose. They differ from glucose in the configuration of groups around C-4 and C-2 respectively. EPIMERISM

15. The α and β cyclic forms of D-glucose are known as anomers . They differ from each other in the configuration only around C1 known as anomeric carbon. If the OH group is on above the plane, form is called as β - anomer of glucose. If the OH group is below the plane, form is called as α - anomer of glucose. ANOMERISM

16. Mutarotation is defined as the change in the specific optical rotation by the interconversion of forms of D-glucose to an equillibrium mixture. When crystalline sample of either α and β anomers is dissolved in water, specific rotation changes with time untill an equillibrium value of +52.7 is attained. This change is called mutarotation . MUTAROTATION

17. CHEMICAL PROPERTIES OF MONOSACCHARIDEs

18. On heating a sugar with mineral acids, the sugar loses water and forms furfural derivatives. These may condense with napthol , thymol or resorcinol to produce colored complxes . This is on the basis of the: Molisch test Seliwanoff test ACTION OF STRONG ACID

19. On treatment with dilute alkalies , both aldoses and ketoses are changed to enediols . Enediol is the enol form of sugar because two OH groups are attached to the double bonded carbon. Enediols are good reducing agents and form basis of the benedict’s and fehling’s test. ACTION OF ALKALIES

20. When aldoses oxidises under proper conditions they may form: aldonic acid, saacharic acids, uronic acids. Oxidation of an aldose with hypobromous acid, which acts as an oxidizing agent gives aldonic acid. Oxidation with nitric acid under proper conditions converts both aldehyde and terminal primary alcohol groups to carboxyl groups, forming saccharic acid. OXIDATION (SUGAR ACID FORMATION)

21. Both aldoses and ketoses may be reduced by enzymes or non- enzymatically to the corresponding polyhydroxy alcohols. Manitol , the sugar alcohol derived from mannose, is frequently used medically to reduce cerebral edema. Sorbitol , the sugar alcohol derived from glucose, often accumulates in the lenses of diabetics and produces cataract. REDUCTION TO FORM SUGAR ALCOHOL

22. Osazones are yellow or orange crystalline derivatives of reducing sugars with phenylhydrazine and have a characteristic crystal structure, which can be used for identification and characterization of different sugars closely similar properties. Osazones formed from glucose, mannose and fructose are identical because these are identical in the lower four carbon atoms. OSAZONE FORMATION

23. The osazone crystals of glucose and of the reducing disaccharides, lactose and maltose differ in forms: Glucosazone is needle shaped. 2. Lactosazone is power puff shaped. 3. Maltosazone is sunflower shaped. Non-reducing sugars like the disaccharide sucrose cannot form osazone due to the absence of a free carbonyl group.

24. Glycosides are formed when the hydroxyl group of anomeric carbon of a monosaccharide reacts with OH or NH group of second compound that may or maynot be a carbohydrate. The bond so formed is known as glycosidic or glycosyl bond. The monosaccharides are joined by glycosidic bonds to form disaccharides, oligosaccharides and polysaccharides. GLYCOSIDE FORMATION

25. Zx

26. Disaccharides consist of two monosaccharide units. They are crystalline, water soluble and sweet to taste. They are subclassified on the basis of the presence or absence of free reducing group. Reducing disachharide with free aldehyde or keto group. Eg . Maltose, lactose Non-reducing disaccharides with no free aldehyde or keto group. Eg . sucrose DISACCHARIDES

27. Maltose contains two glucose residues, joined by glycosidic linkage between C-1 of one glucose residue and C-4 of the other, leaving one free anomeric carbon of the second glucose residue, which can act as a reducing agent. Thus, maltose is called as reducing dissacharide . Maltose is produced as an intermediate product in the digestion of starch and glycogen by the action of the enzyme amylase. MALTOSE

28. It consists of two glucose molecules linked by an glycosidic bond. Isomaltose is a dissacharide derived from the digestion of the starch or glycogen. It is hydrolyzed to glucose in the intestinal tract by an enzyme called isomaltase . ISOMALTOSE

29. It is present in milk. Lactose contains one unit of galactose and one unit of glucose that are linked by a glycosidic linkage. The anomeric carbon of the glucose unit is available for the oxidation and thus lactose is a reducing dissacharide . Lactose is hydrolyzed to glucose and galactose by lactase enzyme in human beings. LACTOSE (MILK SUGAR)

30. Sucrose is a dissacharide of glucose and fructose. It is formed by plant not by human beings. Sucrose is an intermediate product of photosynthesis. Commonly used as table salt. Sucrose contains no free anomeric carbon atom. The anomeric carbon of both glucose and fructose are involved in the glycosidic bond. Therefore, called as non-reducing sugar. And hydrolyzed by enzyme sucrase . SUCROSE

31. Polysaccharides consist of repeat units of monosaccharides or their derivatives, held together by glycosidic bonds. Polysaccharides are linear as well as branched polymers. HOMOPOLYSACCHARIDES which on hydrolysis yield only a single type of polysaccharides. HETROPOLYSACCHARIDES on hydrolysis yield a mixture of a few monosaccharides or their derivatives. POLYSACCHARIDES

32. It is the storage form of glucose in plants. Starch is composed of two constituents ie . Amylose and amylopectin . AMYLOSE- is a linear polymer of D-glucose units joined by α (1-4) glycosidic linkages. AMYLOPECTIN- is structurally identical to those of amylose but with side chains joining them by linkages. STARCH

33. Amylopectin is a branched polymer having both linkages. The branched points in amylopectin are created by α (1-6) bonds and occur at an interval of 20-30 units of glucose.

34. Glycogen is the major storage form of carbohydrate (glucose) in animals, found mostly in liver and muscle. Often called as animal starch. The structure of the glycogen is similar to that of amylopectin , except that it is more highly branched having α (1-6) linkages at intervals of about 8-10 glucose units. GLYCOGEN

35. The function of glycogen is to act as a readily available source for energy within muscle itself. Liver glycogen is concerned with storage and maintenance of the blood glucose.

36. Inulin is a polymer of D-fructose linked together by glycosidic linkage. It occurs in the tubers of some plants, Inulin is not hydrolyzed by amylase but is hydrolyzed by inulinase , which is not present in the humans and so it is not utilized as food. INULIN

37. Partial hydrolysis of starch by acids or amylase produces substances known as dextrins . These also occur in honey. All dextrins have few free aldehyde groups and can show mild reducing property. DEXTRINS

38. Cellulose is the chief constituent of cell wall of plants. It is an unbranched polymer of glucose and consists of long straight chains which are linked by β (1-4) glycosidic linkages. Humans lack an enzyme cellulase , that can hydrolyze the glycosidic linkages, cellulose cannot be digested and absorbed and have no food value like starch. CELLULOSE

39. When the polysaccharides are composed of different types of sugars or their derivatives, they are referred to as heteropolysaccharides or hetroglycans . HETEROPOLYSACCHARIDE

40. Mucopolysaccharides are hetroglycans made up of repeating units of sugars 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. MUCOPOLYSACCHARIDES

41. Heparin is an anticoagulant that occurs in blood, lungs, liver, kidney, spleen. Heparin helps in the release of the enzyme lipoprotein lipase which helps in clearing the turbidity of lipemic plasma. Heparin is composed of alternating units of N- sulfo D-glucosamine 6-sulfate and glucuronate 2-sulfate. HEPARIN

42. Hyaluronic acid is an important GAG found in the ground substance of synovial fluid of joints and vitreous humor of eyes. It serves as a lubricant in joints. Hyaluronic acid is composed of alternate units of D- glucuronic acid and N-acetyl D-glucosamine. These two contains disaccharides units held together by β (1-3) glycosidic bonds. HYALURONIC ACID

43. It is a heterogeneous GAG with a variable sulfate content, besides small amounts of mannose, fructose, sialic acid etc. Keratan sulfate essentially consists of D- galactosamine and N- acetylglucosamine 6-sulfate. KERATAN SULFATE

44. The name dermatan sulfate is derived from the fact that this compound mostly occurs in the skin. Also present in blood vessels valves and heart valves. Dermatan sulfate maintains the shape of tissues. DERMATAN SULFATE

45. Glycoproteins are proteins to which oligosaccharides are covalently attached to their peptide chain. Glycoproteins contain much shorter carbohydrate chain than proteoglycans . The distinction between glycoproteins and proteoglycans may be based on the amount of carbohydrate. Glycoproteins contain less than 4% carbohydrate and proteoglycans contains more than 4% carbohydrate. GLYCOPROTEINS

46. Many integral proteins are glycoproteins . Almost all the plasma proteins of humans are glycoproteins except albumin. Glycoproteins serve as lubricant and protective agent, eg . Mucin of gastrointestinal. Glycoproteins also serve as transport molecules, such as transferrin and ceruloplasmin . FUNCTIONS OF GLYCOPROTEINS