1. 19 19-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March

2. 19 19-2 © 2003 Thomson Learning, Inc. All rights reserved Chapter 19 Carbohydrates Carbohydrates

3. 19 19-3 © 2003 Thomson Learning, Inc. All rights reserved Carbohydrates Carbohydrate: Carbohydrate: a polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis Monosaccharide: Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate C n H 2n O n nmonosaccharides have the general formula C n H 2n O n, where n varies from 3 to 8 aldose:aldose: a monosaccharide containing an aldehyde group ketose:ketose: a monosaccharide containing a ketone group

4. 19 19-4 © 2003 Thomson Learning, Inc. All rights reserved Monosaccharides Monosaccharides are classified by their number of carbon atoms

5. 19 19-5 © 2003 Thomson Learning, Inc. All rights reserved Monosaccharides There are only two trioses often aldo- and keto- are omitted and these compounds are referred to simply as trioses although this designation does not tell the nature of the carbonyl group, it at least tells the number of carbons

6. 19 19-6 © 2003 Thomson Learning, Inc. All rights reserved Monosaccharides Glyceraldehyde, the simplest aldose, contains a stereocenter and exists as a pair of enantiomers

7. 19 19-7 © 2003 Thomson Learning, Inc. All rights reserved Monosaccharides Fischer projection: Fischer projection: a two dimensional representation for showing the configuration of tetrahedral stereocenters horizontal lines represent bonds projecting forward vertical lines represent bonds projecting to the rear

8. 19 19-8 © 2003 Thomson Learning, Inc. All rights reserved D,L Monosaccharides In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde D-monosaccharide:D-monosaccharide: the -OH on its penultimate carbon is on the right L-monosaccharide:L-monosaccharide: the -OH on its penultimate carbon is on the left

9. 19 19-9 © 2003 Thomson Learning, Inc. All rights reserved D,L Monosaccharides the most common D-tetroses and D-pentoses the three common D-hexoses

10. 19 19-10 © 2003 Thomson Learning, Inc. All rights reserved Amino Sugars Amino sugars contain an -NH 2 group in place of an -OH group only three amino sugars are common in nature: D- glucosamine, D-mannosamine, and D-galactosamine

11. 19 19-11 © 2003 Thomson Learning, Inc. All rights reserved Cyclic Structure hemiacetals Aldehydes and ketones react with alcohols to form hemiacetals cyclic hemiacetals form readily when the hydroxyl and carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring

12. 19 19-12 © 2003 Thomson Learning, Inc. All rights reserved Haworth Projections D-Glucose forms these cyclic hemiacetals

13. 19 19-13 © 2003 Thomson Learning, Inc. All rights reserved Haworth Projections a five- or six-membered cyclic hemiacetal is represented as a planar ring, lying roughly perpendicular to the plane of the paper groups bonded to the carbons of the ring then lie either above or below the plane of the ring anomeric carbonthe new carbon stereocenter created in forming the cyclic structure is called an anomeric carbon anomersstereoisomers that differ in configuration only at the anomeric carbon are called anomers the anomeric carbon of an aldose is C-1; that of the most common ketoses is C-2

14. 19 19-14 © 2003 Thomson Learning, Inc. All rights reserved Haworth Projections In the terminology of carbohydrate chemistry,   means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH 2 OH   means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH 2 OH pyranose furanosea six-membered hemiacetal ring is called a pyranose, and a five-membered hemiacetal ring is called a furanose

15. 19 19-15 © 2003 Thomson Learning, Inc. All rights reserved Haworth Projections aldopentoses also form cyclic hemiacetals the most prevalent forms of D-ribose and other pentoses in the biological world are furanoses

16. 19 19-16 © 2003 Thomson Learning, Inc. All rights reserved Haworth Projections D-fructose also forms a five-membered cyclic hemiacetal

17. 19 19-17 © 2003 Thomson Learning, Inc. All rights reserved Chair Conformations chair conformation For pyranoses, the six-membered ring is more accurately represented as a chair conformation

18. 19 19-18 © 2003 Thomson Learning, Inc. All rights reserved Chair Conformations in both a Haworth projection and a chair conformation, the orientations of groups on carbons 1- 5 of  -D- glucopyranose are up, down, up, down, and up

19. 19 19-19 © 2003 Thomson Learning, Inc. All rights reserved Mutarotation Mutarotation: Mutarotation: the change in specific rotation that accompanies the equilibration of  - and  - anomers in aqueous solution example: when either  -D-glucose or  -D-glucose is dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms

20. 19 19-20 © 2003 Thomson Learning, Inc. All rights reserved Physical Properties Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol sweetness relative to sucrose:

21. 19 19-21 © 2003 Thomson Learning, Inc. All rights reserved Formation of Glycosides Treatment of a monosaccharide, all of which exist almost exclusively in a cyclic hemiacetal form, with an alcohol gives an acetal

22. 19 19-22 © 2003 Thomson Learning, Inc. All rights reserved Formation of Glycosides glycosidea cyclic acetal derived from a monosaccharide is called a glycoside glycosidic bondthe bond from the anomeric carbon to the -OR group is called a glycosidic bond mutarotation is not possible in a glycoside because an acetal, unlike a hemiacetal, is not in equilibrium with the open-chain carbonyl-containing compound glycosides are stable in water and aqueous base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide e-ideglycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate in which the ending -e is replaced by -ide

23. 19 19-23 © 2003 Thomson Learning, Inc. All rights reserved Reduction to Alditols The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH 4 and H 2 in the presence of a transition metal catalyst alditolthe reduction product is called an alditol

24. 19 19-24 © 2003 Thomson Learning, Inc. All rights reserved Reduction to Alditols sorbitol is found in the plant world in many berries and in cherries, plums, pears, apples, seaweed, and algae it is about 60 percent as sweet as sucrose (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics these three alditols are also common in the biological world

25. 19 19-25 © 2003 Thomson Learning, Inc. All rights reserved Oxidation to Aldonic Acids the aldehyde group of an aldose is oxidized under basic conditions to a carboxylate anion aldonic acidthe oxidation product is called an aldonic acid reducing sugarany carbohydrate that reacts with an oxidizing agent to form an aldonic acid is classified as a reducing sugar (it reduces the oxidizing agent)

26. 19 19-26 © 2003 Thomson Learning, Inc. All rights reserved Oxidation to Uronic Acids uronic acid Enzyme-catalyzed oxidation of the primary alcohol at C-6 of a hexose yields a uronic acid enzyme-catalyzed oxidation of D-glucose, for example, yields D-glucuronic acid

27. 19 19-27 © 2003 Thomson Learning, Inc. All rights reserved D-Glucuronic Acid D-glucuronic acid is widely distributed in the plant and animal world in humans, it is an important component of the acidic polysaccharides of connective tissues it is used by the body to detoxify foreign phenols and alcohols; in the liver, these compounds are converted to glycosides of glucuronic acid and excreted in the urine

28. 19 19-28 © 2003 Thomson Learning, Inc. All rights reserved Phosphate Esters Mono- and diphosphoric esters are intermediates in the metabolism of monosaccharides for example, the first step in glycolysis is conversion of D-glucose to  -D-glucose 6-phosphate note that at the pH of cellular and intercellular fluids, both acidic protons of a phosphoric ester are ionized, giving it a charge of -2

29. 19 19-29 © 2003 Thomson Learning, Inc. All rights reserved Disaccharides Sucrose (table sugar) sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets sucrose is a nonreducing sugar

30. 19 19-30 © 2003 Thomson Learning, Inc. All rights reserved Disaccharides Lactose lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk it consists of D-galactopyranose bonded by a  -1,4- glycosidic bond to carbon 4 of D-glucopyranose lactose is a reducing sugar

31. 19 19-31 © 2003 Thomson Learning, Inc. All rights reserved Disaccharides Maltose present in malt, the juice from sprouted barley and other cereal grains maltose consists of two units of D-glucopyranose joined by an  -1,4-glycosidic bond maltose is a reducing sugar

32. 19 19-32 © 2003 Thomson Learning, Inc. All rights reserved Polysaccharides Polysaccharide: Polysaccharide: a carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds Starch: Starch: a polymer of D-glucose starch can be separated into amylose and amylopectin amylose is composed of unbranched chains of up to 4000 D-glucose units joined by  -1,4-glycosidic bonds amylopectin contains chains up to 10,000 D-glucose units also joined by  -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by  - 1,6-glycosidic bonds

33. 19 19-33 © 2003 Thomson Learning, Inc. All rights reserved Polysaccharides Glycogen Glycogen is the energy-reserve carbohydrate for animals glycogen is a branched polysaccharide of approximately 10 6 glucose units joined by  -1,4- and  - 1,6-glycosidic bonds the total amount of glycogen in the body of a well- nourished adult human is about 350 g, divided almost equally between liver and muscle

34. 19 19-34 © 2003 Thomson Learning, Inc. All rights reserved Polysaccharides Cellulose Cellulose is a linear polysaccharide of D-glucose units joined by  -1,4-glycosidic bonds it has an average molecular weight of 400,000 g/mol, corresponding to approximately 2200 glucose units per molecule cellulose molecules act like stiff rods and align themselves side by side into well-organized water- insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds this arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength it is also the reason why cellulose is insoluble in water

35. 19 19-35 © 2003 Thomson Learning, Inc. All rights reserved Polysaccharides Cellulose (cont’d) humans and other animals cannot use cellulose as food because our digestive systems do not contain  - glucosidases, enzymes that catalyze hydrolysis of  - glucosidic bonds instead, we have only  -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen many bacteria and microorganisms have  - glucosidases and can digest cellulose termites have such bacteria in their intestines and can use wood as their principal food ruminants (cud-chewing animals) and horses can also digest grasses and hay

36. 19 19-36 © 2003 Thomson Learning, Inc. All rights reserved Acidic Polysaccharides Acidic polysaccharides: Acidic polysaccharides: a group of polysaccharides that contain carboxyl groups and/or sulfuric ester groups, and play important roles in the structure and function of connective tissues there is no single general type of connective tissue rather, there are a large number of highly specialized forms, such as cartilage, bone, synovial fluid, skin, tendons, blood vessels, intervertebral disks, and cornea most connective tissues are made up of collagen, a structural protein, in combination with a variety of acidic polysaccharides

37. 19 19-37 © 2003 Thomson Learning, Inc. All rights reserved Acidic Polysaccharides Hyaluronic acid Hyaluronic acid contains from 300 to 100,000 repeating units it is most abundant in embryonic tissues and in specialized connective tissues such as synovial fluid, the lubricant of joints in the body, and the vitreous of the eye where it provides a clear, elastic gel that maintains the retina in its proper position

38. 19 19-38 © 2003 Thomson Learning, Inc. All rights reserved Acidic Polysaccharides Heparin: a heterogeneous mixture of variably sulfonated polysaccharide chains, ranging in molecular weight from 6,000 to 30,000 g/mol

39. 19 19-39 © 2003 Thomson Learning, Inc. All rights reserved Acidic Polysaccharides Heparin (cont’d) heparin is synthesized and stored in mast cells of various tissues, particularly the liver, lungs, and gut the best known and understood of its biological functions is its anticoagulant activity it binds strongly to antithrombin III, a plasma protein involved in terminating the clotting process

40. 19 19-40 © 2003 Thomson Learning, Inc. All rights reserved End Chapter 19 Carbohydrates