Chem 150 Unit 9 - Biological Molecules II Carbohydrates

1. Chem 150Unit 9 - Biological Molecules IICarbohydrates • Carbohydrates play many important roles in biological systems. They represent the major form of chemical energy for both plants and animals. In plants they represent the end product of photosynthesis, and therefore connect all living systems to the sun’s sustaining light energy. Our discussion of carbohydrates will also introduce us to biopolymers, of which proteins and nucleic acids also belong. One of these polymers, the structural polysaccharide cellulose, ties more of the earth’s organic carbon than any other molecule.

2. Introduction • Polymers are large molecules that are made by stringing together, like beads on a string, smaller units called monomers. • Poly- is the Greek prefix meaning “many”. • The names of may polymers describe what they are made from • Polyethylene is made by stringing together many ethylene units. • Ethylene (ethene) is the monomer • Polypropylene is made by stringing together many propylene units. • Propylene (propene) is the monomer. • Polysaccharides are made by stringing together many monosaccharides. • Monosaccharides (simple sugars) are the monomers.

3. Introduction • Carbohydrates are placed in to one of three catagories, depending on the number of monosaccharide units, or residues, they contain. • Monosaccharides, contain a single monosaccharide residue. • Oligosaccharides, contain 2 to 10 monosaccharide residues. • These include the disaccharides, which contain 2 monosaccharide residues. • Polysaccharides, which contain more than 10 monosaccharide residues. • These can contain thousands of monosaccharide residues.

4. Monosaccharides • Monosaccharides are polyhydroxy aldehydes or ketones. • Monosaccharides contain 3 to 7 carbon atoms.

5. Monosaccharides • Monosaccharides are classified according to the number of carbons and whether they contain an aldehyde or ketone. • The “-ose” ending is used to designate carbohydrates.

6. Question (Clickers) • The monosaccharide shown below is a member of which type of monosaccharide? • Aldotetrose • Aldopentose • Ketopentose • Aldohexose • Ketohexose

7. Question (Clickers) • The monosaccharide shown below is a member of which type of monosaccharide? • Aldotetrose • Aldopentose • Ketopentose • Aldohexose • Ketohexose

8. Monosaccharides • The physical properties of monosaccharides are heavily influenced by the large number of hydroxy groups that they contain. • There ability to participate in numerous hydrogen bonds gives them high melting points and high solubilities in water. • Each hydroxyl group has two hydrogen bonding acceptor sites and one hydogen bonding donor site. • Each carbonyl group has two acceptor sites.

9. Monosaccharides • Monosaccharides contain chiral carbon atoms. • This is what accounts for the large number of different monosacchides. • For each chiral carbon, n, a monosacharide has 2n stereoisomers. • These will be divided among 2n/2 pairs of enantiomers. • Glucose contains 4 chiral carbons • Glucose has 24 = 16 stereoisomers • These stereoisomers can be grouped into 16/2 = 8 pairs of enantiomers. http://www.preparatorychemistry.com/Bishop_Jmol_carbohydrates.htm

10. Fischer projection: Monosaccharides • Fischer projections are used to distinguish the different stereoisomers. • The letters D and L are used to distinguish between the members of a pair of enantiomers. • The D or L designation is based on the chiral carbon furthest from the carbonyl carbon.

11. Question (Clickers) • How many stereoisomers does the the monosaccharide shown below have? • 5 • 32 • 16 • 8 • 3

12. Question (Clickers) • Is the monosaccharide shown below a D- or and L- isomer? • D-isomer • L-isomer

13. Monosaccharides • Diasteriomers are stereoisomers that are not enantiomers. • Diasteriomers have have different names • Enatiomers have the same name and are distinguished by a D or L. enantiomers diastereomers

14. Monosaccharides • Important monosaccharides. • pentose and hexoses are the most abundant • Pentoses • D-ribose and D-2-deoxyribose are found in DNA, RNA and nucleotides such as FADH2 and NADH

15. Monosaccharides • Important monosaccharides. • pentose and hexoses are the most abundant • Hexoses • D-glucose (dextrose or blood surgar) - major metabolite and strorage form of chemical energy. • D-galactose - combines with glucose to produce lactose (milk sugar) • D-fructose (fruit sugar) - major metabolite and sweetest tasting natural sugar. • fructose is a ketose

16. Monosaccharides • Monosaccharide derivatives • Deoxy sugars. • One or more -OH’s are replaced with -H’s

17. Monosaccharides • Monosaccharide derivatives • Amino sugars. • One or more -OH’s are replaced with -NH2’s • Often these are acetylated to form amides. Arthritis relief??

18. Monosaccharides • Monosaccharide derivatives • Alcohol sugars. • The ketone or aldehyde is reduced to an alcohol are reduced to

19. Monosaccharides • Monosaccharide derivatives • Carboxylic acid sugars. • The ketone, aldehyde, or primary alcohol is oxidized to a carboxylic acid.

20. Question • The monosaccharide D-xylose is shown below. Draw the derivative describe: • D-2-deoxyxylose • xylitol • D-xylonic acids (carbon 1 is oxidized to a carboxylic acid)

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22. Reactions of Monosaccharides • Reduction of the carbonyl • In the lab this can be done with H2 and a platinum catalyst. • In the cell, NADH + H+ is used.

23. Reduction of Aldehydes and Ketones (Unit 8) • The same reaction can also be used to reduce aldehydes and ketones to alcohols:

24. Reduction of Aldehydes and Ketones (Unit 8) • In biochemistry, NADH + H+ is used instead of H2 • The reduction of a ketone containing steroid by the enzyme Hydroxsteroid dehydrogenase.

25. Reactions of Monosaccharides • Oxidation of sugars • Oxidation with Cu+ The Benedict’s test • Sugars that are capable of producing a positve Benedicts test are called reducing sugars.

26. Oxidation of Aldehydes (Unit 8) • Aldehydes can also be oxidized with the copper(II) ion (Cu2+) • This reaction oxidizes aldehydes, but not alcohols. • The Cu2+ ion forms a clear blue solution • The Cu+ that is produced in the reaction forms an orange/red precipitate.

27. Oxidation of Aldehydes (Unit 8) • Aldehydes can also be oxidized with the copper(II) ion (Cu2+) • The reaction is called the Benedict’s reaction, and has been used for years in a clinical setting to test for the presence of glucose in the urine. Cu2+ Cu2+ + Cu+ Cu+

28. Reactions of Monosaccharides • Oxidation of sugars • Even though ketones should not give a positive Benedict’s test, ketoses do. • This is because under the basic conditions of the test, the ketoses can isomerize to form aldoses, which the react.

29. Reactions of Monosaccharides • Reactions with alcohols to form hemiacetals and hemiketals • Since monosaccharides contain both hydroxyl groups along with either aldehyde or ketone groups, they can react to form hemiacetals and hemiketals.

30. Reactions of Alcohols with Aldehydes and Ketones (Unit 7) • The first reaction, which is similar to the reduction of aldehydes and ketones, involves adding an alcohol across the carbonyl to form a hemiacetal (from aldehydes) or a hemiketal (from ketones).

31. Reactions of Alcohols with Aldehydes and Ketones (Unit 7) • Hemiacetal and hemiketal formation is catalyzed by acids.

32. Reactions of Alcohols with Aldehydes and Ketones (Unit 7) • As we will see with the carbohydrates, the carbonyl group and the alchohol that react can come from the same molecule. • This will produce a ring molecule.

33. Cyclic Form of Monosaccharides • Monosaccharides form rings by reacting one of the hydroxyls with the carbonyl to form a hemiacetal or hemiketal: http://www.preparatorychemistry.com/Bishop_Jmol_carbohydrates.htm

34. Cyclic Form of Monosaccharides • Usually these are drawn using a Haworth project: • The OH’s that were on the right in the Fisher projection are placed in the down position on the Haworth projection • The OH’s that were on the left in the Fisher projection are placed in the up position on the Haworth projection • The CH2OH on the number 5 carbon points up for D sugars and down for L sugars.

35. Cyclic Form of Monosaccharides • The hemiacetal or hemiketal carbon that forms in the ring is called the anomeric carbon. • The anomeric carbon is also chiral, which increases the number of chiral carbons by 1 and increases the doubles the number of stereoisomers. • The two forms of the anomeric carbon are designated as α or β. • The β-anomer has the -OH pointing up in the ring form. • The α-anomer has the -OH pointing down in the ring form.

36. Cyclic Form of Monosaccharides • The ring formation is a dynamic equilibrium reaction. • The open form can switch back and forth between the two ring forms.

37. Cyclic Form of Monosaccharides • When naming the ring forms of monosaccharides, the endings -pyranose and -furanose to designate the six-member and five-member rings, respectively.

38. Cyclic Form of Monosaccharides • In general, the -OH on the chiral carbon furthest from the the carbonyl is the one that reacts to from the pyranose or furanose ring. This means that • Aldohexoses will form pyranose rings: • Aldopentoses and ketohexoses will form furanose rings: D-glucose(aldohexose) D-ribose(aldopentose) D-fructose(ketohexose)

39. β-D-sorbofuranose α-D-sorbofuranose Question (Clicker) • Shown to the below is the Fischer projection for sorbose • Is the structure shown • D-sorbose • L-sorbose • Draw and name the α and β ring forms for sorbose

40. α-L-galactopyranose β-L-galactopyranose Question (Clicker) • Shown to the below is the Fischer projection for galactose • Is the structure shown • D-galactose • L-galactose • Draw and name the α and β ring forms for sorbose

41. Oligosaccharides • Monosaccharides are connected to one another to form oligosaccharides and polysaccharides by reacting the anomeric (hemiacetal or hemiketal) hydroxyl group on one sugar in its ring form, with a hydroxyl group from another sugar. • We saw in Unit 8 how this leads to the formation of acetals and ketals. • The bond that forms between the two monosaccharides is called a glycosidic bond.

42. Reactions of Alcohols with Aldehydes and Ketones (Unit 7) • A hemiacetal or hemiketal can react with a second alcohol to form an acetal or ketal. • This is a substitution reaction and produces an water molecule:

43. Oligosaccharides • The disaccharideD-maltose forms when the anomeric carbon on a D-glucopyranose molecule in the α form reacts with the hydroxyl group on the forth carbon of a second D-glucopyranose molecule: • The bond that forms is called an α(1→4) glycosidic bond Maltose is produced from the breakdown of the polysaccharides starch and glycogen

44. Oligosaccharides • Maltose is still able to reduce Cu+ in a Benedict’s test, though it is only 1/2 as reactive. • Like monosaccharides, maltose is considered a reducing sugar. • This is because the one monosaccharide is still able to open to expose an aldehyde. nonreducing end reducing end

45. Oligosaccharides • The disaccharideD-cellobiose forms when the anomeric carbon on a D-glucopyranose molecule in the β form reacts with the hydroxyl group on the forth carbon of a second D-glucopyranose molecule: • The bond that forms is called an β(1→4) glycosidic bond Cellobiose is produced from the breakdown of the polysaccharids cellulose. Unlike the α(1→4) glycosidic bond in maltose, most organisms are unable to cleave the β(1→4) glycosidic bond

46. Oligosaccharides • The disaccharideD-lactose forms when the anomeric carbon on a D-galactopyranose molecule in the β form reacts with the hydroxyl group on the forth carbon of a D-glucopyranose molecule: • The bond that forms is called an β(1→4) glycosidic bond Lactose is milk sugar. By the age of 5, some people become unable to break the β(1→4) glycosidic bond in lactose, resulting in lactose intolerance.

47. Oligosaccharides • The disaccharideD-sucrose forms when the anomeric carbon on a D-glucopyranose molecule in the α form reacts with the hydroxyl group of the anomeric carbon of D-fructofuranose in the β form: • The bond that forms is called an α,β(1↔2) glycosidic bond Sucrose is table sugar. Because both anomeric carbons are involved in forming the glycosidic bond, sucrose is not a reducing sugar.

48. Oligosaccharides • There are also oligosaccharides with 3 or more monosaccharides • The blood group antigens are oligosaccharides that are attached to lipids and proteins found on cell surfaces. A: N-Acetyl-D-galactosamine(as shown) B: D-galactose O: none

49. Polysaccharides • Polysaccharides are polymers of 10 or more monosaccharide units • Homopolysaccharides contain a single type of monosaccharide unit. • Heteropolysaccharides contain more than one typee of monosaccharide unit.

50. Polysaccharides • The polysaccharide cellulose is a structural polymer produced by plants: • It is a linear, unbranched polyer, with D-glucopyranose units connected by β(1→4) glycosidic bonds