Ch. 22: Carbohydrates

1. Ch. 22: Carbohydrates

2. Introduction to Carbohydrates • Carbohydrates: a large class of naturally-occurring, polyhydroxy aldehydes and ketones. • Monosaccharides (simple sugars) are the simplest carbohydrates,containing 3-7 carbon atoms. • aldose: sugar containing an aldehyde group • the CHO group is always at the end of the C chain • there is a CH2OH group at the other end of the chain • there are OH groups on all the C atoms in-between • ketose: sugar containing a ketone group • the ketone group is on the second carbon of the chain • there is a CH2OH group at the other end of the chain • there are OH groups on all the C atoms in-between

3. Carbohydrates Cont. • The family name ending “-ose” indicates a carbohydrate. • Simple sugars are known by common names such as glucose, ribose, fructose, etc.

4. Monosaccharides • classified according to: • (1) The number of carbon atoms present in the molecule and • (2) whether they contain an aldehyde or ketone group

5. Mono-, Di-, & Polysaccharides • Monosaccharides are carbohydrates that cannot be hydrolyzed to simpler carbohydrates • Monosaccharides react with each other to form disaccharides and polysaccharides. • Monosaccharides are chiral molecules and exist mainly in cyclic forms rather than the straight chain. • Disaccharide: a carbohydrate composed of 2 monosaccharides • Polysaccharide (complex carbohydrate): a carbohydrate that is a polymer of monosaccharides can be hydrolyzed to two monosaccharides

6. Handedness in Carbohydrates • Carbohydrates are chiral molecules since they have carbon atoms carrying four different groups. • The simplest, three-carbon, naturally-occurring carbohydrate glyceraldehyde lacks a plane of symmetry and exists as a pair of enantiomers – a right-handed D form (dextro from Latin “right) or a left-handed L form (levro from Latin “left”_.

8. Handedness Cont. • The two forms of glyceraldehyde (D and L) have the same physical properties except the way they affect polarized light. • The two forms of glyceraldehyde rotate the plane of a polarized light in the opposite direction by the same amount. • An instrument known as polarimeter can be used to measure the degree of rotation of the plane of a polarized light.

9. A solution of an optically active (chiral) isomer rotates the plane of the polarized light by a characteristic amount. • Each enantiomer of a pair rotates the plane of light by the same amount, but the directions of rotation are opposite. • There is no correlation between the D and L form and direction of rotation of a plane of polarized light. The direction of rotation cannot be predicted for an enantiomer.

10. What About if There is More Than 1 Chiral C Atom? • A monosaccharide whose highest numbered chiral C atom has the same configuration as D-glyceraldehyde is a D sugar • A monosaccharide whose highest numbered chiral C atom has the same configuration as L-glyceraldehyde is an L sugar

11. Multiple Chiral C Atoms in a Molecule • Glyceraldehyde has one chiral carbon atom and can exist as 2 enantiomers (non-superimposable mirror images.) • In general, compounds with “n” chiral carbon atoms have a maximum of 2n possible steroisomers and half that many pair of enantiomers. • Glucose, an aldohexose, has four chiral carbon atoms and a total of 24 = 16 possible stereoisomers (8 pairs of enantiomers).

12. Another Example • Aldotetroses has 2 chiral C atoms, so 4 stereoisomers are possible. • Two pairs of enantiomers (non-superimposable mirror images).

13. Drawing Sugar Molecules • Fisher Projection:a way of representing three-dimensional structures of stereoisomers on a flat page. • A chiral carbon atom is represented in the Fisher projection as the intersection of two crossed lines. • Bond that points up out of the page are shown as horizontal lines. • Bonds that point behind the page are shown as vertical lines. • For sugars, the aldehyde or ketone is at the top.

16. D and L Sugars • In D form, the –OH group on the last chiral C atom comes out of the plane of paper and points to the right. • In L form, the –OH group on the last chiral C atom comes out of the plane of paper and points to the left. • There is no correlation between the D and L and direction of rotation of a plane of polarized light. The D and L relate directly only to the position of –OH group on the bottom carbon in a Fisher projection. • A Fischer projection cannot be removed from the plane of the paper or turned 90o and still represent the molecule accurately. • If given one enantiomer, to draw the other enantiomer you simply reverse the substituents on the left side and right side of each chiral carbon atom. • Most amino acids are “L”. • Most carbohydrates are “D”.

17. Structure of Glucose & Other Monosaccharides • D-Glucose, sometimes called dextrose or blood sugar, is the most widely occurring of all monosaccharides. • In nearly all living organisms, D-glucose serves as a source of energy for all biochemical reactions. • D-glucose is stored in polymeric form as starch in plants and as glycogen in animals.

18. Structure Cont. • Although they can be written with the carbon atoms in a straight chain, monosaccharides with five or six carbon atoms exist mainly in cyclic forms. • Recall… • hemiacetal: a compound with both an alcohol (OH) group and an ether (ROR) group bonded to the same C atom. • Hemiacetals can reversible form from the reaction of an aldehyde or ketone with an alcohol • Since the monosaccharides have both alcohol OH groups and aldehyde or ketone groups on the same molecule, it can react with itself and form a ring. (An intramolecular reaction)

19. Aldopentoses

20. Haworth Projections • The flat cyclic representation of a carbohydrate is called the Haworth Projection. • The cyclic form differs only at C1, where the –OH group is either on the opposite side (a) from the CH2OH or on the same side (B).

22. Drawing Haworth Projections From Fischer Projections • See figure 22.3. • Lay the Fischer projection on is side with C1 to the right and the other end looped around to the back. • Rotate the CH2OH group into a vertical position. (this will put the OH group on C5 near C1) • Connect the oxygen from the OH group on C5 to C1. • Place the hemiacetal OH group on C1.

24. Some Terms • anomers: Cyclic sugars that differ only in the position of the substituents at the hemiacetal carbon • The a-anomer has the C1 hydroxyl on the opposite side to the -CH2OH group • The b-anomer has the C1 hydroxyl on the same side to the -CH2OH group • The a vs. B form can make a huge difference (such as why we can digest starch but not cellulose) • anomeric carbon atom: the hemiacetal C atom in a cyclic sugar (the C atom bonded to an OH group and an O in the ring)

25. Mutarotation • A change in rotation of plane-polarized light resulting from the equilibrium betweeen cyclic anomers and the open-chain form of a sugar. • Takes place when a pure sugar anomer is dissolved

26. Mutarotation Cont. • The a- and b-forms of glucose can be isolated separately • Pure a-glucose has a specific rotation of +112o • Pure b-glucose has a specific rotation of +18.7o • When either form of glucose is allowed to stand in aqueous solution, the specific rotation of the solution slowly changes to +52.7o • It does not matter whether one starts with pure a- or b-glucose • a-D-glucose <--> open-chain D-glucose <--> B-D-glucose • Mutarotation of glucose results in an equilibrium mixture of 36% a-glucose and 64% b-glucose • The more stable b-glucose form predominates

27. The structure of D-galactose: The molecule can exist as an open chain hydroxy aldehyde or as a pair of cyclic hemiacetals.

28. Some Important Monosaccharides • Monosaccharides are generally: • high-melting • white • crystalline solids • soluble in water and insoluble in nonpolar solvents. sweet tasting • digestible • nontoxic

29. Glucose • important for human metabolism • the final product of carbohydrate digestion • provides acetyl groups for entry into the citric acid cycle as acetyl-SCoA

30. Galactose • in plant gums and pectins • a component of the disaccharide lactose (milk sugar) • genetic disorders known as galactosemias result from an inherited deficiency of any of several enzymes needed to metabolize galactose • treated by maintaining a galactose-free diet • otherwise: liver failure, mental retardation, and cataracts

31. Fructose • Levulose or “fruit sugar” • occurs in honey and many fruits • one of the two monosaccharides combined in sucrose • sweeter than sucrose • an ingredient in many sweetened beverages and prepared foods.

32. Ribose and 2-Deoxyribose • ribose is part of RNA (ribonucleic acid) • deoxyribose is part of DNA (deoxyribonucleic acid)

33. Reactions of Monosaccharides • 1. redox reactions • 2. formation of phosphate esters • 3. formation of glycosides and disaccharides

34. 1. Reducing Sugars: Reactions with Oxidizing Agents • Recall: aldehydes can be oxidized to carboxylic acids. RCHO  RCOOH • An open chain aldose monosaccharide is ultimately oxidized to a carboxylic acid group. • reducing sugar: a carbohydrate that reacts in basic solution with a mild oxidizing agent • In basic solution, all monosaccharides, whether they aldoses or ketoses, are reducing sugars.

35. 2. Formation of Phosphate Esters of Alcohols • The –OH group of sugar can add –PO32- group to form phosphate esters. • Phosphate esters of monosaccharides appear as reactants and products throughout the metabolism of carbohydrates. • see p. 647

36. 3. Reaction with Alcohols: Glycoside & Disaccharide Formation • Recall: hemiacetals react with alcohols to yield an acetal and water. • Monosaccharides are cyclic hemiacetals. They also can react with alcohols to form acetals, which are called glycosides. • In a glycoside, the -OH group on the anomeric C atom is replaced by an –OR group. See p. 646 • glycoside: a cyclic acetal formed by reaction of a monosaccharide with an alcohol, accompanied by loss of H2O. • glycosidic bond: the bond between the anomeric C atom of a monosaccharide and an –OR group

37. Example: Glucose + Methanol

38. Disaccharides • Disaccharides are made up of two monosaccharides. -Sucrose (table sugar) is a disaccharide made up of one glucose and one fructose. • In disaccharides monosaccharides are connected to each other by glycosidic bonds. • Disaccharides contain a glycosidic link between the hemiacetal hydroxyl group at C1 of one sugar and the hydroxyl group of another sugar. • If the link is between the hemiacetal hydroxyl group at C1 of one sugar and the hydroxyl group at C4 of another sugar, we call that a 1,4 link • If the link is between the hemiacetal hydroxyl group at C1 of one sugar and the hydroxyl group at C2 of another sugar, we call that a 1,2 link

39. Hydrolysis • The reverse of glycosidic reaction that happens during digestion of all carbohydrates. • Breaks di- or polysaccharides down into monosaccharides. (See p. 646)

40. 3 Most Common, Naturally-Occurring Disaccharides • Maltose: Two a-glucose are joined by an a-1,4-link. • Lactose, also known as milk sugar: The major carbohydrate found in mammalian milk. Two b-monosaccharides are joined by an b-1,4-link. • Sucrose, table sugar: Sugar beets and sugarcane are the most common sources of sucrose. One molecule of D-fructose and one molecule of D-glucose joined together by a 1,2-link between the anomeric carbons.

41. Disaccharides 3 most common: sucrose, maltose, and lactose a link is between C1 and C4 B link is between C1 and C4 link is between C1 of glucose and C2 of fructose

42. Maltose • see p. 648 • malt sugar • present in fermenting grains and by enzyme-catalyzed degradation of starch • used in prepared food as a sweetener. • 2 glucose molecules joined together • a reducing sugar

43. Lactose • see p. 648 • milk sugar • disaccharide composed of galactose and glucose • a reducing sugar • inability to digest lactose leads to lactose intolerance

44. Sucrose • see p. 649 • table sugar • most common sources are sugar beets and sugarcane • disaccharide of glucose and furctose • not a reducing sugar

45. Variations on the Carbohydrate Theme • Monosaccharides with modified functional groups are components of a wide variety of biomolecules. • Short chains of monosaccharides, known as oligosaccharides, enhance the function of proteins and lipids to which they are bonded.

46. Examples • chitin: the shells of lobster, beetles, and spiders are made of chitin. • heparin: an agent that prevents or retards the clotting of blood (anticoagulant). • glycoproteins: proteings that conatin a short carbohydrate (oligosaccharide) chain. • perform important functions at the cell surface. They can function as receptor for molecular messengers or drugs. They are also responsible for the familiar A, B, O system of typing blood.

48. Some Important Polysaccharides • Polysaccharides: polymers of many monosaccharides linked together through glycosidic bonds. • Three of the most important polysaccharidies are cellulose, starch, and glycogen. • Cellulose monosaccharides are linked by B-1,4 links. Starch and glycogen monosaccharides are linked by a-1,4 links.

49. Cellulose • a fibrous substance that provides structure in plants. • consists entirely of several thousand b-D-glucose units joined together in a long straight chain by b-1,4-links. • Humans lack enzymes to cleave the b linkages in cellulose and so cannot use cellulose as a source of glucose

50. Cellulose • In cellulose, glucose units are joined by b-glycosidic linkages • Cellulose chains are relatively straight • The linear chains of cellulose hydrogen bond with each other to give the rigid, insoluble fibers found in plant cell walls • The resulting sheets then stack on top of each other