Carbohydrates

1. Carbohydrates Dr. Mamoun Ahram Nursing Summer semester, 2016

2. What are they? • Carbohydrates are polyhydroxy aldehydes or ketones • The term “carbohydrate” comes from the fact that when you heat sugars, you get carbon and water • Saccharide is another name for a carbohydrate

3. Classifications • Monosaccharides contain a single unit • Glucose, fructose • Disaccharides consist of two monosaccharide units • Sucrose • Oligosaccharides contain from 3 to 10 units • Raffinose • Polysaccharides contain very long chains of hundreds or thousands of monosaccharide units • Cellulose, glycogen, starch

4. Monosaccharides • Monosaccharides are the simplest carbohydrates • They contain two or more hydroxyl groups • Their basic chemical formula is (CH2O)n • They either contain aldehyde or ketone groups • They are called aldoses or ketoses

5. Numbering carbons • Carbons in a monosaccharide are numbered such that the aldehyde group is carbon number one or the ketone group is carbon number two.

6. Drawing carbohydrates • Fischer projections are a convenient way to represent mirror images in two dimensions

7. Another classification • They can be classified by the number of carbon atoms they contain. • The smallest monosaccharides contain three carbons (n = 3). • They are referred to as trioses • Four, five, six, and seven carbon atoms are called tetroses, pentoses, hexoses, and heptoses, respectively.

8. Trioses • There are two trioses: dihydroxyacetone and glyceraldehyde. • Dihydroxyacetone is a ketose, whereas glyceraldehyde is an aldose.

9. Glyceraldehyde • Glyceraldehyde has a single asymmetric carbon. • An asymmetric carbon is one attached to four different chemical groups and is also known as a chiral carbon • A chiral molecule, like glyceraldehydes, is one that cannot be superimposed on its mirror image.

11. Isomerism

12. Stereoisomers • The two forms of glyceraldehydes are known as stereoisomers. • Stereoisomers are molecules that have the same molecular formula and atomic connectivity, but have a different arrangement of the atoms in space.

13. Number of possible stereoisomers • How many stereoisomers can a monosaccharide molecule have? • The number of possible stereoisomers is 2n where n is the number of chiral carbons in a sugar molecule.

14. Isomers of glucose

15. Isomerism

16. Enantiomers • When two stereoisomers are mirror images of each other, they are known as enantiomers.

17. Designation • They are designated with the prefix d- and l- indicating the position of the –OH group in relation to the carbon being right or left (“L” means left and “D” means right) • So, the two isomers are d-glyceraldehyde and l-glyceraldehyde. • The majority of sugars in our bodies are d- sugars.

18. Naming D- or L • When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group: • If the hydroxy group points to right when the carbonyl is “up” it is the D-isomer, and when the hydroxy group points to the left, it is the L-isomer.

19. Dextrose • The form most commonly found in living organisms, D-glucose, medically known as dextrose, has only one mirror image.

20. Cyclic sugars • A monosaccharide that has 5 or 6 C atoms can form very stable ring structures. • In the cell 99% of pentoses and hexoses are in the ring form. • The ring forms when the aldehyde or ketone can react with one of the hydroxyl groups.

21. Formation of cyclic glucose

22. Cyclic fructose • Similarly, fructose mainly forms a furanose when oxygen of the keto carbon (C2) reacts with the C-5 hydroxyl group • The α- and β-anomers refer to the hydroxyl groups attached to C-2.

23. Ring structures • Two ring structures can form: pyranose and furanose rings depending on the carbons involved in the bond formation

24. Anomers • For an aldohexose such as glucose, two ring pyransoe structures can be formed: -d-glucopyranose and -d-glucopyranose • C1 is known as the anomeric carbon and the hydroxyl group attached to it can be either below the ring or above it

25. Anomers • The designation α means that the hydroxyl group attached to C-1 is below the ring; β means that it is above the ring. • The α and β forms are called anomers. • When in solution, these two forms are in equilibrium where they can inter-convert by mutarotation.

26. Other aldohexoses

27. Ribose: an aldopentose

28. Reducing Sugars have free anomeric carbon • Reducing sugars become oxidized forming an acid.

29. Oxidation of monosaccharides • Aldose → aldonic acid

30. Ketoses can also be oxidized • a ketone undergoes rearrangement into an intermediate known as an enediol (ene for the double bond and diol for the two hydroxyl groups). • Theenediol rearranges to give an aldose, which is susceptible to oxidation.

31. Reduction of sugars(Sugar alcohols) • The carbonyl group of aldehydes of ketones are converted to an alcohol group. • Examples include sorbitol, mannitol, and xylitol, which are used to sweeten food products.

32. Glycosides and glycosidic bonds • When a cyclic monosaccharide reacts with an alcohol: • A glycoside is produced (acetal) • The bond is a glycosidic bond (a or b)

33. Stable ring structure • Once the glycoside is formed, the ring can no longer open up to the open-chain form.

34. Common derivatives • Oxidation of aldehyde of aldose • Aldonic acids • Reduction of carbonyl of aldose or ketose • Alditols • Condensation reactions between anomeric –OH and alcohols to form acetals or ketals: • Glycosides

35. Amino sugars • A hydroxyl group is replaced by an amino group such glucosamine and galactosamine, whose carbon 2 is modified. • Amino sugars are present in many polysaccharides and cell membranes. • Amino sugars are often further modified by acetylation (addition of acetyl group) at the amino groups.

36. Sugar esters • An excellent example is phosphate esters where a phosphate group is added at carbon 6 of aldohexoses and ketohexoses. • Phosphate esters of monosaccharides are important in DNA formation and in generation of energy in cells.

37. Deoxy sugars • One or more hydroxyl groups are replaced by hydrogens. • An example is 2-deoxy-2-ribose, which is a constituent of DNA.

38. Disaccharides and oligosaccharides • Because sugars contain many reactive hydroxyl groups, monosaccharides can join one to another via glycosidic bonds forming oligosaccharides.

39. Glycosidic bonds • The glycosidic bondsare formed by elimination of water between the anomeric hydroxyl of a cyclic monosaccharide and the hydroxyl group of the other compound. • Oligosaccharidesdo not undergo mutarotation, so they remain locked in the same configuration.

40. Disaccharide • A disaccharide consists of two sugars joined by a glycosidic bond • They can be of the same sugar, thus termed homodisaccharides • If they are made of two different sugars, they are known as heterodisaccharides

41. Distinctions of disaccharides • The 2 specific sugar monomers involved and their stereoconfigurations (D- or L-) • The carbons involved in the linkage (C-1, C-2, C-4, or C-6) • The order of the two monomer units, if different (example: galactose followed by glucose) • The anomeric configuration of the OH group on carbon 1 of each residue (α or β)

42. Linkages ( and )

43. Abundant disaccharides • Sucrose (table sugar): is made of a glucose unit and a fructose unit • Lactose (milk sugar): is made from a galactose joined to glucose • Maltose (malt sugar): is made from two glucose molecules

44. Maltose

45. Lactose

46. Lactose intolerance • It is a condition caused by defective lactase, which digests lactose. • Because lactose remains in the intestines, it draws in • excess water. • Bacteria also ferment the lactose to produce carbon dioxide and methane. • Symptoms: bloating, cramps, flatulence, and diarrhea. • Treatment: a lactose-free diet, artificial sweeteners lactose-low milk products.

47. Sucrose

48. Reducing vs. nonreducing sugars • Note that the anomeric carbons of the second sugar of maltose and lactose are free. • Therefore, this carbon can participate in reduction reactions and, hence, known as a reducing carbon and the sugar is known as a reducing sugar

49. Sucrose is a non-reducing sugar • On the other hand, the anomeric carbons of both residues of sucrose are involved in the glycosidic bond and, hence, non-reducing.

50. Polysaccharides • Polysaccharides, also called glycans, consist of more than 10 monosaccharides and their derivatives. • If a polysaccharide contains only one kind of monosaccharide molecule, it is a homopolysaccharide, or homoglycan, whereas those containing more than one kind of monosaccharide are heteropolysaccharides. • Polysaccharides differ not only in the nature of their component monosaccharides but also in the length of their chains and in the amount of chain branching that occurs.