Carbohydrates: Fun Facts, Structures, and Reactions

1. Chapter 22 Carbohydrates

2. Carbohydrates • Fun Facts: • Photosynthesis converts more than 100 billion metric tons of CO2 and H20 into carbohydrates annually. • Non-photosynthetic cells can make there own glucose from amino acids, fats and other breakdown products.

3. Carbohydrates • Fun Facts 2 • Mole Ratios 1C, 2H, 1O • Empirical Formula = CH2O • monosaccharides have from 3 to 8 carbons • aldose: linear sugar with an aldehyde group • ketose: linear sugar with a ketone group

4. Carbohydrates • Fun Facts 3 • Three classes of carbohydrates • Monosaccharides • 3 to 8 carbons with carbonyl and alcohol FG • Disaccharides • 2 monosaccharides connected with a ketal or acetal connection • Polysaccharides • Multiple ketal or acetal connections

5. Monosaccharides • Monosaccharides are classified by their number of carbon atoms

6. Monosaccharides • And they differ by the type of carbonyl present • Aldehyde • Ketone

7. Monosaccharides • There are only two trioses • often aldo- and keto- are omitted and these compounds are referred to simply as trioses

8. Monosaccharides • Glyceraldehyde, the simplest aldose, contains a stereocenter and exists as a pair of enantiomers

9. Monosaccharides • 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

10. D,L Monosaccharides • Emil Fischer decided on of D- and L- assignments for the enantiomers of glyceraldehyde • D-monosaccharide: the -OH is on the right • L-monosaccharide: the -OH is on the left

11. D,L Monosaccharides • the most common D-tetroses and D-pentoses

12. D,L Monosaccharides • the three common D-hexoses

13. Amino Sugars • Amino sugars contain an -NH2 group in place of an -OH group

14. Cyclic Structure • Aldehydes and ketones react with alcohols to form hemiacetals • cyclic hemiacetals form readily as five- or six-membered ring

15. Haworth Projections • D-Glucose forms these cyclic hemiacetals

16. Haworth Projections • a five- or six-membered cyclic hemiacetal is represented as a planar ring • groups lie either above or below the plane • the new carbon stereocenter is called an anomeric carbon • stereoisomers 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

17. Haworth Projections • Terminology of carbohydrate chemistry, • b means that the anomeric -OH is on the same side of the ring as the terminal -CH2OH • a means that the anomeric -OH is on the side of the ring opposite the terminal -CH2OH • a six-membered hemiacetal ring is called a pyranose, and a five-membered hemiacetal ring is called a furanose

18. Haworth Projections • aldopentoses also form cyclic hemiacetals • the most prevalent forms of D-ribose and other pentoses in the biological world are furanoses

19. Haworth Projections • D-fructose also forms a five-membered cyclic hemiacetal

20. Mutarotation • Mutarotation: the equilibrium interconversion of a- and b-anomers in aqueous solution

21. Chair Conformations Pg 475 • Lets leave this out. I will be very happy if you can draw Fisher and Hayworth forms.

22. Physical Properties • Monosaccharides are colorless crystalline solids, very soluble in water • sweetness relative to sucrose:

23. Chemical Properties • Monosaccharides • Hemiacetal into acetal – glycosidic bond • A glycosidic bond slows mutarotation to snails pace. • Acid is needed to break acetal or ketal • Aldose’s reduce Cu2+, Fe3+, and cold MnO4- • Only works with the linear aldehyde form • Hemiacetals are in equilibrium with aldehyde form • Called reducing sugars • Glycosides cannot reduce these • Carbonyl can be reduced

24. Formation of Glycosides • Treatment of a monosaccharide with an alcohol gives an acetal

25. Glycosides • a cyclic acetal derived of a monosaccharide is called a glycoside • the bond from the anomeric carbon to the -OR group is called a glycosidic bond • mutarotation is VERY SLOW in a glycoside • glycosides are stable in water and aqueous base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide

26. Oxidation to Aldonic Acids • the aldehyde group of an aldose is oxidized under basic conditions to a carboxylate anion • the oxidation product is called an aldonic acid • reducing sugar (it reduces the oxidizing agent)

27. Oxidation to Uronic Acids • 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

28. Reduction to Alditols • The carbonyl group can be reduced to a hydroxyl group by NaBH4 and H2/Pd • the reduction product is called an alditol

29. 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 • these three alditols are also common in the biological world

30. D-Glucuronic Acid • D-glucuronic acid exists in the plants and animals • in humans, it is an important component of the acidic polysaccharides of connective tissues • it is used to detoxify foreign phenols and alcohols; in the liver, these compounds are converted to glycosides of glucuronic acid and excreted in the urine

31. Phosphate Esters • Mono- and diphosphoric esters are intermediates in metabolism of monosaccharides • the first step in glycolysis is conversion of D-glucose to a-D-glucose 6-phosphate

32. Disaccharides • Sucrose • most abundant disaccharide • sucrose is a nonreducing sugar (why)

33. Disaccharides • Lactose • lactose is the principal sugar present in milk • it consists of D-galactopyranose bonded by a b-1,4-glycosidic bond to carbon 4 of D-glucopyranose • lactose is a reducing sugar (why)

34. Disaccharides • Maltose • present in malt • two D-glucopyranose joined by an a-1,4-glycosidic bond • maltose is a reducing sugar (Why)

35. Polysaccharides • Polysaccharide: lots of monosaccharide units • Also called glycans • Can be a or b linked anomers • One we can digest “a” • The other we cannot “b”

36. Polysaccharides - a • Starch: an energy storage polymer of D-glucose found in plants • starch can be separated into amylose and amylopectin • amylose is D-glucose units joined by a-1,4-bonds • Amylopectin - D-glucose units joined by a-1,4 bonds; at branch points, new chains every 24 to 30 units are started by a-1,6-glycosidic bonds

37. Polysaccharides - a • Glycogen is the energy-reserve carbohydrate for animals • glycogen - glucose units joined by a-1,4- and a-1,6-glycosidic bonds (branches occur every 8 to 12 residues - more compact than starch) • 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

38. Polysaccharides - a • Why Store sugar as starch or glycogen? • Osmolarity • Individual sugars would be 0.4 M • Polymers (mostly insoluable) 10-8 M • Cells would burst with water running into the to equilibrate osmotic pressure!

39. Polysaccharides - b • Cellulose is a linear polysaccharide of D-glucose units joined by b-1,4-glycosidic bonds • it has an average molecular weight of 400,000 g/mol, approximately 2200 glucose units • cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups hydrogen bond with each other rather than water. • 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

40. Polysaccharides - b • Cellulose (cont’d) • animals cannot digest cellulose • no contain b-glucosidases, enzymes that catalyze hydrolysis of b-glucosidic bonds • we have only a-glucosidases; hence we can digest starch and glycogen • many bacteria and microorganisms have b-glucosidases and can digest cellulose • termites have such bacteria in their intestines and can use wood as their principal food

41. Acidic Polysaccharides • Acidic polysaccharides: contain carboxyl groups and/or sulfuric ester groups • play important roles in the structure and function of connective tissues • there are a large number of highly specialized forms of connective tissue • 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

42. Acidic Polysaccharides • Hyaluronic acid • Found in embryonic tissues, synovial fluid, lubricant of joints in the body, and the vitreous of the eye

43. Acidic Polysaccharides • Heparin:a heterogeneous mixture of variably sulfonated polysaccharide chains, ranging in molecular weight from 6,000 to 30,000 g/mol

44. 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

45. Carbohydrates End Chapter 19