4. Carbohydrates Learning Objectives
What Are the Structures and the Stereochemistry of Monosaccharides?
How Do Monosaccharides React?
What are Disaccharides ?
What Are Some Important Oligosaccharides?
What Are the Structures and Functions of Polysaccharides?
What Are Glycoproteins?

5. Carbohydrates
Carbohydrate: a polyhydroxyaldehyde or polyhydroxyketone, or a substance that contain C:H:O and H:O ratio is 2:1 (as water)
~ose designate sugars.
Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate
they have the general formula CnH2nOn, where n varies from 3 to 8
Monosaccharide maybe
aldose: a monosaccharide containing an aldehyde group
ketose: a monosaccharide containing a ketone group

6. Monosaccharides
Monosaccharides are classified by their number of carbon atoms

7. Fischer Projections
Fischer projection: a two dimensional representation for showing the configuration of tetrahedral
horizontal lines represent bonds projecting forward
vertical lines represent bonds projecting to the rear

8. Monosaccharides There are only two trioses
aldo- and keto- are often 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 D
ihydroxyacetone
(a ketotriose)
Glyceraldehyde
(an aldotriose) C H O 2 =

9. D- Aldoses

10. D-Ketoses

11. Hexoses of physiologic importance.
Sugar
Source
Importance
Clinical
Significance
D-Glucose
Fruit juices. Hydrolysis of starch, cane sugar, maltose, and lactose.
The sugar of the body. carried by blood the principal one used by the tissues
Present in the urine (glucosuria)
in diabetes mellitus owing to
raised blood glucose (hyperglycemia)
D-Fructose
Fruit juices. Honey. Hydrolysis of cane sugar and of inulin.
Can be changed to glucose in the liver
Hereditary fructose intolerance
leads to fructose accumulation
and hypoglycemia.

12. Hexoses of physiologic importance.
Sugar
Source
Importance
Clinical
Significance
D-Galactose
Hydrolysis of lactose.
Can be changed to glucose in the liver
and metabolized. Synthesized in the
mammary gland to make the lactose of
milk. A constituent of glycolipids and
glycoproteins.
Failure to metabolize leads to galactosemia and cataract.
D-Mannose
Hydrolysis of plant mannans and gums.
A constituent of many glycoproteins.

13. Sucralose ( Splenda )
Discovered in 1976, sucralose is 600 times sweeter than sugar and does not metabolize to produce energy, thus it does not contain calories. It is the only low calorie sweetener that is made from sugar, which has been changed so passes through the body unchanged and unmetabolized. Substituting for three alcohol groups on the sugar
molecule with 3 chlorine atoms.

14. Stevia
Stevia is a sweetener and sugar substitute extracted from the leaves of the plant species Stevia rebaudiana which is grown in Brazil and Paraguay.
Stevia has no calories, and it is 200 times sweeter than sugar in the same concentration.

15. Monosaccharides
Glyceraldehyde contains a stereocenter (chiral C) and exists as a pair of stereoisomers (enantiomers)
Note: Number of stereoisomers= 2n
n is the number of stereocenters (chiral carbon)

16. D, L Monosaccharides According to the conventions proposed by Fischer
D-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon (the one before the last= the highest numbered chiral carbon) on the right
L-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon (the one before the last= the highest numbered chiral carbon) on the left

17. The four aldotetroses
Enantiomers: stereoisomers that are mirror images .
example: D-erythrose and L-erythrose are enantiomers
Diastereomers: stereoisomers that are not mirror images .
Epimers: diastereomers that differ from each other in configuration of one chiral carbon
example: D-erythrose and D-threose are diastereomers

18. D, L Monosaccharides Following are the two most common
D-aldotetroses and the two most common
D-aldopentoses

19. C H O D-Glucosamine D-Glucose D-Galactose N D-Fructose
The three most common D-aldohexoses. Note that the third of these is an amino sugar.
also shown is the most common 2-keto-D-hexose C H O 2
D-Glucosamine
D-Glucose
D-Galactose N
D-Fructose

20. Cyclic Structure
Monosaccharides especially pentoses and hexoses have -OH and C=O groups in the same molecule and exist almost entirely as five- and six-membered cyclic hemiacetals or hemiketal
Hemiacetal : bond between C=O on C1 (aldehyde) and
-OH on last chiral C
Hemiketal: bond between C=O on C2 (ketone) and -OH on last chiral C
Cyclic strusture will display extra chiral carbon (anomeric carbon)
anomeric carbon: the new stereocenter resulting from cyclic hemiacetal or hemiketal formation
anomers: monosaccharides that differ in configuration only at their anomeric carbons

21. Haworth Projections Haworth projections
five- and six-membered hemiacetals are represented as planar pentagons or hexagons
most commonly written with the anomeric carbon on the right and the hemiacetal oxygen to the back right
the designation -means that -OH on the anomeric carbon is cis to the terminal -CH2OH (above the plane); - means that it is trans to the terminal -CH2OH (below the plane)
The OH groups on the right in Fischer below in Haworth. Those to the left in Fischer upward in Haworth.
Counting is clockwise starting from anomeric carbon

22. Haworth Projections

23. Haworth Projections
a six-membered hemiacetal ring is shown by the infix -pyran-
a five-membered hemiacetal ring is shown by the infix -furan-
Pyranose: a six-membered ring sugar
Furanose: a five-membered ring sugar

24. Haworth Projections
Five-membered rings are so close to being planar that Haworth projections are adequate to represent furanoses.

25. Comparison of the Fischer and Haworth Representations

26. Comparison of the Fischer and Haworth Representations

27. Oxidation Reducing sugar: one that reduces an oxidizing agent
Reducing sugar: has free aldehyde or ketone group
oxidation of a cyclic hemiacetal form gives a lactone
when the oxidizing agent is Tollens’ reagent (used to detect reducing sugars), Ag precipitates as a silver mirror

28. Ascorbic Acid (Vitamin C)
L-Ascorbic acid (vitamin C) is synthesized both biochemically and industrially from oxidation of D-glucose

29. Ascorbic Acid (Vitamin C)
L-Ascorbic acid is very easily oxidized to L-dehydroascorbic acid
both are physiologically active and are found in most body fluids

30. Reduction
The carbonyl group of a monosaccharide can be reduced to a hydroxyl group by a variety of reducing agents
reduction of the CHO group of a monosaccharide gives a polyhydroxy compound called an alditol
Commercial sweetener in sugarless gum and candy

31. Reduction Reactions

32. Formation of deoxy sugars
B-L-Fucose in ABO blood group antigens, polypeptide chain
D-2-deoxyribose in DNA

33. Phosphoric Esters
Phosphoric esters are particularly important in the metabolism of sugars
phosphoric esters are frequently formed by transfer of a phosphate group from ATP H O C 2 b
-D-glucose - P A d e n o s i +
ATP 3
-D-glucose-6-phosphate D
enzyme

34. Formation of Glycosides
Glycoside: a carbohydrate in which the -OH of the anomeric carbon is replaced by -OR
those derived from furanoses are furanosides; those derived from pyranoses are pyranosides
O-glycosidic bond: the bond in the glycoside, between the anomeric carbon and the -OR group
(methyl alcohol)

35. Amino Sugars
The bond involved is N-glycosidic bond

36. Glycosidic linkages
Glycosidic linkages can take various forms; the anomeric carbon of one sugar to any of the -OH groups of another sugar to forma an - or -glycosidic linkage

37. Disaccharides Sucrose
table sugar; obtained from the juice of sugar cane and sugar beet
one unit of D-glucose and one unit of D-fructose joined by an a-1,2-glycosidic bond
nonreducing sugar: no reducing ends

38. Disaccharides Lactose
about 5% - 8% in human milk, 4% - 5% in cow’s milk
one unit of D-galactose and one unit of D-glucose joined by a b-1,4-glycosidic bond
reducing sugar: has reducing ends
Lactose intolerance

39. Disaccharides Maltose
two units of D-glucose joined by an a-1,4-glycosidic bond
Resulted from hydrolysis of starch
reducing sugar: has reducing ends

40. Disaccharides Cellobiose
Two units of β-glucose linked by β-(1-4) glycosidic bond

41. Reducing disaccharides

42. Polysaccharides
Cellulose: the major structural component of plants, especially wood and plant fibers
a linear polymer of approximately 2800 D-glucose units per molecule joined by b-1,4-glycosidic bonds
fully extended conformation with alternating 180° flips of glucose units
extensive intra- and intermolecular hydrogen bonding between chains

43. Starch
A polymers of a-D-glucose units and used for energy storage in plants, 2 forms
amylose: continuous, helical unbranched chains of up to 4000 a-D-glucose units joined by -1,4-glycosidic bonds
amylopectin: a highly branched helical polymer consisting of units of D-glucose joined by
_1,4-glycosidic bonds and branches created by
-1,6-glycosidic bonds
amylases catalyze hydrolysis of -1,4-glycosidic bonds
debranching enzymes catalyze the hydrolysis of

44. Starch

45. Polysaccharides Branching in amylopectin and glycogen
The difference between amylopectin and glycogen is no. of branches
Glycogen every 10
Amylopectin every 25

46. Polysaccharides
Chitin: the major structural component of the exoskeletons of invertebrates, such as insects and crustaceans; also occurs in cell walls of algae, fungi, and yeasts
composed of units of N-acetyl--D-glucosamine joined by -1,4-glycosidic bonds

48. Polysaccharides
Bacterial cell walls: prokaryotic cell walls are constructed on the framework of the repeating unit NAM-NAG joined by b-1,4-glycosidic bonds

49. Bacterial Cell Walls The N-acetyl-D-glucoseamine and
N-acetylmuramic acid polysaccharide is in turn cross-linked by small peptides
in Staphylococcus aureus, the cross link is a tetrapeptide
this tetrapeptide is unusual in that it contains two amino acids of the D-series, namely D-Ala and D-Gln
each tetrapeptide is cross linked to an adjacent tetrapeptide by a pentapeptide of five glycine units
Peptidoglycan: is the resulted cross linking of polysaccharides by peptides

50. Bacterial Cell Walls

51. Bacterial Cell Walls The peptidoglycan of a bacterial cell wall
Staphylococcus
aureus

52. Polysaccharides
Glycosaminoglycans: polysaccharides based on a repeating disaccharide where one of the monomers is an amino sugar and the other has a negative charge due to acid such as a sulfate or carboxylate group
heparin: natural anticoagulant
hyaluronic acid: a component of the vitreous humor of the eye and the lubricating fluid of joints
chondroitin sulfate and keratan sulfate: components of connective tissue

53. The repeating unit of heparin

54. The repeating unit of hyaluronic acid

55. Chondroitin sulfate
The repeating unit of chondroitin sulfate

56. Glycoproteins antibodies are glycoproteins
Glycoproteins contain carbohydrate units covalently bonded to a polypeptide chain
antibodies are glycoproteins
carbohydrates play a role as antigenic determinants, the portions of the antigenic molecule that antibodies recognize and to which they bond.

57. Blood Group Substances
Membranes of animal plasma cells have large numbers of relatively small carbohydrates bound to them:
these membrane-bound carbohydrates act as antigenic determinants
among the first antigenic determinants discovered were the blood group substances
in the ABO system, individuals are classified according to four blood types: A, B, AB, and O
at the cellular level, the biochemical basis for this classification is a group of relatively small membrane-bound carbohydrates

58. ABO Blood Classification

59. ABO Blood Classification
in type A, the nonreducing end is NAGal
in type B it is Gal
in type AB, both types are present
in Type O, neither of these terminal residues is present

60. End
Chapter 16