1. Carbohydrates
IUG, Fall 2017
Dr. Tarek Zaida

2. Carbohydrates Most abundant class of biological molecules on Earth
Originally produced through CO2 fixation during photosynthesis

3. Roles of Carbohydrates
Energy storage (glycogen, starch)
Structural components (cellulose, chitin)
Cellular recognition
Carbohydrate derivatives include DNA, RNA, co-factors, glycoproteins, glycolipids

4. Carbohydrates
Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions
Oligosaccharides = "a few" - usually 2 to 10
Polysaccharides are polymers of the simple sugars

5. Monosaccharides Polyhydroxy ketones (ketoses) and aldehydes (aldoses)
Aldoses and ketoses contain aldehyde and ketone functions, respectively
Ketose named for “equivalent aldose” + “ul” inserted
Triose, tetrose, etc. denotes number of carbons
Empirical formula = (CH2O)n

6. Monosaccharides are chiral
Aldoses with 3C or more and ketoses with 4C or more are chiral
The number of chiral carbons present in a ketose is always one less than the number found in the same length aldose
Number of possible stereoisomers = 2n (n = the number of chiral carbons)

7. Monosaccharides
Monosaccharides are classified according to the number of carbon atoms present
(triose, tetrose, pentose, hexose, and so on) and according to whether the carbonyl group is present as an aldehyde (aldose) or as a ketone (ketose).
There are only two trioses:
1. Glyceraldehyde and
2. Dihydroxyacetone
Each has two hydroxyl groups, attached to different carbon atoms, and one carbonyl group.

9. Glyceraldehyde is the simplest aldose Dihydroxyacetone is the simplest ketose.
Each is related to glycerol in that each has a carbonyl group in place of one of the hydroxyl groups.
Other aldoses or ketoses can be derived from glyceraldehyde or dihydroxyacetone by adding carbon atoms, each with a hydroxyl group.
In aldoses, the chain is numbered from the aldehyde carbon. In most ketoses, the carbonyl group is located
at C-2.

11. Name each of the following monosaccharides as an aldose or ketose & according to its number of C atoms.

12. Origin of Carbohydrates
1. Plants catch up CO2 from air & H2O from soil
6CO2 + 6H2O sun light C6H12O O2
The above reaction is called photosynthesis (formation of carbohydrates from CO2, H2O and sun light)
enzymes, chlorophyll
glucose

13. Plants build disaccharides from monosaccharides:
2. The carbohydrates produced during photosynthesis are monosaccharides.
Plants build disaccharides from monosaccharides:
C6H12O6 + C6H12O C12H22O11 + H2O
glucose
glucose
Disaccharide

14. 3. Plants as well as animals can combine many molecules of monosaccharides into large polysaccharide molecules:
nC6 H12O polymerization (C6H10O5)n nH2O
Polysaccharides in plants are:
- Cellulose in stalk and stem
- Starch in roots and seeds
- Mono & disaccharides are found in fruits

15. Oxygen – Carbon dioxide cycle
1. Plants pick up CO2 from air, water from soil to build carbohydrates (monosaccharides).
2. Animals can not do that, so they have to rely on plants to get their carbohydrates
C6H12O6 + 6O CO H2O + energy
The above reaction is the reverse of photo-synthesis.
Krebs cycle

16. During Photosynthesis:
Energy from the sun needed for aerobic reactions.
During catabolism of carbohydrates in animals same amounts of energy is liberated from oxidation of food.
So energy from metabolism in animals comes from the sun through plants that store solar energy in carbohydrates.

17. Stereoisomerism
Stereoisomers:
Are compounds having the same molecular formula but a different structural formula.
Stereoisomers have structures that are mirror image of one another
If you put your left hand on top of your right hand, you will see that they are not superimposable

18. Both methane and mirror image are said to be superimposable (achiral)
Some molecules are superimposable with their mirror images and others are not:
H-C-H H-C-H
Both methane and mirror image are said to be superimposable (achiral) H H H H
methane
Mirror image

19. In general any object with a plane of symmetry is achiral
Any object or molecule does not have a plane of symmetry it is chiral
If there are 4 different atoms attached to a central carbon atom, it is said to be chiral

20. Stereoisomers in Carbohydrates
Carbohydrates are chiral molecules since they have carbon atoms carrying four different groups.
The simplest three carbon sugar is glyceraldehyde.
This sugar exists as a pair of enantiomers.

21. Stereochemistry Enantiomers = mirror images
Pairs of isomers that have opposite configurations at o more than one chiral centers but are NOT mirror images are diastereomers
Epimers = Two sugars that differ in configuration at only one chiral center

22. Two forms of glyceraldehyde (D and L) have the same physical properties except they behave differently in the way they rotate polarized light and the way they are affected by catalysts.

23. Remember: Compounds with n chiral carbon atoms has a maximum of 2n possible stereoisomers and half that many pairs of enantiomers (mirror images).
•This aldotetrosose, has 2 chiral carbon atoms and a total of 22 = 4 possible stereoisomers (2 pairs of enantiomers).

24. Fischer Projections of Sugar Molecules
Example

26. Monosaccharides are divided into two families: D form and L form sugars.
D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right.
L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the left.
The D and L relate directly only to the position of –OH group on the bottom carbon in a Fischer projection.

27. D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right. L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the left.

28. Chiral Drugs
Many enzymes will interact with only one particular enantiomer
In human body, enzymes will react with carbohydrates of D- form, but will react with proteins of L-form.
Some drugs are enantiomers:
1. Ibuprofen used in pain relief
2. Indinavir used in Aids
3. Levofloxacin used as antibiotic

29. some important pentoses and hexoses, and their derivatives
1. D-glucose, also called dextrose or blood sugar , is the most important monosaccharide in human metabolism.

30. 2. D-fructose, or fruit sugar, is most common natural ketose.
Honey contains about 38% fructose

31. In deoxy sugars a hydrogen atom replaces one or more of the -OH groups in a monosaccharide.
D-ribose and its derivative D-2-deoxyribose (deoxy = minus one oxygen atom) are found in various coenzymes and in DNA.

32. The Cyclic Form of Monosaccharides 1. Glucose
– A monosaccharide contains both an alcohol and an aldehyde group.
– It can react with itself to form a cyclic hemiacetal.

36. Glucose is prepared by:
1. hydrolysis of sucrose
2. hydrolysis of starch
Glucose is found in blood stream
Found in tissue fluids
If found in urine, it is an indicator of diabetes (glucosuria)
Glucose may show up in urine during extreme excitement (emotional glucosuria) or after ingestion of a large amount of sugar (alimentary glucosuria)

37. 2. Galactose

38. Glucose & Galactose are epimers.
Epimers: are 2 sugars that differ only in about a single carbon atom.
D-galactose epimerase D-glucose
Galactosuria
Is a severe inherited disease results in the inability of infants to metabolize galactose because of a deficiency in either:
1. Galactose-1-phosphate-uridyl transferase (GALT)
2. Galactokinase (GALK)
As a result the galactose concentration will increase in blood and urine (galactosuria)

39. 3. Fructose

40. Fructose is often called fruit sugar or levulose
Can be prepared by hydrolysis of sucrose
It also can be prepared by hydrolysis of Inulin
Fructose is 1.7 times sweeter than sucrose
Fructosemia:
An inherited disease (prevents the conversion of fructose to glucose) due to a deficiency of the enzyme fructose-1-phosphate aldolase.
As a result fructose will increase in blood.
symptoms may include vomiting, hypoglycemia, severe malnutrition

42. Hexoses which are either aldoses or ketoses have reducing properties.
Reactions of hexoses
Hexoses which are either aldoses or ketoses have reducing properties.
This reducing property of hexoses is the basis of the test for sugar in urine or blood.
Gluconic acid

45. Reduction of Monosaccharides
Note:
Accumulation of sorbitol in the eye is a major factor in the formation of cataracts due to diabetes.

46. Fermentation
In the presence of yeast (or enzymes of yeast) glucose forms ethanol and carbon dioxide
Fructose can ferment but galactose can not.
Pentoses do not ferment in the presence of yeast.

47. Derivatives of Monosaccharides

48. Sugar Phosphates Phosphate Esters

49. Amino Sugars
Amino sugars (hexosamines) contain an amino group on place of an –OH group.
Examples of natural amino sugars:
D- Glucosamine
D-galactosamine
D-mannosamine
Amino sugars are components of some antibiotics such as Erythromycin and Carbomycin

51. Sugar alcohols

52. Protein-Sugar Interactions
When the glucose level in the blood is elevated over a period of time, normal hemoglobin binds to glucose covalently.
The amount of glucosylated hemoglobin HbA1c in blood is used as a measure of the effectiveness of blood glucose control in a diabetic patient.

53. The concentration of HbA1c directly reflects the elevation of blood glucose over the preceding several days.
In diabetic patient HbA1c is about 7 – 11% of the total hemoglobin (HbA)
In nondiabetic person this value ranges 4 – 6%.

54. Disaccharides

55. A disaccharide consists of two sugars joined by an O-glycosidic bond.
The three common Disaccharides are:
Maltose
Lactose
Sucrose

56. 1. Maltose
Maltose comes from the hydrolysis of large polymeric oligosaccharides such as starch and glycogen and is in turn hydrolyzed to glucose by maltase.
Sucrase, lactase, and maltase are located on the outer surfaces of epithelial cells lining the small intestine

57. Cellobiose is an isomer of maltose.
It differs from maltose only in having the β configuration at C-1 of the left glucose unit. Otherwise, all other structural features are identical, including a link from C-1 of the left unit to the hydroxyl group at C-4 in the right unit.

58. 2. Lactose
Lactose, the disaccharide of milk, consists of galactose joined to glucose by β-1,4-glycosidic linkage.
Lactose is hydrolyzed to these monosaccharides by lactase in human beings and by
β-galactosidase in bacteria.
Lactose is formed in the urine of pregnant women, and it gives a positive test with Cu2+ containing reagents since it is a reducing sugar

59. 3. Sucrose Sucrose is the common table sugar
Is obtained commercially from sugar cane or sugar beets.
The anomeric carbon atoms of a glucose unit and a fructose unit are joined in this disaccharide;
The configuration of this glycosidic linkage is α for glucose and β for fructose.
Sucrose can be cleaved into its component monosaccharides by the enzyme sucrase.

60. Reducing Properties of Disaccharides
Disaccharides with 1,4 linkage are reducing because they have a free aldehyde group in one of the two monosaccharides.
Lactose & maltose are reducing sugars.
Sucrose with 1,2 linkage, no aldehyde or ketone group is free, therefore sucrose is a nonreducing sugar

61. Fermentation
Sucrose & maltose will ferment in the presence of yeast, because yeast does contain sucrase & maltse.
Lacotse on the other hand will not ferment when yeast is added, because it does not contain lactase.

62. Sweetness and Sweeteners (Sugar Substitutes)
Sweetness is literally a matter of taste.
Although individuals vary greatly in their sensory perceptions, it is possible to make some quantitative comparisons of sweetness.
For example, we can take some standard sugar solution (say 10% sucrose in water) and compare its sweetness with that of solutions containing other sugars or sweetening agents.

63. If a 1% solution of some compound tastes as sweet as the 10% sucrose solution, we can say that the compound is 10 times sweeter than sucrose.
D-Fructose is the sweetest of the simple sugars—almost twice as sweet as sucrose.
D-Glucose is almost as sweet as sucrose.
On the other hand, sugars like lactose and galactose have less than 1% of the sweetness of sucrose.

64. Many synthetic sweeteners are known
The most common sugar substitutes are:
Saccharin is made commercially from toluene.
Cyclamates: they are not used anymore because they might cause cancer.

65. 3. Aspartame became the first new sweetener to be approved by the U. S
3. Aspartame became the first new sweetener to be approved by the U.S. Food and Drug Administration (FDA) 25 years ago.
It is about 160 times sweeter than sucrose.
Structurally, aspartame is the methyl ester of a dipeptide of two amino acids that occur naturally in proteins—aspartic acid and phenylalanine—and is sold under the trade name NutraSweet.

66. Polysaccharides

67. Polysaccharides
Contain many linked monosaccharides and vary in chain length and molecular weigh.
Most polysaccharides give a single mono-saccharide on complete hydrolysis.
The monosaccharide units may be linked linearly, or the chains may be branched.
Polysaccharides formed from pentoses are called pentosans, while the ones made from hexoses are called hexosans

68. Comparison of Polysaccharides to Mono- & Disaccharides
Property
Di- & Monosaccharides
Polysaccharides
Taste
sweet
tasteless
Solubility in water
soluble
insoluble
Molecular Weight
low
Very high
Size of particles
pass through membranes
Do not pass through membranes
Test with Cu2+
positive except for sucrose
negative

69. Examples of Polysaccharides 1. Starch
Is the energy-storing carbohydrate of plants.
It is the form in which glucose is stored by plants for later use.
Is made up of glucose units joined mainly by 1,4-α-glycosidic bonds, although the chains may have a number of branches attached through 1,6- α –glycosidic bonds.
Partial hydrolysis of starch gives maltose, and complete hydrolysis gives only d-glucose.

70. Amylose and Amylopectin

71. Starch can be separated into two fractions:
amylose and amylopectin.
In amylose, which constitutes about 20% of starch, the glucose units (50 to 300) are in a continuous chain, with 1,4 linkages

72. Amylopectin is highly branched.
May contain 300 to 5000 glucose units,
Chains with consecutive 1,4 links average only 25 to 30 units in length.
These chains are connected at branch points by 1,6 linkages.
Because of this highly branched structure, starch granules swell and eventually form colloidal systems in water.

74. 2. Glycogen Is the energy-storing carbohydrate of animals.
Like starch, it is made of 1,4- and 1,6-linked glucose units.
Glycogen has a higher molecular weight than starch (perhaps 100,000 glucose units), and its structure is even more branched than that of amylopectin, with a branch every 8 to 12 glucose units.
Glycogen is produced from glucose, transported to the liver, muscles, where it is stored for later use.
Glycogen helps maintain the glucose balance in the body by removing and storing excess glucose from ingested food and later supplying it to the blood when various cells need it for energy.

75. glycogen

76. 3. Cellulose
Cellulose is an unbranched polymer of glucose joined by 1,4-β-glycosidic bonds.

77. Other Polysacchrides
4. Dextrin: Produced during the hydrolysis of starch.
5. Heparin:
Made up of repeated units of glucuronic acid and glucosamine, used as a blood anti-coagulant. Is the strongest organic acid in the body.
6. Dextran: produced by bacteria when grow on sucrose.

78. Dextrans

79. 7. Pectin: Obtained from fruits and berries, used in making jellies.
They are linear polymers of d-galacturonic acid, linked with 1,4-α-glycosidic bonds.