based on undergraduate syllabus

1. Carbohydrates
Raj Krishna Dangol
Department of Biochemistry
Lumbini Medical College

3. Objectives
At the end of session, you will able to
– Define carbohydrate
– Describe biomedical importance and function of
carbohydrates
– Classify carbohydrate
– Study of Isomerism
– Study of structure and properties of physiological
important carbohydrates
– Study of structure and importance of
Glycosaminoglycans and Glycoproteins

4. Introduction
• Carbohydrates = Carbon + Hydrates
• General emperical formula: Cn(H2O)n

5. • Exception

6. Definition
 Polyhydroxy aldehydes or Polyhydroxy ketones
or Compounds which yields these derivatives on
hydrolysis.

7. Biomedical importance
Structural and metabolic roles
Glucose is most important carbohydrate
Dietary fibers
Disease associated with carbohydrate
metabolism include diabetes mellitus,
galactosemia, glycogen storage diseases and
lactose intolerance

8. Function
Source of energy

9. Precursors for other organic compounds

10. Storage form of energy

11. Structural component

12. Glycoproteins and glycolipids are participate
in cell membrane and cellular function

13. Classification
Monosaccharides
• Glucose
• Fructose
• Galactose
• Ribose
Disaccharides
• Sucrose
• Maltose
• Lactose
• Trehalose
Oligosaccharides
• Raffinose
• Stachyose
• Verbascose
Polysaccharides
• Starch
• Glycogen
• Hyaluronic
acid
• Heparin

14. ISOMERISM (isosG=equal; merosG=part)
• Different compounds with the same
molecular formula

15. • Two types
– Structural: same molecular formula but possess
different structure
– Sterioisomer: same molecular formula and
same structure but differ only in spatial
configuration
• Asymmetric carbon (chiral center)
• 2n

16. • More important types of isomerism found with
glucose are as follows
– D and L isomerism
– Epimers
– Pyranose and furanose ring structure
– Alpha and beta isomers
– Aldose and ketose isomerism

17. D and L isomerismD and L isomerism
Non-Superimposable
COMPLETE mirror
image (differ in
configuration at
EVERY CHIRAL
CENTER.

18. • Mirror-image stereoisomers are also called
enantiomers
• D & L form of monosaccharide is based on the
configuration of the asymmetrical carbon atom
located farthest (penultimate carbon)from the
carbonyl functional group

19. • Non-mirror-image stereoisomers are called
diasteromers
• Most of stereoisomers that occur in the body
are of ‘D’ series

20. Optical Isomers
• Optically active compounds that differ in their
direction of rotation of plane polarized light
– Dextrorotation (d or +): rotates plane polarized light
to the right
– Levorotation (l or -): rotates plane polarized light to
the left
• Stereoisomerism and optical isomerism are
independent properties
• D (+) glucose, D (-) fructose
• Equimolecular mixture of optical isomers has
no net rotation (racemic mixture)

21. Epimer
• Pairs of sugars which differ only in the
configuration around a single carbon atom
• Differ in configuration of –OH group on 2nd,
3rd and 4th carbon atoms of monosaccharides

22. Pyranose and furanose ring
structure
• Monosaccharides with five or more carbons
occur predominantly in cyclic forms
• Formation of ring is due to reaction of
carbonyl carbon and alcohol

23. • Pyranose form: Glucose, Galactose
• Furanose form: Ribose, Fructose

25. ANOMER
• Same composition but differ in the orientation
of groups around anomeric carbon atom
– α form: OH group is below the plane
– β form: OH group is above the plane

26. Mutarotation
• Change in the optical rotation by the
interconversion of α and β anomeric forms of
monosaccharides to an equilibrium mixture

27. Functional isomerism
• Same molecular formula but different
functional groups

28. MONOSACCHARIDE
Simple sugars
Consist of a single polyhydroxy aldehyde or
ketone unit, and thus cannot be hydrolyzed
into a simpler form
General formula: CnH2nOn

29. May be subdivided into different groups
– Depending upon the number of carbon atoms they
possess, e.g. trioses, tetroses, pentoses, hexoses
and heptoses
– Depending upon the functional aldehyde or ketone
group present: aldoses or ketoses

30. Trioses (C=3)
• D-glyceraldehyde and dihydrooxyacetone
occur in the form of phosphate esters, as
intermediates in glycolysis

31. Tetroses
• Erythrose 4-P occurs as an intermediate of
HMP shunt

32. • Pentoses
Sugar Source Biochemical and Clinical
importance
D-Ribose Nucleic acids
and metabolic
intermediates
Structural components of nucleic
acids and coenzymes, including
ATP, NAD(P) and flavin
coenzymes
D-Ribulose Metabolic
intermediate
Intermediate in the pentose
phosphate pathway
D-xylose Plant gums,
proteoglycans
Constituents of glycoproteins
L-Xylulose Metabolic
intermediate
Excreted in the urine in essential
pentouria

33. D-Glucose
• Source: Fruit juices, hydrolysis of starch, cane
or beet sugar, maltose and lactose
• Biochemical importance: main metabolic fuel
for tissue
• Clinical significance: excreted in the urine
(glycosuria) in poorly controlled diabetes
mellitus as a result of hyperglycemia

34. D-Fructose
• Source: fruit juices, honey, hydrolysis of cane or
beet sugar and inulin, enzymic isomerization of
glucose syrups for food manufacture
• Biochemical importance: readily metabolized
either via glucose or directly
• Clinical significance: Hereditary fructose
intolerance leads to fructose accumulation and
hypoglycemia

35. D-Galactose
• Source: hydrolysis of lactose
• Biochemical importance: readily metabolized
to glucose; synthesis of milk; constituents of
glycoprotein
• Clinical significance: Hereditary galactosemia
as a result of failure to metabolize galactose
leads to cataracts

36. D-Mannose
• Source: hydrolysis of plant mannan gums
• Biochemical importance : constituent of
glycoproteins

37. Reaction of Monosaccharides
• Some important chemical properties of
monosaccharides are
1. Action of Strong Acids: Furfural formation
2. Action of Alkalies: Enolization
3. Oxidation: Sugar acid formation
4. Reduction: Sugar alcohol formation
5. Action of phenylhydrazine: Osazone
formation

38. • Action of strong acids (Furfural formation)
– Basis for Molisch’s test, Seliwanoff’s test, Bial’s
test

39. • Action of Alkalies (Enolization)
– Basis for benedict’s test, Fehling’s test

41. • Oxidation (Sugar Acid Formation)

42. • Reduction to form sugar alcohol

43. • Action of phenylhydrazine (Osazone
formation)

44. Glycosides
Glycosides are formed when the hemiacetal or
hemiketal hydroxyl group of a carbohydrate
reacts with a hydroxyl group of another
carbohydrate or a non-carbohydrate

45. The bond so formed is known as glycosidic
bond and the non carbohydrate moiety is
referred to as aglycone
E.g. Glucovanillin, cardiac glycosides (digoxin
& digitoxin), streptomycin, Ouabain

46. DISACCHARIDES
Sugars which yield two molecules of the same
or different molecules of monosaccharide on
hydrolysis
General formula: Cn(H2O)n-1

47. Two types
Reducing sugar
– Maltose, lactose,
Non-reducing sugar
– Sucrose, trehalose

48. Maltose
Source:
– germinating cereals and barley, also formed
during hydrolysis of starch
Structure:
– contains two glucose units
– Linked by α(1→4) glycosidic bond

49. Lactose
Source:
– Milk
Structure:
– Contains galactose and glucose
– Linked by β(1→4) glycosidic bond

50. Lactulose
Source:
– Synthetic disaccharide
Structure:
– Contains galactose and fructose
– Linked by β(1→4) glycosidic bond
Clinical importance: used in hepatic
encephalopathy

51. Sucrose
Also called household sugar
Source:
– Sugarcane, beet, maple, also present in pineapple
and carrot
Structure:
– Contains glucose and fructose
– Linked by α,β(1→2) glycosidic bond

52. • Invert sugar
Sucrose (+66.5˚)
Invert sugar (-27.5˚)
Mixture of glucose (+52.7˚)
and fructose (-92.4˚)
HCl
Acid
hydrolysis
Sucrase
Enzymatic
hydrolysis

53. Other disaccharides are:-
– Isomaltose
– Cellobiose
– Trehalose

54. OLIGOSACCHARIDE
Sugars which yield 3 to 10 monosaccharides
units on hydrolysis, e.g.Maltotriose,
rhamninose, gentianose, raffinose, stachyose,
scorodose, verbascose

55. POLYSACCHARIDE
Sugars which yield more than ten molecules of
monosaccharides on hydrolysis
General formula: (C6H10O5)n

56. Classification
• Polymer of same
monosaccharide unit
• E.g. starch, glycogen,
cellulose, chitin, inulin
Homopolysaccharides
• Polymer of different
monosaccharide units or their
derivatives
• E.g. hyaluronic acid,
chondroitin sulfate, dermatan
sulfate, heparin, keratan sulfate
Heterosaccharides

57. Starch
o Reserve carbohydrate of plant kingdom
o Sources: Potatoes, tapioca, cereals (rice,
wheat) and other food grains
o Composed of amylose and amylopectin

58. – Amylose is made up of glucose units with alpha-
1,4 glycosidic linkages
– Accounts for 15-20% of starch composition
– Water soluble
– Gives blue colour with iodine solution

59. • Amylopectin is also made up of
glucose but is highly branched. The
branching points are made by alpha-
1,6 linkage and branching occurs at
every 25 to 30 glucose residue
• Accounts for 80-85% of starch
composition
• Insoluble in water
• Gives reddish-violet colour with
iodine solution

60. • Function
– Storage polysaccharide in plants
– Major source of energy

61. Glycogen
o Reserve carbohydrate in animal
o Structure similar to amylopectin, except
that it is more highly branched
o Found mostly in liver and muscle
o Muscle glycogen act as a readily
available source of glucose for energy
within muscle itself
o Liver glycogen is concerned with storage
and maintenance of the blood glucose
o Inherited deficiency of enzymes for
glycogen metabolism leads to glycogen
storage diseases

62. Cellulose
o Chief constituent of cell wall of plant
o Unbranched polymer of glucose and consists of
long straight chains which are linked by β-(1→4)
glycosidic linkage
o Cellulose cannot be digested and absorbed and
has no food value unlike starch.
o Good source of dietary fiber

63. Inulin
o Polymer of D-Fructose linked together by β-
(1→2) glycosidic linkage
o Occurs in the tubers of some plants, e.g.
chicory, bulb of onion and garlic
o It is used in the studies of glomerular
filtration rates

64. Dextrins
• Low molecular weight polymers of glucose
• Sources: obtained during partial hydrolysis of
starch by enzymes (amylase) and acids
• Uses: as paste (mucilages). Baby foods contain
dextrins as component of starch hydrolysates

65. Dextrans
• Similar to amylopectin
• Glucose units are linked through
α 1-6 in linear part and α 1-3
linkages is present in branch part
• Sources: synthesized by the
action of bacterial enzyme on
sucrose molecules present in a
sucrose medium
• Uses: as plasma expanders in
conditions associated with
decreased plasma volume
• Dental plaque

66. Chitin
• Polymer of N-acetyl D-glucosamine units that
are linked by α (1-4) glycosidic linkages
• Sources: present in the exoskeleton of
invertebrates such as crabs and lobsters

67. Heteropolysaccharides
• Composed of repeating disaccharide units of
different monosaccharides, e.g.
glycosaminoglycans, agar and pectins

68. • Glycosaminoglycans: consisting of repeating
disaccharide units
– Sugars such as galactose
– Amino sugars such as glucosamine, N-acetyl-
glucosamine or N-acetylgalactosamine
– Sugar acids such as D-glucuronic acid or L-
iduronic acid, and
– Sulfate group attached to either amino sugars or
sugar acids

69. Chondroitin sulphate
 Composed of repeating units of D-glucuronic
acid → β (1-3)-N-acetyl galactosamine
sulphate → β (1-4) and so on
 Found at sites of calcification in bone and
cartilage, certain neurons
 Provide an endoskeletal structure helping to
maintain their shape
 Have role in compressibility of cartilage in
weight bearing

70. Dermatan sulfate
 Composed of L-iduronic acid and N-acetyl
galactosamine in beta -1, 3 linkages
 Found in skin, blood vessels and heart valves
 Transparancy of cornea and maintain the
overall shape of the eye

71. Heparin
 Contains repeating units of sulfated
glucosamine → α-1, 4-L-iduronic acid or
glucuronic acid → and so on.
 Present in liver, lungs, spleen and monocytes
 Serves as an anticoagulant (binds antithrombin
III) causes release of lipoprotein lipase from
capillary walls

72. Hyaluronic acid
 Composed of repeating units of N-acetyl-
glucosamine →beta-1,4-Glucuronic acid →
beta-1,3-N-Acetyl glucosamine and so on
 Present in synovial fluid of joints, vitreous
humour of the eye, loose connective tissue and
umbilical cord
 Serve as lubricant and shock absorber,
facilitate cell migration in embryogenesis,
morphogenesis, wound healing

73. Keratan sulfate
 Contains repeating units of galactose and N-
acetyl glucosamine
 Present in Cornea, loose connective tissue and
cartilage
 Transparancy of cornea

74. Agar
• Polymer of sulfated galactose and glucose
molecules
• Sources: sea weeds
• Uses: as dietary fibers, main components of
medium used in bacterial culture, as support
medium in immunodiffusion and
immunoelectrophoretic techniques

75. Glycoproteins
• Complex carbohydrates in which
oligosaccharide units are covalently linked to
proteins.
• Monosaccharides: D-N-acetylgalactosamine,
D-N-acetyl glucosamine, D-galactose, D-
glucose, D-mannose, L-fucose and sailic acid

76. • Type
– O-linked
– N-linked
• Role
– Stabilization and extension of polypeptide chain or
protection against proteolysis
– Antigenic role and cell-cell recognition.

77. The general formula of monosaccharides is
a. CnH2nOn
b. C2nH2On
c. CnH2O2n
d. CnH2nOn

78. Two sugars which differ from one another
only in configuration around a single
carbon atom are termed
a. Epimers
b. Anomers
c. Optical isomers
d. Stereoismers

79. A pentose sugar is
a. Dihydroxyacetone
b. Ribulose
c. Erythrose
d. Glucose

80. Isomers differing as a result of variations in
configuration of the –OH and –H on carbon
atoms 2, 3 and 4 of glucose are known as
a. Epimers
b. Anomers
c. Optical isomers
d. Sterioisomers

81.  α-D-glucose +112˚ → +52.5˚ ← +19˚ β-D-
glucose for glucose above represents
a. Optical isomerism
b. Mutarotation
c. Epimerization
d. D and L isomerism

82. In glucose the orientation of the –H and –
OH groups around the carbon atom 5
adjacent to the terminal primary alcohol
carbon determines
a. D or L series
b. Dextro or levorotatory
c. α and β anomers
d. Epimers

83. The monosaccharide units are linked by
β1→4 glycosidic linkage in
a. Maltose
b. Sucrose
c. Cellulose
d. Cellobiose

84. Which of the following is a reducing sugar?
a. Sucrose
b. Trehalose
c. Isomaltose
d. Agar

85. The polysaccharide used in assessing the
glomerular filtration rate (GFR) is
a. Glycogen
b. Agar
c. Inulin
d. Hyaluronic acid

86. Starch is
a. Polysaccharide
b. Monosaccharide
c. Disaccharide
d. None of these

87. Repeating units of hyaluronic acid are
a. N-acetyl glucosamine and D-glucuronic acid
b. N-acetyl galactosamine and D-glucuronic acid
c. N-acetyl glucosamine and galactose
d. N-acetyl galactosamine and L-iduronic acid

88. REFERENCES