Metabolism of Carbohydrates

1. Carbohydrates
• Most abundant organic molecules on
Earth
• Poly hydroxyl aldehyde or ketone.
• Source – Rice, potato, legumes.
• Originally produced through CO2
fixation during photosynthesis.

2. Roles of Carbohydrates
• Energy storage (glycogen, starch)
• Structural components (cellulose,
chitin)
• Carbohydrate derivatives include
DNA, RNA, co-factors, glycoprotein,
glycolipids

3. General Properties:
• Sugars can exhibit isomerism due to the
presence of asymetric carbon atom.
• Asymetric C-atom - refers a carbon
containing four different atom or group.
• Isomers are molecules having same
molecular formulas but differ in
arrangement of their atoms.
A. Stereo (space) isomers
• These are compounds having same
composition but differ only in their spatial
configuration.

4. Stereo isomers- Types
1. Enantiomers (mirror images): Two such isomers
of D-glyceraldehyde and L-glyceraldehye are
‘mirror’ image of each other.
• Number of possible isomers (I) of any compound
depends upon the number of asymmetric carbon atom
(n) of the molecule possess. Van hoff’s rule : I = 2n
• E.g. for Glucose, I = 2 4 = 16 (8 D & 8 L form).

5. Stereochemistry
Enantiomers
C
C*
O
H
H OH
HO H
H OH
H OH
C*
C*
C*
CH2OH
C
C*
O
H
HO H
H OH
HO H
HO H
C*
C*
C*
CH2OH
C
C*
O
Epimers
H
H OH
HO H
H OH
H OH
C*
C*
C*
CH2OH
C
C*
O
HO H
HO H
H OH
H OH
C*
C*
C*
CH2OH
Diastereomers
H
HO H
HO H
H OH
H OH
H
H OH
HO H
HO H
H OH
2. Diastereomers - Pairs of isomers that have
opposite configurations at two or more chiral centers
but are not mirror images. Eg. Mannose & Glucose.
H
C
C*
O
C*
C*
C*
CH2OH
C
C*
O
C*
C*
C*
CH2OH
L-glucose D-glucose
D-glucose D-mannose
D-mannose D-galactose

6. 3. Epimers
• When sugars differ from one another in
configuration at only one (chiral center ) asymmetric
atom they are called epimers.
• E.G. Glucose and mannose are the epimers with
respect of C2, similarly glucose and galactose are
epimers of C4

7. 4. Anomers
• Two stereo isomers (a and b from) of a given
sugar that differ only in the configuration about
the carbonyl (anomeric) carbon atom.
– the a-isomer has the H-atom in C1 above the plane of
the ring in the Haworth projection (b=hydrogen
below).

8. B. Geometric isomers
• Arise from a peculiar geometry of a
compound having a double bond with in the
carbon chain.
• It is of 2 types –
1. Cis form and 2.trans form.
HO
H
C C
H OH
HO OH
C C
H H

9. B. Geometric isomers -Types
• If the radicals are on the same side of the
double bond, the compound - cis form.
• If the radicals are on the opposite side of the
double bond, the compound - trans form.
HO
H
C C
H OH
HO OH
C C
H H

10. Optical activity
• The presence of asymmetric carbon atom
causes optical activity.
• When a beam of plane polarized light is
passed through a solution, exhibiting optical
activity, it will be rotated to the right or left
in accordance with the type of compound.
• When it is rotated to right (clock wise), the
compound is called “Dextrorotatory”, when
rotated to left (Anti clock wise), the
compound is called “laevorotatory”.

11. Racemic mixture
• Racemic mixture is a compound contains equal
amounts of d & l isomers. It has no optical activity.
• Since the optical activity of each isomers will cancel
each other.
• A change in optical rotation is called Muta rotation.
• It occurs due to change of one ring form to another
ring form. i.e. α- form to β- form.
+ 112° + 52.5° + 19°
α -D-Glucose Equilibrium stage β -D-Glucose

12. CLASSIFICATION OF
CARBOHYDRATES
CARBOHYDRATES
MONOSACCHARDIES DISACCHARIDES OLIGOSACCHARIDES POLYSACCHARIDES

13. Classifications
of monosaccharides
# Carbons Category Name Relevant examples
3 Triose Glyceraldehyde, Dihydroxyacetone
4 Tetrose Erythrose
5 Pentose Ribose, Ribulose, Xylulose
Glucose, Galactose, Mannose,
6 Hexose Fructose
7 Heptose Sedoheptulose
Neuraminic acid
9 Nonose (sialic acid)

14. D & L designate absolute configuration
of the asymmetric carbon atom
farthest from the aldehyde or ketone
group

15. Classifications of monosaccharides based on
functional groups
Monosaccharides
Aldoses
(Glucose)
Ketoses
(Fructose)

16. Glucose
• Glucose - found in blood of animals
• Constituent of some disaccharides and
polysaccharides.
• Immediate energy source to cells.
• Stored as glycogen in liver and muscles.
• Glucose can exist in 2 common form – as a
Straight chain or a ring structure.

17. Isomers of glucose

18. Haworth Projections
1
2
3
4
6
5

19. Haworth Projections

20. Monosaccharide can cyclize to
form Pyranose / Furanose forms

21. Fructose
• It is a keto hexose.
• Commonly called as “fruit
sugar”.
• It is sweeter than
sucrose and glucose.
• It is a constituent of
sucrose and the
polysaccharide inulin.
• Seminal fluid is rich in
fructose and sperms
utilize fructose for
energy.

22. Galactose
• Not found -free in nature
• Occurs as a constituent
of milk sugar lactose
• Epimer of glucose
• Less sweet than glucose.
• Less soluble in water.

23. The Glycosidic Bond
• Glycosidic bond - The bond between the
anomeric carbon of a carbohydrate and
some other group or molecule; the C-O in
disaccharides and the C-N in nucleosides
are two examples. aka glycosidic link.
• Note: The glycosidic bond involves only
the anomeric carbon.

24. If the -OCH3 is replaced by an -NR2, an N-glycosidic
bond would be formed.
Note: such modified sugars (-OCH3/-NR2) are non-reducing because
the open chain form does not have a free aldehyde group.

25. Monosaccharide structures
you need to know
• Glucose
• Fructose
• Ribulose
• Glyceraldehyde
• Dihydroxyacetone

26. Derivatives of
Monosaccharide

27. Sugar Phosphates

28. Deoxy Acids

29. Amino Sugars

30. Sugar alcohols

31. Oligosaccharides
• Oligosaccharides contain 2 to 10
monosaccharide molecules which are
liberated on hydrolysis.
• Based on the number of monosaccharide
units present, the Oligosaccharides are
further subdivided as disaccharides,
trisaccharides, tetrasaccharides or
Pentasaccharides

32. Disaccharides
• Disaccharides are formed by the union
of two monosaccharide with the
elimination of one molecule of water.
• General molecular formula is C12H22O11.

33. Disaccharides - Types
• 2 Types - reducing & non reducing sugars.
• Reducing sugars contains free aldehyde or
keto group.
• E.g. Maltose, Lactose.
• Non reducing sugars have no free aldehyde
or keto group, as they are involved in the
glycosidic linkage of the two sugars.
• E.g. Sucrose, Trehalose.

34. Disaccharide - Sugars
linked by an
O-glycosidic bond.
Sucrose- Non-reducing sugar
Lactose - Reducing sugar
Maltose - Reducing sugar

35. Sugars Can Be Reducing
Agents
• Reducing Sugars - A sugar that will
reduce inorganic ions such as Cu++
(Fehling’s reagent).
• All monosaccharides whether an aldose or
ketose are reducing sugars.
• Most disaccharides are reducing sugars;
sucrose is a notable exception.

36. Lactose
• Found exclusively in the milk of mammals, so it is
called as “Milk sugar”.
• consists of galactose and glucose in a
β-(1 4) glycosidic bond.
• It is less sweet and less soluble.
• Lactose is a reducing sugar & forms powder puff shape
osazones crystals.

37. Maltose
• It is called as “malt sugar”.
• Intermediate product of hydrolysis of starch.
• It is a disaccharide with an α (1 4) glycosidic linkage
between the C1 hydroxyl of one glucose and the C4 hydroxyl
of a second glucose.
• Highly soluble in water.
• It can form Sun flower shape osazones crystals.

38. Sucrose
• Occurs in sugar cane and sugar beets.
• It is commonly called as “table sugar” & cane sugar .
• Highly sweet and soluble.
• Composed of glucose and fructose through an
α-(1 2)β-glycosidic bond.
• On hydrolysis the D-sugars inverted into L-sugars, so it
called as a “invert sugar”.