Biochemistry lecture notes carbohydrates

1. CARBOHYDRATES
Dr.B.RENGESH | M.Tech., Ph.D.
Associate Professor, Department of Pharmaceutical Technology,
Mahendra Engineering College (Autonomous),
Namakkal District, Tamil Nadu, India

2. v Carbohydrates are compounds of tremendous biological importance:
• they serve as a form of stored chemical energy
• they provide energy through oxidation
• they supply carbon for the synthesis of cell components
• they form part of the structures of some cells and tissues
v Carbohydrates are polyhydroxy aldehydes or ketones, or substances
that yield such compounds on hydrolysis
v Carbohydrates have the empirical formula (CH2O)n.

3. Three Main Classes:
o Monosaccharides: contain a single polyhydroxy aldehyde or ketone unit (saccharo is
Greek for “sugar”) (e.g., glucose, fructose).
o Disaccharides: consist of two monosaccharide units linked together by a covalent bond
(e.g., sucrose)
o Oligosaccharides: contain from 3 to 10 monosaccharide units (e.g., raffinose).
o Polysaccharides: contain very long chains of hundreds or thousands of monosaccharide
units, which may be either in straight or branched chains (e.g., cellulose, glycogen,
starch)

4. v Polysaccharides are of two types based on their function and
composition. Based on function, polysaccharides of two types are:
ü Storage polysaccharide - starch.
ü Structural polysaccharide - cellulose.

5. STEREOCHEMISTRY OF CARBOHYDRATES
Stereoisomers:
• Glyceraldehyde, the simplest carbohydrate, exists in two isomeric forms
that are mirror images of each other:
• These forms are stereoisomers of each other.
• Glyceraldehyde is a chiral molecule — it cannot be superimposed on its
mirror image. The two mirror image forms of glyceraldehyde are
enantiomers of each other

6. STEREOCHEMISTRY OF CARBOHYDRATES
Chiral Carbons:
• Chiral molecules have the same relationship to each other (that your left
and right hands have) when reflected in a mirror.
• Achiral objects can be superimposed on the mirror images
• Any carbon atom which is connected to four different groups will be
chiral, and will have two nonsuperimposable mirror images; it is a chiral
carbon or a centre of chirality.

7. STEREOCHEMISTRY OF CARBOHYDRATES
Chiral Carbons:
• Molecules which are enantiomers of each other have exactly the same
physical properties (melting point, boiling point, index of refraction, etc.)
but not their interaction with polarized light.
• Polarized light vibrates only in one plane; it results from passing light
through a polarizing filter

8. STEREOCHEMISTRY OF CARBOHYDRATES
Optical Activity:
• A levorotatory (–) substance rotates polarized light to the left.
[E.g., l-glucose; (-)-glucose]
• A dextrorotatory (+) substance rotates polarized light to the right.
[E.g., d-glucose; (+)-glucose]
• Molecules which rotate the plane of of polarized light are optically active.
• Most biologically important molecules are chiral, and hence are optically
active. Often, living system contain only one of all of the possible
stereochemical forms of a compound. In some cases, one form of a molecule
is beneficial, and the enantiomer is a poison (e.g., thalidomide).

9. STEREOCHEMISTRY OF CARBOHYDRATES
Fischer Projections:
• Fischer projections are a convenient way to represent mirror images in two
dimensions.
• Place the carbonyl group at or near the top and the last achiral CH2OH at
the bottom.

10. STEREOCHEMISTRY OF CARBOHYDRATES
Naming Stereoisomers:
• When there is more than one chiral centre in a carbohydrate, look at the
chiral carbon farthest from the carbonyl group: if the hydroxy group points
to right when the carbonyl is “up” it is the D-isomer, and when the
hydroxy group points to the left, it is the L-isomer.

11. STEREOCHEMISTRY OF CARBOHYDRATES

12. MONOSACCHARIDES
• The word “Monosaccharides” derived from the Greek word “Mono” means
Single and “saccharide” means sugar
• Monosaccharides are polyhydroxy aldehydes or ketones which cannot be
further hydrolysed to simple sugar.
• Monosaccharides are simple sugars. They are sweet in taste. They are soluble
in water. They are crystalline in nature.
• They contain 3 to 10 carbon atoms, 2 or more hydroxyl (OH) groups and one
aldehyde (CHO) or one ketone (CO) group.

13. MONOSACCHARIDES
Physical Properties:
o Most monosaccharides have a sweet taste (fructose is sweetest; 73% sweeter
than sucrose).
o They are solids at room temperature.
o They are extremely soluble in water:
§ Despite their high molecular weights, the presence of large numbers of
OH groups make the monosaccharides much more water soluble than
most molecules of similar MW.
• Glucose solubility is 1 g / 1 mL H2O

14. MONOSACCHARIDES
• Monosaccharides are classified according to the number of carbon atoms they
contain:
• The presence of an aldehyde is indicated by
prefix aldo- and a ketone by the prefix keto-
ü glucose is an aldohexose (aldehyde + 6 Cs)
ü ribulose is a ketopentose (ketone + 5 Cs)]

15. MONOSACCHARIDES

16. MONOSACCHARIDES – Family of D-aldoses

17. MONOSACCHARIDES
Family of D-aldoses

18. MONOSACCHARIDES – Family of D-ketoses

19. MONOSACCHARIDES – Family of D-ketoses

20. MONOSACCHARIDES - Anomers
Monosaccharides do not usually exist in solution in their “open-chain” forms

21. MONOSACCHARIDES
Identify the anomeric form

22. MONOSACCHARIDES – Oxidation reaction
• Aldehydes and ketones that have an OH group on the carbon next to the carbonyl group
react with a basic solution of Cu2+ (Benedict’s reagent) to form a red-orange
precipitate of copper(I) oxide (Cu2O).
• Sugars that undergo this reaction are called reducing sugars. (All of the
monosaccharides are reducing sugars.)

23. MONOSACCHARIDES – Phosphate Esters

24. MONOSACCHARIDES
Glycoside Formation
Once the glycoside is formed, the ring
can no longer open up to the open-
chain form. Glycosides, therefore, are
not reducing sugars.
Methyl-α-D-Glucoside
Methyl-β-D-Glucoside

25. MONOSACCHARIDES
Identify the glycoside form

26. MONOSACCHARIDES – Significant Sugars

27. MONOSACCHARIDES – β-D-ribose derivatives
Adenosine
Cytidine
Guanosine
Uridine

28. MONOSACCHARIDES – β-D-deoxyribose derivatives
deoxyadenosine
deoxycytidine
deoxyguanosine
deoxythymidine

29. DISACCHARIDES
• The word “Disaccharides” derived from the Greek word “Di” means Two and
“saccharide” means sugar joined together by an O-glycosidic bond /linkage.
Classification:
Homo-disaccharides
and
Hetero-disaccharides

30. HOMO-
DISACCHARIDES
Maltose Isomaltose Cellobiose
Structure 2-α-glucose 2-α-glucose 2-β-glucose
Type of bond α-1-4- glucosidic bond α-1-6-glucosidic bond β-1-4- glucosidic bond
Anomeric Carbon Free Free Free
Reducing Property Reducing Reducing Reducing
Produced by
It is produced from
starch by the action of
amylase
by the hydrolysis of
some polysaccharides
such as dextran
by the acid hydrolysis
of cellulose
Cellobiose
Isomaltose
Maltose

31. HETERO-DISACCHARIDES Sucrose Lactose
Composition α-D-glucose + β-D-fructose β-D-galactose + β-D-glucose
Type of bond α-1-β-2-glucosidic bond β-1-α-4-glucosidic bond
Anomeric Carbon No Free aldehyde or ketone group Free
Reducing Property Non-Reducing Reducing
Effect of hydrolysis Yields glucose and fructose
Lactase aids in yielding galactose
and glucose
Present in Table sugar, Cane sugar, beet sugar
Milk sugar - It may appear in urine in
late pregnancy and during lactation
LactoseSucrose

32. OLIGOSACCHARIDES
• The word “Oligosaccharides” derived from the Greek word “Oligo” means
‘a few’ and “saccharide” means sugar. They contain contain from 3 to 10
monosaccharide units joined together by an O-glycosidic bond /linkage.
Galactose Glucose Fructose
Raffinose

33. POLYSACCHARIDES
• The word “Polysaccharides” derived from the Greek word “Poly” means
‘many’ and “saccharide” means sugar. They contain contain from 10 to 100s
and 1000s of monosaccharide units joined together by an O-glycosidic bond
/linkage.
• Polysaccharides are not reducing sugars, since the anomeric carbons are
connected through glycosidic linkages.
• Classification: Homopolysaccharide and Heteropolysaccharide
• Homopolysaccharide: made of one type of monosaccharide units
Eg.: starch, cellulose, chitin etc.,
• Homopolysaccharide: made of more than one type of monosaccharide units
Eg.: hyaluronic acid, chondroitin-4-sulfate, heparin etc.,

34. POLYSACCHARIDES - Starch
• Starch is a glucose polymer in which glucopyranose units are bonded by
alpha-linkages
• Starches (and other glucose polymers) are usually insoluble in water because
of the high molecular weight, but they can form thick colloidal suspensions
with water.
• They can be digested by breaking the alpha-linkages (glycosidic bonds).
Both humans and other animals have amylases to digest starches.
• Starch is a storage compound in plants, and made of glucose units
• made of two components: amylose and amylopectin.
• Most starch is 10-30% amylose and 70-90% amylopectin
• Major source: Potato, rice, wheat, and maize

35. POLYSACCHARIDE
Starch
Amylose
Amylopectin

36. POLYSACCHARIDE – Glycogen
• Glycogen, also known as animal starch, is structurally similar to amylopectin, containing
both α(1→4) glycosidic linkages and α(1→6) branch points.
• Glycogen is even more highly branched, however, with branches occuring every 8 to 12
glucose units.
• Glycogen is abundant in the liver
and muscles; on hydrolysis it forms
glucose, which maintains normal
blood sugar level and provides
energy.

37. POLYSACCHARIDE – Cellulose
• Cellulose is a polymer consisting of long, unbranched chains of D-glucose connected by
β(1→4) glycosidic linkages; it may contain from 300 to 3000 glucose units in one
molecule.
• cellulose has a different
overall shape from
amylose, forming
hydrogen bond to each
other, resulting in a very
rigid structure
• plant cell walls that
provides strength and
rigidity; wood is 50%
cellulose

38. POLYSACCHARIDE – Cellulose
• Cellulose microfibrils arrange themselves into thicker bundles called microfibrils.
(These are usually referred to as fibres)
• Most animals lack the enzymes needed to digest cellulose, although it does provide
needed roughage (dietary fiber) to stimulate contraction of the intestines and thus help
pass food along through the digestive system
• Cellulose is also important industrially, from its presence in wood, paper, cotton,
cellophane, rayon, linen, nitrocellulose (guncotton), photographic films (cellulose
acetate), etc
POLYSACCHARIDE – Others
• Chitin, Inulin, Pectin, Hyaluronic acid, Chondroitin, Heparin

39. MUTAROTATION
• Mutarotation is the change in the optical rotation because of the change in the
equilibrium between two anomers, when the corresponding stereocenters interconvert.
• Cyclic sugars show mutarotation as α and β anomeric forms interconvert.
• The optical rotation of the solution depends on the optical rotation of each anomer and
their ratio in the solution.

40. GLYCOSAMINOGLYCANS
• Glycosaminoglycans (GAGs) or mucopolysaccharides are long linear polysaccharides
consisting of repeating disaccharide (double sugar) units. Except for keratan, the
repeating unit consists of an amino sugar, along with a uronic sugar or galactose
Chondroitin Sulfate
Hyaluronan