1. Carbohydrates
Dr Imran Siddiqui PhD, MPhil, MBBS

2. Objectives
• Define Carbohydrates
• List the biomedical importance of
carbohydrates
• Write down the Empirical formula of
Carbohydrates
• Classify with examples various types of
carbohydrates
• Explain isomerism in carbohydrates

3. Significance of Carbohydrates
• Most abundant organic compound in nature
• A major source of energy from our diet
• Storage form of energy in plants and animals
• Cell membrane component
• Structural component of many organisms
• Composed of the elements C, H and O and
empiric formula is C(H2O)n
• Also called hydrates of carbon OR saccharides,
which means “sugars”

4. Definition
Carbohydrates are polyhydric alcohols (-OH)
with aldehyde or ketone.

5. Figure 7.1. Examples of monosaccharides found in humans, classified
according to the number of carbons they contain.

6. Isomers and epimers
• Compounds that have same chemical formula but have
different structures = ISOMERS
• e.g., fructose, glucose, mannose, & galactose are all isomers
of each other, having same formula C6H12O6
• If 2 monosacch differ in configuration around only one
specific C atom (with exception of carbonyl C), they are
defined as epimers of each other (of course they are also
isomers) e.g.,
– glucose & galactose are C-4 epimers, their structures
differ only in the position of –OH group at C 4.
– Glucose & mannose are C-2 epimers
Note: carbons in sugars are numbered beginning at end
containing the carbonyl C i.e., aldehyde or keto group

7. Figure 7.4
C-2 and C-4 epimers and an isomer of
glucose.

8. B. Enantiomers
- A special type of isomerism is found in the pairs of structures
that are mirror images of each other. These mirror images =
enantiomers, & the 2 members of the pair are designated as
D- & L-sugar. Vast majority of sugars in humans are D-sugars
Figure 7.5
Enantiomers (mirror
images) of glucose.

9. C. Cyclization of monosaccharides
- Less than 1% of each of the monosacch with 5 or
more C’s exist in the open-chain (acyclic) form.
Rather, they are predominantly found in ring form,
in which aldehyde (or ketone) group has reacted
with an alcohol group on the same sugar

10. Two Cyclic Forms of Glucose

11. Figure 7.6. The interconversion of the α and β anomeric forms of
glucose (mutarotation).

12. • Anomers are isomers which differ in placement of
hydroxyl on C1
• The –OH is drawn down for the -anomer, and up for
the -anomer
-D-Glucose -D-Glucose
O
CH2OH
OH
OH
OH
OH
O
CH2OH
OH
OH
OH
OH
and Anomers for D-Glucose

13. Types of Carbohydrates
 Monosaccharides are the simplest carbohydrates
 Empirical formula = (CH2O)n
 Disaccharides consist of two monosaccharides
 Polysaccharides contain many monosaccharides

14. Monosaccharide
Unbranched chain of 3-8 C atoms
One is carbonyl; others attached to -OH
Aldoses
contain an aldehyde group (carbon 1)
Ketoses
contain a ketone group (carbon 2)
CHO
HO H
CH2OH
CHO
HO H
OHH
CH2OH
CH2OH
O
HHO
OHH
OHH
OHCH2OH
Classification of Monosaccharides

15. Structural representation of sugars
• Fischer projection: straight chain
representation
• Haworth projection: simple ring in perspective
• Conformational representation: chair and boat
configurations

16. Rules for drawing Haworth projections
• draw either a six or 5-membered ring including
oxygen as one atom
• most aldohexoses are six-membered
• aldotetroses, aldopentoses, ketohexoses are 5-
membered
O O

17. Rules for drawing Haworth projections
• for D-sugars the last alcohal group (farthest
from the carbonyl) is drawn up.
• For L-sugars, it is drawn down
• for D-sugars, the OH group at the anomeric
position is drawn down for and up for .
• For L-sugars is up and is down

18. Chair and boat conformations of a pyranose sugar
2 possible chair conformations
of -D-glucose

19. Oxidation reactions
• Aldoses may be oxidized to 3 types of acids
– Aldonic acids: aldehyde group is converted to a carboxyl
group ( glucose – gluconic acid)
– Uronic acids: aldehyde is left intact and primary alcohol
at the other end is oxidized to COOH
• Glucose --- glucuronic acid
• Galactose --- galacturonic acid
– Saccharic acids (glycaric acids) – oxidation at both ends
of monosaccharide)
• Glucose ---- saccharic acid
• Galactose --- mucic acid
• Mannose --- mannaric acid

20. Special monosaccharides: deoxy sugars
• These are monosaccharides which lack one or
more hydroxyl groups on the molecule
• deoxy sugar is 2’-deoxy ribose which is the
sugar found in DNA

21. Special monosaccharides: amino sugars
Constituents of mucopolysaccharides

22. D-Glucose
• Most common hexose
• Found in fruits, corn syrup,
and honey
• An aldohexose with the
formula C6H12O6
• Known as blood sugar in the
body
• Building block for many
disaccharides and
polysaccharides

23. D-Fructose
• Ketohexose C6H12O6
• Differ from glucose at C1 and C2 (location
of carbonyl)
• The sweetest carbohydrate (2x sucrose)
• Found in fruit juices and honey
• Formed from hydrolysis of sucrose
• Converts to glucose in the body
D-Fructose
CH2OH
C
C
O
C
H
OHH
HO
C
CH2OH
OHH

24. D-Galactose
• Aldohexose
• Differ from D-glucose at C4
• Not found in the free form in
nature
• Obtained from lactose, a
disaccharide (milk products)
• Important in cellular membranes
in CNS
CHO
C
C
OH
C
H
HHO
HO
C
CH2OH
OHH
H
D-Galactose

25. Memorize!

26. Memorize!

27. •Most common disaccharides are
•Sucrose, lactose, and maltose
•Maltose hydrolyzes to 2 molecules of D-glucose
•Lactose hydrolyzes to a molecule of glucose and a
molecule of galactose
•Sucrose hydrolyzes to a molecules of glucose and a
molecule of fructose
Disaccharides

28. •galactose- -(1,4)-fructose
•a semi-synthetic disaccharide (not naturally
occurring)
•not absorbed in the GI tract
•used either as a laxative (Chronulac) or in the
management of portal systemic encephalopathy
(Cephulac)
•metabolized in distal ileum and colon by bacteria to
lactic acid, formic acid and acetic acid (remove
ammonia)
Lactulose

30. Less common glucose disaccharides
Isomaltose (alpha 1,6)
Gentiobiose (beta 1,6)
Laminaribiose (beta 1,3)
Cellobiose (beta 1,4)

31. Cellobiose
Cellobiose consists of 2 molecules of glucose linked by a beta-1,4 glycosidic bond
It is usually obtained by the partial hydrolysis of cellulose

32. Trehalose is a disaccharide that occurs naturally in insects, plants, fungi, and bacteria. The
major dietary source is mushrooms. Trehalose is used in bakery goods, beverages,
confectionery, fruit jam, breakfast cereals, rice, and noodles as a texturizer, stabilizer with a low
sweetening intensity
O
CH2OH
H
O O
H
H
H
HO
H
OH
OH
H
OH H
H
OH
OH
H
HOH2C
TREHALOSE
Trehalose

33. Sucralose (Splenda)
About 600 times more sweet than sucrose

34. Figure 7.11
Abnormal lactose metabolism.

35. Oligosaccharides
• Trisaccharide: raffinose (glucose, galactose
and fructose)
• Tetrasaccharide: stachyose (2 galactoses,
glucose and fructose)
• Pentasaccharide: verbascose (3 galactoses,
glucose and fructose)
• Hexasaccharide: ajugose (4 galactoses, glucose
and fructose)

36. Honey also contains glucose and fructose along with
some volatile oils

37. Polysaccharides or glycans
Types
• homoglycans / homopolysaccharides (starch,
cellulose, glycogen, inulin)
• heteroglycans / heteropolysaccharides (gums,
mucopolysaccharides)
Characteristics
• Polymers (MW from 200,000)
• White and amorphous products (glossy)
• not sweet
• form colloidal solutions or suspensions

38. Tertiary structure - sterical/geometrical
conformations
• Rule-of-thumb: Overall shape of the chain is
determined by geometrical relationship within each
monosaccharide unit
 1 4) - zig-zag - ribbon like
 1 3) & 4) - U-turn - hollow helix
 1 2) - twisted - crumpled
 (1 6) - no ordered conformation

39. Starch
• most common storage polysaccharide in
plants
• composed of 10 – 30% amylose and 70-
90% amylopectin depending on the source
• the chains are of varying length, having
molecular weights from several thousands to
half a million

40. Starch
• Main sources of starch are rice, corn, wheat,
potatoes
• A storage polysaccharide
• Starch is used as an excipient, a binder in
medications to aid the formation of tablets.
• Industrially it has many applications such as
in adhesives, paper making, biofuel, textiles

41. Cellulose
• Polymer of -D-glucose attached by (1-->4) linkages
• Only digested and utilized by ruminants (cows, deers,
giraffes, camels)
• A structural polysaccharide
• Yields glucose upon complete hydrolysis
• Partial hydrolysis yields cellobiose
• Most abundant of all carbohydrates
• Cotton flax: 97-99% cellulose
• Wood: ~ 50% cellulose
• Gives no color with iodine

42. Products obtained from cellulose
• Microcrystalline cellulose : used as binder-
disintegrant in tablets
• Methylcellulose: suspending agent and laxative
• Oxidized cellulose: hemostat
• Sodium carboxymethyl cellulose: laxative
• Cellulose acetate: rayon; photographic film; plastics
• Cellulose acetate phthalate: enteric coating
• Nitrocellulose: explosives;

43. Glycogen
• also known as animal starch
• stored in muscle and liver (mostly)
• present in cells as granules (high MW)
• contains both (1,4) links and (1,6) branches at
every 8 to 12 glucose unit (more frequent than in
starch)
• complete hydrolysis yields glucose
• hydrolyzed by both and -amylases and by
glycogen phosphorylase

44. Inulin
• -(1,2) linked fructofuranoses
• linear only; no branching
• lower molecular weight than starch
• colors yellow with iodine
• hydrolysis yields fructose
• sources include onions,and garlic, dandelions and
jerusalem artichokes
• used as diagnostic agent for the renal function test
Jerusalem artichokes

45. Chitin
• Chitin is the second most
abundant carbohydrate
polymer
• Present in the cell wall of
fungi and in the
exoskeletons of crustaceans,
insects and spiders
• Chitin is used commercially
in coatings (extends the
shelf life of fruits and meats)

46. Dextrans
• products of the reaction of glucose and the enzyme
transglucosidase
• contains (1,4), (1,6) and (1,3) linkages
• MW: 40,000; 70,000; 75,000
• used as plasma extenders (treatment of shock)
• components of dental plaques

47. Dextrins
• produced by the partial hydrolysis of starch
along with maltose and glucose
• dextrins are often referred to as either
amylodextrins or erythrodextrins
• used as mucilages (glues)
• also used in infant formulas (prevent the
curdling of milk in baby’s stomach)

48. Glycoproteins and proteoglycans
• Glycoproteins are proteins conjugated to
saccharides lacking a serial repeating unit
• In glycoprotein the protein>>>carbohydrate
• Example include enzymes, immunoglobulins or antibodies,
certain hormones
• In Proteoglycans proteins are conjugated to
polysaccharides with serial repeating units
• Here carbohydrate>>> protein
• Proteoglycans modulate cell processes and make cartilage
flexible and resilient

49. Glycosaminoglycans
• they are the polysaccharide chains of proteoglycans
• they are linked to the protein core via a serine or
threonine (O-linked)
• the chains are linear (unbranched)
• the glycosaminoglycan chains are long (over 100
monosaccharides)
• they are composed of repeating disaccharides