Carbohydrates: Significance in the
Frontier area of Biological/Medicinal
Chemistry
Ravindranathan Kartha
O
NIPER

Why, of all, Carbohydrates?
2
Are they only just sweet?

Carbohydrates/Sugars
Ubiquitous!
Plants: Storage/structural polysaccharides
(starch/cellulose).
(Starch in turn a major nutrient for animals.)
Fungus: Mono-, oligo- and/polysaccharides.
Bacteria: e.g. Capsular polysaccharides.
Animals: Glycogen, storage polysaccharide.
Glycosaminoglycans (mucopolysaccharides), in the
extracellular spaces of connective tissues (such as
cartilage, tendon, skin).
Glycoproteins. 3

The Nature’s choice! Carbohydrtes are the ‘forms’ (molecules) in which
the Sun’s energy is stored in plants, by photosynthesis:
CO2 + H2O → Carbohydrates + O2
In animals, energy is stored as Glycogen.
In plants there are two dominating glycans (polysaccharides):
Starch (amylose + amylopectin) and Cellulose.
In plants and animals carbohydrates serve as
energy source.
4

CO2 + H2O
h
Chlorophyll
D-Glucose + O2
Starch Cellulose
Glucose
Glycogen
Glucose
Fats Proteins
CO2 + H2O + h 5

Carbohydrates are important for the storage of genetic
information.
O
OH
OH
HO
HO
O
HO
HO N
N
N
N
NH2
D-Ribose Adenosine
DNA
6

In mammalian systems carbohydrates present on the cell
surface (of egg, sperm and embryo) play vital roles in
the life-cycle of organisms.
Important in disease and infection.
Thus carbohydrates have an important role to play
in all aspects of the life cycle of plants, microbes
and animals.
It’s presence in a system therefore marks the
existence of life/once existed life!!
7

“Thamevabhaanthamanubhaathi sarvam
Thasya bhaasaa sarvamidam vibhaathi”
(That alone shines, everything else shines after it.)
8

Decorations are everyone’s favorite!!
9

Cell surface carbohydrate interactions
Cell surface carbohydrate interactions
10

11

Carbohydrates serve as recognition site for other biomacromolecules
12

Trimeric structure of a C-type mannose-binding protein
Lectin-carbohydrate interaction
13

Model of the galabiose-binding site of PapG196
(pancreatic-associated C-type protein acts as an adhesion molecule for hepatocytes)
Lectin-carbohydrate interaction, more
14

Essential for normal inflammatory response. Over-
expression of fucosyl transferase (and over-production of
sLex) causes chronic inflammatory disease. Aberrant
fucosylation facilitates the metastasis of cancer cells 15

Important in xenotransplantation.
• e.g. Human ‘anti-pig’ antibodies believed to
recognize a pentasaccharide ceramide.
O
O
OH
O
O
OH
OH
HO
OH
AcNH
O
O
HO
OH
O O
HO
OH
O
OH
OH
HN
O
HO
(CH2)10CH3
(CH2)10CH3
O
HO
HO
HO
OH
16
Beware: Immune rejection & xenozoonosis!!

Cells, tissues, organs and organisms are immensely complex.
Represent a complex chemical system.
Despite the complexity, cells are composed of relatively
few types of atoms.
Understanding the differences between these atoms is essential.
Determines: Type of bonds possible.
How molecules are constructed.
The nature/properties of the molecule.
How and why a molecule behaves as it does.
Sugars, fats, proteins and nucleic acids are the important
macromolecular components in biology.
17

18

Molecular Bonds in Biology
Bonds are formed either:
Covalent - Electrons are shared between atoms
Ionic - Electron is donated from one atom to another
Weak
Strong 19

Hydrogen Bonds
Very important in biology
Termed “polar” covalent bonds
An electron spends the majority of time around one atom
Leads to the creation of a partial charge (d+ and d-)
Opposites attract (i.e. d+ and d-)
20

Hydrophobic Bonds
Water is the “universal solvent” in Biology
Water has “structure” due to charge and polar covalent bonds
Molecules can be “hydrophobic” due to composition
Molecules can be forced together due repulsion from the charge of water
21

Most significantly, it is able to readily bond with other carbon atoms
C-C bond is both strong and stable
In addition carbon is routinely found in certain combinations
with other atoms
-CH3 Methyl
-OH Hydorxyl
-COOH Carboxyl
-C=O Carbonyl
-PO3
2- Phosphate
-NH2 Amino
}Major factors in
influencing biological
function
22

Macromolecules
4 Major classes depending on monomeric components
Sugars Sugars Sugars Sugars
Fatty Acids Fatty Acids Fatty Acids Fatty Acids
Amino Acids Amino Acids Amino Acids Amino Acids
Nucleotides Nucleotides
Nucleotides
Nucleotides
Polysaccharides
Fats/Lipids/Membranes
Proteins
Nucleic acids
23

Carbohydrates
“Hydrates of carbon” e.g. C6H12O6 = C6(H2O)6
OR in general, Cn(H2O)n or (CH2O)n.
24

Structure (general)
Originally thought to have the formula (CH2O)n.
Now known that only simple monosaccharides
obey this rule.
Carbohydrate- polyhydroxy aldehyde or ketone
or a larger molecule which can be hydrolyzed to a
polyhydroxy aldehyde or ketone.
25

Classification
Monosaccharides- one sugar residue. Most well
known is glucose, C6H12O6. Can’t be hydrolyzed to
smaller molecules.
Di- to Oligosaccharides- a few (2-10-14) sugar
residues . Most well known is cane sugar or sucrose,
C12H22O11.
Polysaccharides- many sugar residues. Most common
are glycogen, starch and cellulose, from animals and
plants.
Simple ……. Complex
26

Monosaccharides, contd.
27

Most 5 and 6 carbon sugars are in ring
form (think of chair conformation)
• Based on molecules
Pyran and Furan
• Carbohydrates can exist
in either, but generally
one is much more
common.
28

In aqueous solution aldehyde or ketone group reacts with the -OH
group to “close” the chain into a ring structure.
Monosaccharide Ring Formation
29

Minor differences in the spatial arrangement of atoms leads to the
formation of isomers.
e.g. Glucose, mannose and galactose have identical formulas
(C6H12O6) but different structures.
Allows for specific enzyme recognition and different biological
effects!!
Monosaccharide Ring Formation
30

a- and b-D-Glucose
• when “C1” interacts with C5, the H and OH groups can assume
two different configurations 31

a b
carbon 1
a- and b-D-Glucose
• when “C1” interacts with C5, the H and OH groups can assume
two different configurations
32

An equilibrium exists between the
possible structures
33

Disaccharides
• formed by a covalent bond between two sugar residues
• dehydration reaction --> a water molecule is removed
• occurs between C1 and an OH from another sugar residue
• common disaccharides:
• (1,4) linkage of two glucose = maltose (malt sugar, used in
brewing)
• (1,2) linkage of glucose + fructose = sucrose (table sugar)
34

Disaccharides
• maltose is formed by an a-(1,4)- glycosidic linkage
• what is the difference between a- and b- glucoses ?
• hint: it has to do with C1 (the business end of the molecule)
35

Thus, the disaccharide formed depends on the type of
monosaccharides involved and the way in which the glycosidic
bond is configured.
e.g.
Glucose + Glucose = Maltose
Glucose + Glucose = Cellobiose
Glucose + Galactose = Lactose
Glucose + Fructose = Sucrose
Now think of the oligo- and polysaccharides!!
36

Anti-Tuberculosis Drugs:
New Approaches to Meet an Old
Problem
Examples!
O
NIPER
37

Tuberculosis remains the main bacterial
human killer
• In 1997 more people died from TB than in any other
year in history
• 8 Million new infections and 3 million deaths from TB
every year
• The World Health Organization (WHO) described the
situation as a “global emergency”.
38

Problem solving, what chemists do?
Is retrosynthesis.
That is, try to ‘reduce’ the molecule (problem) to its
more fundamental components and assemble the
components to generate the molecule.
Philosophically, this is nothing but reductionism!
(Assemble & disassemble or reduce and integrate.)
39

Some of the most successful
anti-tuberculosis drugs have been targeted
against the mycobacterial cell wall
40

Cholera Toxin/Verotoxin Assembly
Inhibitors:
Synthesis and Evaluation
Moving on….
O
NIPER
41

Cholera (Introduction)
•Caused by Vibrio cholerae
•Bacteria produce cholera toxin (CT)
•CT activates adenylate cyclase enzyme in
the intestinal cells
•Stimulated cells ‘pumps’ Cl- into the
intestinal contents
42

•Osmotic and electrical gradients thereby generated
cause transport of water and other electrolytes from
blood and tissues to the intestinal lumen
•Leads to the sudden onset of diarrhoea (loss of fluids)
•Can quickly lead to dehydration and anuria, acidosis
and shock if untreated (with potential to cause death)
•Loss of K+ ions can cause cardiac complications
Cholera (Introduction, contd.)
43

44

45

46

O
OH
HO
HO
OH
O
O
HO OH
AcHN
O
O
OH
OH
O O
OH
HO
OH
O
O
OH
AcHN
HO
OH
HO2C
O
HO
Ganglioside GM1
HN
O
OH
(CH2)10CH3
(CH2)10CH3
O
O
HO
HO
O
OH
O
HO OH
HO
HO
O
HO
HO
OH
Globotriosyl ceramide Gb3
O
HN (CH2)10CH3
O
(CH2)10CH3
OH
47

HN
CH
C
CH2
H
N
O
HO CH C
CH2
NH
O
CH
C
H2
C
NH
O
OH
CH
C
CH
2
NH
O
CH
C
CH2
NH O OH
CH
C
CH2
N
H O
CH
C
CH2
HN
O
HO
CH
C
H2C
HN
O
CH
C
C
H2
HN
O
HO
CH
C
H2C
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
48

Discovering New Glycosidases/Glycosyl
Transferases and Agglutinins Present in
Nature & Synthesize New Glycosidase
Inhibitors
O
NIPER
49

Glycosidases
• Glycosidases are enzymes that cleave glycosides. The
process involves the splitting of a glycoside into a sugar
and an alcohol.
• Glycosidase inhibitors are of interest in: Simple cold to
diabetes to neurological disorders.
O
O
~~~~~~~~
R
HO
O
O
H
~~
~~~
~~
~
O O
O
O
~~~~~~~~
R
HO
O
O
H
~~
~~
~~~
~
O O
O
HO
O O
~~
~~
~
~~~~~
O
O
O
H
H O
O
~~~~~~~~
H
HO
O
O
H
~~
~~
~~
~~
O O
O
O
~~~~~~~~
H
HO
O
O
H
~~
~~~
~~
~
O O
ROH
50




















ACARBOSE
(an anti-hyperglycaemic agent) 
HN
O CO2
AcNH
HO OH
HO
NH2
H2N
4-deoxy-4-guanidino-Neu5Ac2en.
(an anti-viral agent)
In diabetes therapy the main class of carbohydrates of interest are
aza-sugars and carba-sugars which inhibit the glycosidases that are
responsible for the degradation of poly- and oligo-saccharides.
Inhibition of these enzymes reduces the amount of glucose entering
the bloodstream. Such drugs can be used in conjunction with
conventional agents, which aid either the release or effect of
insulin. The carba-tetrasaccharide acarbose is used in the treatment
of hyperglycaemia.
51

Glycosyl Transferases
O
HO
HO HNAc
CO2
OH
O
OH
O
OH OH
OH
Parasite
HO
O
OH OH
OH
Host
O
HO
HO HNAc
CO2
OH
O
OH
O
OH OH
OH
Parasite
O
HO
HO HNAc
CO2
OH
OH
OH
O
OH OH
OH
Host
HO
Hydrolase
(Desialiated)
Transferase
+
trans-sialidase
Glycosyl transferases transfer a sugar moiety from a donor
substrate to an acceptor substrate. The Process is extremely
important in, for e.g., infection by parasites, etc.
52

Anti-Influenza Drugs:
Examples!
(A rational drug design success story)
53

Influenza viruses continue to be a major cause of morbidity and
mortality worldwide. During outbreaks of influenza, the highest attack
rates are among school-aged children. Secondary spread occurs to adults
and other children in the family. Children younger than five years of age
have the second highest rates of infection with influenza and
hospitalization, which are exceeded only by persons older than 65 years
of age. Besides admissions to hospitals, influenza accounts for a
substantial number of outpatient visits and antibiotic prescriptions in
children.
Influenza, the problem:
54

Other problems associated with developing drugs against viruses are:
* The drugs must get inside the body’s cells.
* Hard to get broad spectrum agents. There are many strains of
influenza to target.
* Virus replication may decline before symptoms occur.
* Difficult to establish in vitro assays.
* Difficult to achieve selective toxicity since viruses share many
pathways with their host cells.
55

The primary method of preventing influenza is through annual active
immunization with the vaccine.
The antiviral agents amantadine or rimantadine have been used for
prophylactic treatment, but these agents are effective only against
strains of influenza type A virus.
The influenza neuraminidase inhibitors represent a new class of drugs
that show efficacy against influenza A and B viruses. Zanamavir is used
as a nasal spray and Oseltamivir is administered orally.
Influenza, the solution:
56

Influenza neuraminidase inhibitors:
As influenza viruses are studded with neuraminidase proteins
inhibitors were developed to capitalize on the essential role of
neuraminic acid in influenza virus replication. The drugs are sialic
acid analogues, Zanamivir (Relenza) by Glaxo Wellcome Inc) and
Oseltamivir (Tamiflu) by Hoffmann-La Roche, Canada), effective in
stopping the virus from replication.
O
C
OH
OH
AcHN
HO
HO
OH
O
HO
57

58

59

The neuraminidase story, continued:
O
O OH
NH
NH2
NH
NHCOCH3
HO
HO
OH
Relenza
(Zanamivir)
O OC2H5
NH2
NHCOCH3
O
Tamiflu
(Oseltamivir)
60

O
HN
O
COOH
HO
OH
HO
N
N
N R
61

62

Cancer vaccine
In normal tissues mucins can be expressed on the surface of
epithelial cells facing the lumen of the organ. In malignant
epithelial cells mucins lose their correct topology and
polarisation. Due to these changes mucins can also be
expressed on the outer cell surface and soluble mucins can
enter the bloodstream. The presence of mucins in the
extracellular matrix may lead to the production of metastatic
tumour cells.
Altered glycosylation is another common feature in
malignant cells. The O-glycosyl side chains bound to serine
or threonine of the peptide backbone become shortened in
the mucins of malignant cells.
63

GalNAc-R R = Ser or Thr
Gal-b-1,3-GalNAc
b 1,3 galactosyltransferase
b 1,6 GlcNAc transferase
Gal-b-1,3-GalNAc
GlcNAc
a 2,6-sialyltransferase
NeuNAc-a-2,6-GalNAc
(Sialyl TN)
a 2,3-sialyltransferase
NeuNAc-a-2,3-Gal-b-1,3-GalNAc
(Sialyl T)
Cancer: chain termination
Normal: chain extension
b-1,6
Contd….
64

65
 Most of the chemical compounds identified as specific agents for
killing cancer cells have also been found toxic to normal cells
 Most of the potential anticancer drugs have considerable side
effects and have little clinical use
 Hence, discovery of newer safer drugs which are active against
tumors becomes an important goal in medical research
 The enhancement or potentiation of host defense mechanism
emerges as possible means of inhibiting tumor growth without
harming the host
Curr. Med. Chem. 2000, 7, 715-729

 Considering this view, numerous antitumor polysaccharides have
been discovered from mushrooms, fungi, yeasts, algae, lichens
and plants
 These bioactive polysaccharides (glycans) or polysaccharide-
protein complexes belong to either homoglycans or heteroglycans
 The first four major mushroom-derived antitumor polysaccharide
drugs to hit the market were:
- Lentinan from Lentinula edodes (Shiitake)
- Krestin from Trametes (Coriolus) versicolor
- Schizophyllan from Schizophyllum commune (Split gill fungus)
- Grifron-D from Grifola frondosa ( Maitake)
66
Appl. Microbiol. Biotechnol. 2002, 60, 258-274

O
O
O
HO
HO
O
OH
HO
HO
HO
HO
HO
HO
O
OH
HO
HO
O
OH
HO
HO
O
O
O
O
O
O
HO
HO
HO
HO
HO
HO
O
O
HO
HO
HO
O
OH
HO
HO
O
O
O
O
n
67

O
O
HO
HO
HO
O
O
HO
HO
HO
O
O
O
HO
HO
OH
HO
HO
HO
O
O
O
HO
HO
HO
O
O
HO
HO
HO
O
O
O
HO
HO
OH
HO
HO
HO
O
O
3
n
O
OH
HO
HO O
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
1
0
3
0
5
0
7
0
9
0
1
1
0
1
3
0
1
5
0
Elution volume (mL)
Absorbance
(485
nm)
68

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1
0
3
0
5
0
7
0
9
0
1
1
0
1
3
0
1
5
0
1
7
0
1
9
0
Elution volume (mL)
Absorbance
(485
nm)
69
O
HO
HO
HO
O
HO
O
HO
O
HO
HO
HO
O
O
HO
HO
OH
O
HO
HO
HO
O
O
O
HO
HO
HO
O
O
HO
O
HO
O
HO
HO
HO
O
O
HO
O
HO
O
HO
HO
HO
O
O
HO
OH
HO
O
Fucose
n

Cytotoxic activity
NA – not active
Values are expressed by percentage cytotoxicity
70

Sugar alcohol-based molecular straw & molecular origami
71

Water
Hexane CH2Cl2
CH2Cl2 +Hexane
Gel in Methanol
Methanol
x1700 x2500
x1700
x1100
x 1100
Octane
x5000
Toluene
x1100
72

In CH2Cl2
(Inset portion expanded)
Origami in palm leaf for
comparison of the structure
obtained
73

Drugs can only repress symptoms. They cannot eradicate
the disease. The true remedy for all diseases is natures
remedy …. Nature has provided, in the white corpuscles
as you call them-in the phagocytes as we call them-a
natural means of devouring and destroying all disease
germs. There is at bottom only one genuinely scientific
treatment for all diseases, and that is to stimulate the
phagocytes. Stimulate the phagocytes … they devour the
disease, and the patient recovers. Unless, of course, he’s
too far gone.
George Bernard Shaw
Sir Bloomfield Bonnington in “The Doctor’s Dilemma”
Royal Gold Theatre, London
20 November 1906
The power lies with in!
74

Carbohydrate chemistry is indeed sweeeet!!
Demand more!!
Thank you! 75