Carbohydrates

Carbohydrates
i. Their Classification
ii. Their Structure
iii. Their Functions
1
Chapter# 3 Biochemistry -2031
Taken From: Lehinger Principles of Biochemistry
(6th Edition)
Prepared By: M.Fazal-ur-Rehman
UE Lahore,Vehari Campus

Carbohydrates
Carbohydrates are the most abundant biomolecules on Earth.
Each year, photosynthesis converts more than 100 billion metric
tons of CO2 and H20 into cellulose and other plant products.
Carbohydrates are polyhydroxy aldehydes and ketones, or
substances that yield such compounds on hydrolysis. Many, but
not all have the empirical formula (CH2O)n, but some also
contain nitrogen, phosphorus, or sulfur. Carbohydrates occur in
three main size classes: monosaccharides, disaccharides,and
polysaccharides. The most abundant monosaccharide in nature
is D-glucose, which is also known as dextrose. A common
disaccharide, sucrose, consists of the six-carbon sugars D-
glucose and D-fructose. Common polysaccharides include
cellulose and starches. Both of these are homopolymers of D-
glucose units, but with different linkages between residues.
More complex carbohydrate polymers attached to a protein or
lipid moiety (glycoconjugates) are also prevalent in nature.
General
Formula is
Cn(H2O)n
2

Carbohydrates
Monosaccharide Disaccharides Polysaccharides

Monosaccharides
Monosaccharides are colorless, crystalline solids that are very soluble in
water, but insoluble in nonpolar solvents. Most have a sweet taste. The
backbones of common monosaccharides are unbranched carbon chains in
which all the carbons are linked by single bonds. In this open-chain form,
one of the carbon atoms is double-bonded to an oxygen atom to form a
carbonyl group. Each of the other carbons has a hydroxyl group. If the
carbonyl group is at an end of the carbon chain (that is, in an aldehyde
group) the monosaccharide is called an aldose. If the carbonyl group is at
any other position (a ketone group) the monosaccharide is a ketose.
Monosaccharides with three, four, five, six, and seven carbons in their
backbones are called trioses, tetroses, pentoses, hexoses, and heptoses.
Many of the carbon atoms to which hydroxyl groups are attached are chiral
centers. This gives rise to the many stereoisomers found in
monosaccharides.
4
Physical Properties
 Colorless
 Crystalline solids
 Soluble in water (H-bond because of OH
groups)
 Insoluble in nonpolar solvents

Common Monosaccharides
Common aldoses and ketoses of three-, five-, and six-carbon lengths are
shown in Fig. 7-1. The simplest monosaccharides are the two three-
carbon trioses: D-glyceraldehyde, an aldotriose; and dihydroxyacetone, a
ketotriose. The most common monosaccharides in nature are the
aldohexose D-glucose, and the ketohexose D-fructose. The aldopentoses
D-ribose and 2-deoxy-D-ribose are components of nucleotides and
nucleic acids.
5

D & L Stereoisomers
All of the monosaccharides except
dihydroxyacetone contain one or more asymmetric
(chiral) carbon atoms and thus occur in optically
active isomeric forms. The simplest aldose,
glyceraldehyde, contains one chiral center (the
middle carbon atom) and therefore has two
different optical isomers, or enantiomers. One of
the two enantiomers of glyceraldehyde is, by
convention, designated the D isomer and the other
is the L isomer. In general, a molecule with n
chiral centers can have 2n stereoisomers.
Glyceraldehyde has 21 = 2; the aldohexoses with
four chiral centers have 24 = 16. The stereoisomers
of monosaccharides of each carbon-chain length
are divided into two groups that differ in the
configuration about the chiral carbon that is most
distant from the carbonyl carbon. Those in which
the configuration of this reference carbon is the
same as that of D-glyceraldehyde are designated D
isomers. Those with the same configuration as L-
glyceraldehyde are L isomers. Thus of the 16
possible aldohexoses, eight are D forms and 8 are
L forms. The reason D forms predominate in
nature is unknown.
6

Structures of the D Monosaccharides
The structures of the D stereoisomers of all the aldoses and ketoses having
three to six carbon atoms are shown in Fig.(next two slides). The carbons of
a sugar are numbered beginning at the end of the chain nearest the carbonyl
group. Each of the eight aldohexoses, which differ in the stereochemistry at
C-2, C-3, and C-4, has its own name: D-glucose, D-mannose, D-galactose,
and so forth. The four- and five-carbon ketoses are designated by inserting
“ul” into the name of the corresponding aldose; for example, D-ribulose is
the ketopentose corresponding to the aldopentose D-ribose. The
ketohexoses are named otherwise: for example, fructose is named from the
Latin fructus, “fruit”.
7

Epimers of D-Aldohexoses
Two monosaccharides that differ only in the configuration around one
chiral carbon atom are called epimers. D-glucose and D-mannose are
epimers which differ in the configuration at C-2. D-glucose and D-
galactose are epimers that differ in the configuration at C-4.
Due to Change in Position of
-OH group, their names
change.
10

Common L Stereoisomers
Some sugars occur naturally in their L form. Some examples are L-
arabinose (below) and the L isomers of some sugar derivatives that are
common components of glycoconjugates.
11

Formation of Hemiacetals and Hemiketals
Aldotetroses and all monosaccharides with five or more carbon atoms occur
predominantly as cyclic ring structures in which the carbonyl group has
formed a covalent bond with the oxygen of a hydroxyl group along the
chain. The formation of these ring structures is the result of a general
reaction between alcohols and aldehydes or ketones to form derivatives
called hemiacetals or hemiketals. Actually, two molecules of an alcohol can
add to a carbonyl carbon. The product of the first reaction for an aldose is
a hemiacetal, and the product of the first reaction for a ketose is a
hemiketal. If the -OH and carbonyl groups are from the same molecule, a
five- or six-membered ring results. The addition of the second alcohol
molecule produces the full acetal or ketal, and the bond formed is a
glycosidic linkage. When the two reacting molecules are both
monosaccharides, the acetal or ketal produced is a disaccharide.
12

Cyclization of D-Glucose
The reaction of the first alcohol with an aldose or
ketose creates an additional chiral center at what
was the carbonyl carbon. Because the alcohol can
add to the carbonyl carbon by attacking either
from the “front” or the “back”, the reaction can
produce either of two stereoisomeric
configurations, denoted  and ß. For example, D-
glucose exists in solution as an intramolecular
hemiacetal in which the free hydroxyl group at C-
5 has reacted with the aldehyde C-1, rendering the
latter carbon asymmetric and producing two
possible stereoisomers, designated  and ß. These
two isomeric forms, which differ only in their
configuration about the hemiacetal carbon atom
are called anomers, and the carbonyl carbon is
called the anomeric carbon. The same
nomenclature is used to describe anomeric forms
of hemiketals such as formed by fructose .The 
and ß anomers of D-glucose interconvert via the
linear form in aqueous solution by a process called
mutarotation. In solution, an equilibrium mixture
forms which consists of about one-third -D-
glucopyranose, two-thirds ß-D-glucopyranose, and
trace amounts of the linear and five-membered
glucofuranose ring forms.
13

Pyranoses and Furanoses
Six-membered monosaccharide ring
compounds are called pyranoses because
they resemble pyran. Five-membered
monosaccharide ring compounds are called
furanoses because they resemble furan. The
systematic names for the two ring forms of
D-glucose are therefore -D-glucopyranose
and ß-D-glucopyranose. Ketohexoses such
as fructose also occur as cyclic compounds
with  and ß anomeric forms. In these
compounds the hydroxyl group at C-5 (or
C-6) reacts with the keto group at C-2
forming a furanose (or pyranose, not shown)
ring containing a hemiketal linkage. D-
fructose readily forms a furanose ring. The
more common anomer of this sugar in
combined forms or in derivatives is ß-D-
fructofuranose.  Cyclic form of glucose is a pyranose
 Cyclic form of fructose is a furanose
 Glucose serves as the essential energy
source, and is commonly known as blood
sugar or dextrose.
 Fructose is the sweetest, occurs naturally in
honey and fruits, and is added to many foods
in the form of high-fructose corn syrup.
 Galactose rarely occurs naturally as a single
sugar.

Fisher Projection & Haworth Perspective
Formulas
Cyclic sugar structures are more accurately represented in Haworth
perspective formulas (see below) than in Fischer projections used for
linear sugar structures. In Haworth formulas the six-membered ring is
tilted to make its plane almost perpendicular to that of the paper. The
bonds closest to the reader are drawn thicker than those farther away. To
convert the Fisher projection formula of any linear D-hexose to a
Haworth perspective formula, draw the six-membered ring (five carbons,
and one oxygen at the upper right), number the carbons in a clockwise
direction beginning with the anomeric carbon, then add the hydroxyl
groups as follows. If a hydroxyl group is to the right in the Fischer
formula, it is placed pointing down in the Haworth formula. If a hydroxyl
group is to the left in the Fischer formula, then it is placed pointing up in
the Haworth
formula. The terminal -CH2OH group
projects upward for the D-enantiomer, and
downward for the L-enantiomer. When the
hydroxyl group on the anomeric carbon of a
D-hexose is on the same side of the ring as
C-6, the structure is by definition ß. When it
is on the opposite side from C-6, the
structure is .
15

Conformational Formulas of Pyranoses
It is important to keep in mind the actual
conformational structures of the ring
forms of monosaccharides. For example
the six-membered pyranose ring is not
actually planar, as suggested by Haworth
representations, but instead tends to
assume either of two chair conformations.
The inter-conversion of the two chair
forms (conformers) does not require bond
breakage and does not change the
configurations of substituents attached to
any of the ring carbons. However, it does
require a considerable input of energy.
The actual three-dimensional structures
of monosaccharide units are important in
determining the biological properties and
functions of some polysaccharides, as
shown below.
16

Important Hexose DerivativesIn addition to simple hexoses such
as glucose, galactose, and mannose,
there are many sugar derivatives in
which a hydroxyl group in the
parent compound is replaced with
another substituent, or a carbon
atom is oxidized to a carboxyl
group. In addition, hexoses in
metabolic pathways commonly are
phosphorylated on hydroxyl
groups. In amino sugars, an -NH2
group replaces one of the -OH
groups in the parent hexose.
Substitution of -H for -OH
produces a deoxy sugar, some of
which occur in nature as L isomers.
The acidic sugars contain a
carboxylate group, which confers a
negative charge at neutral pH.
Lactones result from the formation
of an ester linkage between the C-
1 carboxylate group and the C-5
hydroxyl group of the sugar. Some
notable functions of hexose
derivatives in biology are 1) N-
acetylglucosamine and N-
acetylmuramic acid, components
of the bacterial cell wall; and 2) N-
acetylneuraminic acid (sialic acid)
and fucose, components of the
oligosaccharide chains of
mammalian glycoproteins.

Reactions of Monosaccharides
• Oxidation of sugars
– Even though ketones should not give a positive Benedict’s
test, ketoses do.
– This is because under the basic conditions of the test, the
ketoses can isomerize to form aldoses, which the react.
18
Benedict’s Test

19
Oxidation by Tollens Reagent
• Tollens reagent reacts with aldehyde, but the base
promotes enediol rearrangements, so ketoses react too.
• Sugars that give a silver mirror with Tollens are called
reducing sugars.
Prepared By : M.Fazal-ur-Rehman UE Lahore,Vehari Campus

20
• All monosacchs are reducing sugars.
• They can be oxidised by weak oxidising
agent such as Benedict’s reagent
• Benedict's reagent is a solution of copper
sulfate, sodium hydroxide, and tartaric
acid.
• Aqueous glucose is mixed with
Benedict's reagent and heated. The
reaction reduces the blue copper (II)
ion to form a brick red precipitate of
copper (I) oxide. Because of this,
glucose is classified as a reducing
sugar.

21
Formation of Glycosides
The acetal (or ketal) of a sugar is called a glycoside

Reaction of Ketoses with Phenylhydrazine

Osazone Formation
Aldoses and ketoses react with three equivalents of
phenylhydrazine

Reduction of Monosaccharides
Sugars alcohols: sweetners in many sugar-free (diet drinks & sugarless gum).
HHO
H
CH2OH
OHC
H
H
OH
OH
C
C
C
OH
C
H2 HHO
H
CH2OH
OHC
H
H
OH
OH
C
C
C
OH
C
D - glucose D – Sorbitol (D – glucitol)
CH2OH
Problem: diarrhea and cataract
Alditols
Transition metals

25

Oxidation of Monosaccharides
Reducing sugars: reduce another substance.
HHO
H
CH2OH
OHC
H
H
OH
OH
C
C
C
OH
C
+ 2Cu2+ Oxidation HHO
H
CH2OH
OHC
H
H
OH
OH
C
C
C
OH
C
+ Cu2O(s)
OH
D - glucose D – gluconic acid
Benedict’s
Reagent (blue)
(Brike red)
Aldonic acids

7) Action of acids:
The sugar molecule is very stable towards dilute acids, but with concentrated
acids it is distorted:
A- Conc. Sulphuric acid removes the elements of water and converts the
sugar into carbon.
B- Conc. Nitric acid oxidizes the aldose into dicarboxylic acid.
C- Conc. Hydrochloric acid reacts with aldoses giving different products:
i) With pentoses it yields furfural which is steam volatile and can
therefore be used for the estimation of pentoses (this reaction is used
in wood analysis).
Conc.HCl
furfuraldehyde
O CHOCH2OH-(CHOH)3CHO
ii) With methyl pentoses we obtain methyl furfural which
is steam volatile.
Conc.HCl
Methylfurfuraldehyde
O CHO
CH3CHOH-(CHOH)3CHO H3C

Barfoed's Test
Shows positive test for:
Reducing monosaccharides.
Reactions:
Reducing monosaccharides are
oxidized by the copper ion in
solution to form a carboxylic acid
and a reddish precipitate of
copper (I) oxide within three
minutes. Reducing disaccharides
undergo the same reaction, but
do so at a slower rate.
28
How to perform the test:
One ml of a sample solution is placed in a
test tube. Three ml of Barfoed's reagent (a
solution of cupric acetate and acetic acid) is
added. The solution is then heated in a
boiling water bath for three minutes.
A positive test is indicated by:
The formation of a reddish precipitate
within three minutes.
a negative test (left) and a positive
test (right)

Salivonoff's Test
Ketoses
Reactions:
The test reagent dehydrates ketohexoses to
form 5-hydroxymethylfurfural.
5-hydroxymethylfurfural further reacts with
resorcinol present in the test reagent to produce
a red product within two minutes (reaction not
shown). Aldohexoses react to form the same
product, but do so more slowly.
29
How to perform the test:
One half ml of a sample
solution is placed in a test
tube. Two ml of Seliwanoff's
reagent (a solution of
resorcinol and HCl) is added.
The solution is then heated in
a boiling water bath for two
minutes.
A positive test is indicated by:
The formation of a red
product.
a negative test (left) and a
positive test (right)

Molisch’s test
Molisch test is a qualitative test for
determining the presence of carbohydrates in
a solution.It is named after an Austian
botanist “Hans Molisch” who discovered this
test for the presence of carbohydrates.
It is a general test for the presence of
carbohydrates and is positive for all kinds of
carbohydrates which in free from or in
combined form.
All carbohydrates. Monosaccharides give a
rapid positive test. Disaccharides and
polysaccharides react slower.
Reactions:
The test reagent dehydrates pentoses to form
furfural (top reaction) and dehydrates hexoses
to form 5-hydroxymethyl furfural (bottom
reaction). The furfurals further react with -
naphthol present in the test reagent to
produce a purple product (reaction shown on
next).
30

Tollen’s Phloroglucinol Test
This reaction is positive for pentoses, but not a specific
indication of them since hexoses and glycuronic acid also
react. The distinction between the groups is in a carefully
conducted test. If the cherry red color develops very
promptly, and if this is soluble in amyl alcohol and
alcoholic solution shows a specific absorbance band
consistent with resource data, them it can be considered
a fairly reliable test for pentoses and glycuronic acid. The
colored compounds formed are condensation products of
the various intermediate and final acid decomposition
products from carbohydrates with the phenolic
compound phloroglucinol.
31

Anthron when mixed with
conc.Sulphuric Acid and
Carbohydrate ,gives a blue or
green color.
Anthron Test

Reducing sugars
The carbohydrates, which reduce Fehling's and Benedict's reagents
are called as reducing sugars. They have a free aldehyde or keto
group.
Examples: Maltose, lactose
Non-reducing sugars
The carbohydrates, which do not reduce Fehling's and Benedict's
reagents are called as non-reducing sugars. They have no free
aldehyde or keto group.
Examples: Sucrose, trehalose
The most common disaccharides are:
 Sucrose (cane sugar) made up of glucose + fructose
 Maltose (Malt sugar) made up of glucose + glucose
 Lactose (milk sugar) made up of glucose + galactose.
33

Disaccharides (I)
A disaccharide (e.g., maltose,) is formed from two
monosaccharides (two D-glucose molecules for
maltose) when an -OH alcohol group of the right D-
glucose condenses with the intramolecular hemiacetal
of the left D-glucose. Water is eliminated, and a
glycoside with a glycosidic bond is formed. The reversal
of this reaction is hydrolysis by attack of a water
molecule on this bond--a reaction which is readily
catalyzed using dilute acid. The oxidation of a sugar by
cupric ion occurs only with its linear form, which exists
in equilibrium with its cyclic forms. Thus, the anomeric
carbon of the D-glucose residue on the left can no longer
react with Cu2+ because it is tied up in a glycosidic bond.
In contrast, the hemiacetal linkage in the right D-
glucose molecule can open up, and react with Cu2+. For
this reason, the right end of maltose is called its
reducing end. Because mutarotation interconverts the 
and ß forms of the right hemiacetal linkage, the bonds
at this position are sometimes depicted with wavy lines
to indicate that either configuration at the anomeric
carbon is possible. In maltose, the configuration of the
anomeric carbon atom in the glycosidic linkage is . In
maltose, (14) shows that C-1 of the first D-glucose
unit is joined to C-4 of the second. 4) Name the second
residue. Following this convention, maltose is -D-
glucopyranosyl-(14)-D-glucopyranose. Because most
sugars in the textbook are the D enantiomers and the
pyranose form of hexoses predominates, a shortened
version of the formal name of compounds, such as
maltose, can be used which gives the configuration of
the anomeric carbon and names the carbons joined by
the glycosidic bond.
Molecular Formula For all is
“C12H22O11”.
Maltose is a
Reducing Sugar,
which gives +ve test
34

Disaccharides (III)
The chemical structures and full
systematic names of the common
disaccharides, lactose (milk sugar),
sucrose (table sugar), and trehalose (a
sugar occurring in insect hemolymph)
are shown in Fig.. Lactose is composed
of D galactose and D glucose, sucrose
is composed of D fructose and D
glucose, and trehalose is composed of
two D glucose residues. Lactose is a
reducing sugar, and its reducing end is
located on the glucose unit on the
right. Sucrose and trehalose are both
nonreducing sugars because the
anomeric carbons of both
monosaccharides in these compounds
are tied up in glycosidic linkages.
Sucrose:
 Major Sweetening agent, flavoring agent, persevering
agent.
 Can be Hydrolyzed with Water.
 Used as a Diet, Compatible with human body. 35

Polysaccharides
Most carbohydrates found in nature
occur as polysaccharides, polymers of
medium to high molecular weight (Mr
>20,000). Polysaccharides, also called
glycans, differ from each other in the
identity of their recurring
monosaccharide units, in the lengths
of their chains, in the types of bonds
linking the monosaccharide units, and
in the degree of branching.
Homopolysaccharides contain only a
single monomeric species, whereas
heteropolysaccharides contain two or
more different kinds. Unlike proteins,
polysaccharides generally do not have
defined molecular weights. This is
because polysaccharides are not
synthesized from a template. Instead,
there is no specific stopping point for
the enzymes involved in their
biosynthesis.
36
There are two types.
1. Storage polsaccharides
i. Starch
ii. Glycogen
2. Structural Polysaccharides
i. Cellulose
ii. Chitin

Starches (I)
Starch is a storage homopolysaccharide of D glucose residues
that is found in the cytoplasm of plant cells. Starch (and
glycogen) is extensively hydrated because it has many exposed
hydroxyl groups available to hydrogen-bond with water. Starches
consist of two types of polymers called amylose and
amylopectin.Amylose is a linear polymer of D glucose residues
that all are connected via (14) linkages (as in maltose). The
molecular weights of amylose chains vary from a few thousand
to more than a million. Amylopectin is a branched polymer of D
glucose residues that can weigh up to 200 million Da. The
glycosidic linkages between D glucose residues in amylopectin
chains are also (14); the branch point linkages between D
glucose units, however, are (16) linkages (Fig. next slide).
Branch points occur about every 24 to 30 residues.
General Formula is
(C6H10O5)n
 Amylose form
Collidle Soln.
with water.
 It gives Blue
color with
Iodine.
37

Starches (II)
A cluster of amylose and amylopectin molecules, like that believed to be
present in the starch granules in plant cells, is shown in Fig. (right).
Strands of amylopectin (black) form double-helical structures with each
other or with amylose strands (blue). Amylopectin has (16) branch
points (red). Glucose resides at the non-reducing ends of the outer
branches are removed enzymatically during the mobilization of starch for
energy production. Glycogen has a structure that is similar to
amylopectin, but is more highly branched and more compact.
38

Glycogen
Glycogen is the main storage polysaccharide occurring in animal cells.
Its structure is very similar to amylopectin, in that main chain
linkages between D glucose units are (14) and the linkages at
branch points are (16). Branch points occur more frequently in
glycogen (about every 8 to 12 residues) than in amylopectin. Glycogen
is especially abundant in hepatocytes of the liver where it may
constitute as much as 7% of the wet weight of the tissue. Slightly less
glycogen (about 2% by wet weight) is stored in skeletal muscle cells.
Glycogen molecules occur in large granules that can be observed in
the cytoplasm of cells by electron microscopy. A single glycogen
molecule can weigh several million Da. Like amylopectin, glycogen
molecules have many nonreducing ends at the ends of the branches,
but only one reducing end. The enzymes of glycogen metabolism
build up and break down glycogen to glucose units at the nonreducing
ends of the molecule. Simultaneous reactions at the many
nonreducing ends speed up the metabolism of the polysaccharide.The
storage of glucose units in glycogen molecules has a much smaller
osmotic effect on cells than would the storage of an equivalent
amount of glucose as the free monosaccharide.
o It contains
Enzymes
responsible for
degradation of
glycogen called as
Glycogen
Phosphorylated
Enzymes.
o Specified to act
on non-reducing
ends.
o Highly soluble in
water than
Starch.
o In Sliva,  -
amylase on
glycogen obtained
by eating the food
stuff.
o Glucose polymer,
similar to
amylopectin, but
even more highly
branched.
o Energy storage in
muscle tissue and
liver.
o The many
branched ends
provide a quick
means of putting
glucose into the
blood.
39

Cellulose
Cellulose is a linear homopolysaccharide composed exclusively
of D glucose units held together in (ß14) linkages. A single
chain of cellulose can contain 10-to-15,000 residues. Due to the
presence of ß linkages, cellulose chains fold quite differently
than chains of D glucose in the starches and glycogen (see
below). Cellulose molecules are insoluble in water and form
tough fibers. Cellulose is found in the cell walls of plants,
particularly in stalks, stems, trunks, and all the woody portions
of the plant body. Cellulose constitutes much of the mass of
wood, and cotton is almost pure cellulose. Vertebrate animals
lack the hydrolytic enzymes (cellulases) that can cleave the
(ß14) linkages between glucose units in cellulose. These
enzymes are produced by many cellulolytic microorganisms.
These microorganisms, such as Trichonympha, a symbiotic
protist that resides in the termite gut, allow the host to derive
energy from the glucose units stored in cellulose. Similarly,
cellulases produced by microorganisms living in the rumens of
cattle, sheep, and goats allow these animals to obtain energy
from cellulose present in soft grasses in the diet.
o It is useful for
production of
rubbers,tires,
plastic
o Extensive
interchanging H-
binding is absent.
40

Chitin
Chitin is a linear homopolysaccharide composed of N-acetylglucosamine
residues in (ß14) linkage . The only chemical difference from cellulose is
the replacement of the hydroxyl group at C-2 with an acetylated amino
group. Chitin also forms extended fibers similar to those of cellulose. Like
cellulose, chitin cannot be digested by enzymes found in vertebrates. Chitin is
the principal component of the hard exoskeletons of nearly a million species
of arthropods--insects, lobsters, and crabs, for example--and is probably the
second most abundant polysaccharide in nature.
o It has less water
content.
o It is present in
Algae, Fungi,
Yeast.
41

Folding of Homopolysaccharides
The folding of polysaccharides in three
dimensions follows the same principles as
those governing the folding of
polypeptides. Weak noncovalent
interactions, particularly hydrogen bonds
between -OH groups, are important in
stabilizing structures. In addition, rotation
about the  and  bonds adjacent to the
oxygen atoms of glycosidic bonds between
monosaccharide units have steric
constraints as they do for the comparable
bonds on either side of the  carbons in the
polypeptide backbone. Analogous to
polypeptides, polysaccharides can be
represented as a series of rigid pyranose
rings connected by an oxygen atom
bridging the rings. Certain conformations
are much more stable than others, as can be
shown on a Ramachandran-like plot of
energy as a function of  and  angles.
42

Helical Structure of Starch (Amylose)
The most stable three-dimensional structure for the (14) linked chains
of starch and glycogen is a tightly coiled helix (Fig.b). The helix is
stabilized by interchain hydrogen bonds. The glucose residues in the chain
are also able to form hydrogen bonds to the surrounding solvent, which
keep the polymer in solution. The average plane of each residue along the
amylose chain forms a 60˚ angle with the average plane of the preceding
residue (Fig.a), so the helical structure has six residues per turn. These
tightly coiled helical structures produce the dense granules of stored
starch or glycogen seen in many cells.
43

Interactions Between Cellulose Chains
For cellulose, the most stable conformation is
that in which each chair is turned 180˚ relative
to its neighbors, yielding a straight extended
chain (Fig. above). All -OH groups are
available for hydrogen bonding with
neighboring chains. With several chains lying
side by side, a stabilizing network of
interchain and intrachain hydrogen bonds
produces straight, stable supramolecular fibers
of great tensile strength. The water content
of cellulose fibers is low because extensive
interchain hydrogen bonding between
cellulose molecules satisfies their capacity for
hydrogen bond formation.
44

The Extracellular Matrix
The extracellular space in the tissues of multicellular animals is filled with
a gel-like material, the extracellular matrix (ECM), which holds cells
together and provides a porous pathway for the diffusion of nutrients and
oxygen to individual cells. The ECM that surrounds fibroblasts and other
connective tissue cells is composed of an interlocking meshwork of
heteropolysaccharides and fibrous proteins such as fibrillar collagens,
elastins, and fibronectins. These heteropolysaccharides, the
glycosaminoglycans, are a family of linear polymers composed of repeating
disaccharide units (next two slides). They are unique to animals and
bacteria and are not found in plants. One of the two monosaccharides is
always either N-acetylglucosamine or N-acetylgalactosamine. The other
monosaccharide is in most cases a uronic acid, usually D-glucuronic acid or
its 5-epimer, L-iduronic acid. Some glycosaminoglycans contain sulfate
groups attached to hydroxyl groups in ester linkage. The combination of
sulfate groups and the carboxylate groups of the uronic acids gives
glycosaminoglycans a very high density of negative charge, and an extended
rod-like structure in solution. Glycosaminoglycans are specifically
recognized by a number of proteins that bind them via electrostatic
interactions. As discussed in the 7B lecture slide file, the sulfated
glycosaminoglycans are attached to extracellular proteins to form the
proteoglycans.
46

Glycosaminoglycans (I)
The glycosaminoglycan hyaluronan (hyaluronic
acid) consists of alternating residues of D-
glucuronic acid and N-acetylglucosamine. A
single hyaluronan molecule contains up to
50,000 repeats of this disaccharide unit and has
a molecular weight of several million. It forms
clear, highly viscous solutions that serve as
lubricant in the synovial fluid of joints and give
the vitreous humor of the vertebrate eye its
jellylike consistency.
(The Greek hyalos means “glass”). Hyaluronan is also a component of the
ECM of cartilage and tendons. In many species, a hyaluronidase enzyme in
sperm hydrolyzes an outer glycosaminoglycan coat around the ovum,
allowing sperm entry.
Other glycosaminoglycans differ from hyaluronan in three respects: they
are generally much shorter polymers, they are covalently linked to specific
proteins forming proteoglycans, and one or both monomeric units differ
from those of hyaluronan. Chondroitin sulfate (Greek, chondros,
“cartilage”) is a polymer of repeating D-glucuronic acid and sulfated N-
acetylgalactosamine units. It contributes to the tensile strength of
cartilage, tendons, ligaments, and the wall of the aorta. 47

Glycosaminoglycans (II)
Keratin sulfates (Greek keras, “horn”) lack
uronic acid and their sulfate content is variable.
The species shown in Fig. 7-22 is a repeating
polymer of D-galactose and sulfated N-
acetylglucosamine residues. Keratin sulfates
are present in the cornea, cartilage, bone, and a
variety of horny structures formed of dead
cells: horn, hair, hoofs, nails, and claws.
Heparan sulfate (Greek hepar, “liver”) is
produced by all animal cells and contains
variable arrangements of sulfated and
nonsulfated sugars. The species shown in Fig.
is a repeating polymer of sulfated L-iduronate
and sulfated
D-glucosamine residues. The sulfated segments of the polymer allow it to
interact with a large number of proteins, including growth factors and
ECM components, as well as various enzymes and factors present in serum.
Heparin is a fractionated form of heparan sulfate that is a therapeutic
agent used to inhibit blood coagulation. Heparin binds to the protease
inhibitor antithrombin, and causes it to bind to and inhibit thrombin, a
protease essential to blood clotting. Heparin has the highest charge density
of any known biological macromolecule. 48

49
1) Hyaluronic acid - glassy and translucent
- lubricants in joints, cartilage, and tendons
- hyaluronidase in pathogenic bacteria and
sperm
2) Chondroitin sulfate
- cartilage, tendon, ligament, and walls of the
aorta
3) Dermatan sulfate
- skin, blood vessels, and heart valves
4) Keratan sulfate
- cornea, bone, horn, hair, hoofs, nails and claws
5) Heparin - natural anticoagulant made in mast
cells
- bind antithrombin, then bind and inhibit
thrombin

Glycoconjugate (complex saccharide)
Glycoconjugate
Sugar + lipid
Sugar + protein
Glycolipid
Lipopolysaccharide (LPS)
Glycoprotein
Proteoglycan
Glycoconjugates: carbohydrates covalently linked with
other chemical species.
•Glycoconjugates are very important compounds. They are
involved in cell-cell interactions, including cell-cell recognition,
and cell-matrix interactions.

52
Glycoproteins - oligosaccharides + proteins
- outer face of membrane ; extracellular
matrix, blood
- Golgi complexes ; secretory granules ;
lysosomes
Glycolipids – oligosaccharides + membrane lipids
- oligosaccharides: hydrophilic head groups
- specific sites for recognition
Glycoproteins: proteins that
contain oligosaccharide chains
(glycans) covalently attached to
their polypeptide side-chains.
Glycoproteins play essential roles
in the body. For instance, in the
immune system almost all of the
key molecules involved in the
immune response are
glycoproteins.
Glycolipids are carbohydrate-
attached lipids.
Their role is to provide energy and
also serve as markers for cellular
recognition.

Proteoglycans
glycosaminoglycans + proteins
A special type of glycoprotein with sugar
weighing about 95%.
- glycosaminoglycans: main site of biological
activity
- hydrogen bonding and electrostatic
interactions
- major components of connective tissues
 Component of the bacterial cell wall.
Proteoglycan superfamily: at least 30 types
 tissue organizers
 influence the development of
specialized tissues
 mediate activities of growth factors
 regulate extracellular assembly of
collagen fibrils
Basic proteoglycan unit:
core protein + trisaccharide linker +
glycosaminoglycan
It is Secreted into the extracellular matrix
(basal lamina)

54
Bacterial cell wall.
Proteoglycan structure of an
integral membrane protein

Four types of protein interactions
with S-domains of heparan sulfate

Carbohydrate moiety in glycoproteins are smaller but more structurally diverse
than the glycosaminoglycans of proteoglycans.
O-linked: -OH of Ser or Thr (GalNAc) ,, N-linked: amide nitrogen of Asn (GlcNAc)
Membrane protein: glycophorin A
Secreted proteins: immunoglobulins and certain hormones, lactalbumin,
ribonuclease, etc.
Proteins in lysosomes, Golgi complexes, and ER.
Glycoproteins have covalently attached oligosaccharides
Oligosaccharide
linkages in
glycoproteins

Functions of Carbohydrates
Carbohydrates participate in a wide range of functions:
 Carbohydrates are most abundant dietary source of energy for all
organisms.
 They supply energy and serve as storage form of energy.
 Carbohydrates such as glucose, fructose, starch, glycogen, etc.
provide energy for functioning of living organisms.
 Carbohydrates are utilized as raw materials for several
industries. For e.g., paper, plastics, textiles etc.
 Polysaccharides like cellulose act as chief structural material for
cell walls in plants.
 Carbohydrates participate in cellular functions such as cell
growth, adhesion and fertilization.
 Structural building blocks of cells
 Components of several metabolic pathways
 Recognition of cellular phenomena, such as cell recognition and
binding (e.g., by other cells, hormones, and viruses)
57

 Monosaccharides serve as a major fuel for cells and as raw
material for building molecules.
 Though often drawn as a linear skeleton, in aqueous
solutions they form rings.
 Chitin, another structural polysaccharide, is found in the
exoskeleton of arthropods.
 Chitin also provides structural support for the cell walls of
many fungi.
 Chitin can be used as surgical thread.
 Starch, a storage polysaccharide of plants, consists entirely
of glucose monomers.
 Plants store surplus starch as granules within chloroplasts
and other plastids .
 Glycogen is a storage polysaccharide in animals.
 Humans and other vertebrates store glycogen mainly in liver
and muscle cells.

 Cellulose is a major component of the tough wall of plant cells.
 Enzymes that digest starch by hydrolyzing alpha linkages can’t
hydrolyze beta linkages in cellulose.
 Cellulose in human food passes through the digestive tract as insoluble
fiber.
 Some microbes use enzymes to digest cellulose.
 Many herbivores, from cows to termites, have symbiotic relationships
with these microbes.
 As a major energy source for living organisms (glucose is a principal
energy source in animal and plants)
 As a means of transporting energy ( exp: sucrose in plant tissues)
 As a structural material ( cellulose in plants, chitin in insects, building
blocks of nucleotides).
 As a precursor for other biomolecules (purine, pyrimide)
 Carbohydrates are the most abundant biomolecules in nature, having a
direct link between solar energy and the chemical bond energy in living
organisms.
 Source of rapid energy production

Note: Some things are added from different sources other than Lehniger’s biochemistry PDF Book.So Don’t
worry,because those things are added just for reader’s ease.

Carbohydrates

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