Carbohydrates
Polysaccharides
Dr. Nikhat Siddiqi
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Contents
• Polysaccharides – Classification
• Homoglycans
• Heteroglycans
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Joining of monosaccharides
• Monosaccharides can be joined to form disaccharides,
oligosaccharides, and polysaccharides.
• Important disaccharides include lactose (galactose +
glucose), sucrose (glucose + fructose), and maltose
(glucose + glucose).
• Important polysaccharides include branched glycogen
(from animal sources) and starch (plant sources) and
unbranched cellulose (plant sources); each is a polymer
of glucose.
• The bonds that link sugars are called glycosidic bonds.
These are formed by enzymes known as
glycosyltransferases.
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Naming glycosidic bonds
• Glycosidic bonds between sugars are named
according to the numbers of the connected
carbons, and also with regard to the position
of the anomeric hydroxyl group of the sugar
involved in the bond.
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Naming glycosidic bonds
• If this anomeric hydroxyl
is in the α configuration,
the linkage is an α-bond.
• If it is in the β
configuration, the linkage
is a β-bond.
• Lactose, for example, is
synthesized by forming a
glycosidic bond between
carbon 1 of β-galactose
and carbon 4 of glucose.
The linkage is, therefore,
a β(1→4) glycosidic bond.
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Polysaccharides
• Polysaccharides are also called as glycans.
• Made up of large number of monosaccharides
linked by glycosidic linkages.
• Smaller glycans are called as oligosaccharides
(10-15 monomers) found attached to
popypeptides in glycoproteins and some
glycolipids.
• Found in membrane and secretary proteins.
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Polysaccharides - Classification
Homoglycans
Polysaccharides
Heteroglycans
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Polysaccharides - Classification
Structural
Polysaccharides
Polysaccharides
Storage
Polysaccharides
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Homoglycans
• Found in starch, glycogen, cellulose and chitin.
• Starch, glycogen, cellulose give D-glucose
when hydrolysed.
• Starch –energy storage molecule in plants.
• Glycogen –energy storage molecule in
animals.
• Chitin – component of exoskeleton of insects,
cell wall of fungi, yields glucose derivative Nacetylglucosamine when hydrolyzed.
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Starch (Storage Polysacchardes)
• Significant source of carbohydrate in human
diet.
• Source- potatoes, rice, wheat, corn.
Amylose
Starch
Amylopetin
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Amylose
• Composed of long,
unbranched chains of Sglucose residues that
are linked by (1,4)
glycosidic bonds.
• Have one reducing end
in which the ring can
open to form free
aldehyde group with
reducing properties.
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Amylose
• Contains several thousand glucose residues
has molecular weight 150,000 to 600,000.
• Linear amylose molecule forms tight helices.
• Gives blue colour with iodine due to
interaction between iodine molecules and the
helically arranged glucose rsidues.
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Amylopectin
• is branched polymer
containing (1,4) and
(1,6)glycosidic
linkages.
• The (1,6) points may
occur every 20 to 35
residues and prevent
helix formation.
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• Starch digestion begins in the mouth where
the salivary enzyme - amylase initiates
hydrolysis of glycosidic bonds.
• Digestion continues in the small intestine
where pancreatic -amylase hydrolyzes all
the (1,4) glycosidic bonds except the branch
points.
• The products of - amylase maltose,
trisaccharide maltotriose and  limit dextrin.
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Starch - Structure
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Glycogen (Storage Polysaccharides)
• Storage carbohydrate in vertebrates.
• Found in liver and skeletal muscle.
• Structure similar to amylopectin except that it
has more branch point possibly at every
fourth glucose residue.
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Glycogen-Structure
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Glycogen- Structure
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Cellulose (Structural Polysaccharides)
• Polymer of D-glucopyranose residues linked by
(1,4) glycosidic bonds. Unbranched
• Structural polysaccharide in plants.
• Pairs of unbranched cellulose molecules may
contain as many as 12,000 glucose units each
held together by hydrogen bonding to form
sheet like strips called microfibrils. These
structures are found in primary and secondary
cell walls of plants.
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• The ability to digest cellulose is found in
microorganism which contain the enzyme
cellulase.
• Cellulose can be hydrolyzed to its constituent
glucose units by microorganisms that inhabit
the digestive tract of termites and ruminants.
• Cellulose makes up dietary fiber.
• Paper, wood, textiles are some of cellulose
containing products.
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Cellulose - Structure
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Chitin (Structural Polysaccharide)
• Chitin is an unbranched polymer of N-Acetyl-Dglucosamine.
• It is found in fungi and is the principal component
of arthropod and lower animal exoskeletons, e.g.,
insect, crab, and shrimp shells.
• It may be regarded as a derivative of cellulose, in
which the hydroxyl groups of the second carbon
of each glucose unit have been replaced with
acetamido (-NH(C=O)CH3) groups.
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Chitin - Structure
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Polysaccharides
Heteroglycans
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Heteroglycans
• High molecular weight carbohydrate polymers
that contain more than one kind of
monosaccharides.
• Major classes found in animals are N and Olinked glycans attached to proteins.
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Glycosaminoglycans (GAGs)
• Glycosaminoglycans (GAGs) are large
complexes of negatively charged
heteropolysaccharide chains.
• They are generally associated with a small
amount of protein, forming proteoglycans,
which typically consist of over 95%
carbohydrate.
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Glycosaminoglycans (GAGs)
• Glycosaminoglycans have the special ability to
bind large amounts of water, thereby producing
the gel-like matrix that forms the basis of the
body's ground substance, which, along with
fibrous components such as collagen, make up
the extracellular matrix.
• The viscous, lubricating properties of mucous
secretions also result from the presence of
glycosaminoglycans, which led to the original
naming of these compounds as
mucopolysaccharides.
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Structure of Glycosaminoglycans
• Glycosaminoglycans are
long, unbranched,
heteropolysaccharide
chains generally
composed of a
repeating disaccharide
unit [acidic sugar–
amino sugar]n.
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Structure of Glycosaminoglycans
• The amino sugar is either Dglucosamine or Dgalactosamine, in which the
amino group is usually
acetylated, thus eliminating
its positive charge.
• The amino sugar may also
be sulfated on carbon 4 or 6
or on a nonacetylated
nitrogen.
• The acidic sugar is either Dglucuronic acid or its
carbon-5 epimer, L-iduronic
acid.
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Structure of Glycosaminoglycans
• A single exception is keratan sulfate, in which
galactose rather than an acidic sugar is
present.
• These acidic sugars contain carboxyl groups
that are negatively charged at physiologic pH
and, together with the sulfate groups, give
glycosaminoglycans their strongly negative
nature.
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Some monosaccharide units found in
glycosaminoglycans.
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Functions of GAGs
• Because of their large number of negative
charges, these heteropolysaccharide chains tend
to be extended in solution.
• They repel each other, and are surrounded by a
shell of water molecules.
• When brought together, they “slip” past each
other, much as two magnets with the same
polarity seem to slip past each other.
• This produces the “slippery” consistency of
mucous secretions and synovial fluid.
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Functions of GAGs---cont
• When a solution of glycosaminoglycans is
compressed, the water is “squeezed out” and the
glycosaminoglycans are forced to occupy a
smaller volume.
• When the compression is released, the
glycosaminoglycans spring back to their original,
hydrated volume because of the repulsion of
their negative charges.
• This property contributes to the resilience of
synovial fluid and the vitreous humor of the eye.
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Classification of the
glycosaminoglycans
• The six major classes of glycosaminoglycans
are divided according to monomeric
composition, type of glycosidic linkages, and
degree and location of sulfate units.
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Structure and distribution of
glycosaminoglycans (GAGs).
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Hyaluronic Acid
• Hyaluronate molecules may consist of as
many as 25,000 disaccharide units, with
molecular weights of up to 107.
• Hyaluronates are important components of
the vitreous humor in the eye and of synovial
fluid, the lubricant fluid of joints in the body.
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• The chondroitins and keratan sulfate are
found in tendons, cartilage, and other
connective tissue, whereas dermatan sulfate,
as its name implies, is a component of the
extracellular matrix of skin.
• Heparin, with the highest net negative charge
of the disaccharides shown, is a natural
anticoagulant substance.
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Proteoglycans
• High carbohydrate content (about 95%).
• Occur on cell surfaces or are secreted into
extracellular matrix.
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Structure of proteoglycans
• All of the glycosaminoglycans, except
hyaluronic acid, are found covalently attached
to protein, forming proteoglycan monomers.
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Structure of proteoglycan monomers
• A proteoglycan monomer found in cartilage consists of
a core protein to which the linear glycosaminoglycan
chains are covalently attached.
• These chains, which may each be composed of more
than 100 monosaccharides, extend out from the core
protein, and remain separated from each other
because of charge repulsion.
• The resulting structure resembles a “bottle brush”. In
cartilage proteoglycan, the species of
glycosaminoglycans include chondroitin sulfate and
keratan sulfate.
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“Bottle-brush” model of a cartilage
proteoglycan monomer.
• A proteoglycan
monomer found in
cartilage consists of a
core protein to which
the linear
glycosaminoglycan
chains are covalently
attached.
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Linkage region of glycosamino-glycans
• Linkage between the
carbohydrate chain and
the protein: This linkage is
most commonly through
a trihexoside (galactosegalactose-xylose) and a
serine residue,
respectively.
• An O-glycosidic bond is
formed between the
xylose and the hydroxyl
group of the serine.
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Proteoglycan aggregates
• The proteoglycan
monomers associate with a
molecule of hyaluronic acid
to form proteoglycan
aggregates. The association
is not covalent, but occurs
primarily through ionic
interactions between the
core protein and the
hyaluronic acid. The
association is stabilized by
additional small proteins
called link proteins .
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Proteoglycans
• Proteoglycans consist of
a protein core (brown)
and one or more
covalently attached
glycosaminoglycan
chains ([blue].
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Examples of proteoglycans
• Examples include syndecans, glycipcans and
affrecans
• The syndecans are a class of heparan sulfate
and chondroitin sulfate containing
proteoglycans in which the core protein is a
transmembrane protein.
• Aggregans is found in cartilage.
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Mucopolysaccharidoses
• The mucopolysaccharidoses are hereditary disorders
(1:25,000 births) that are clinically progressive.
• They are characterized by accumulation of
glycosaminoglycans in various tissues, causing varied
symptoms, such as skeletal and extracellular matrix
deformities, and mental retardation.
• Mucopolysaccharidoses are caused by a deficiency of
any one of the lysosomal hydrolases normally involved
in the degradation of heparan sulfate and/or dermatan
sulfate .
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Functions of Proteoglcans
• Organizing extracellular matrix.
• Membrane bound syndecans, glycipcans bind
to specific signal molecules like growth factors
involved in cell cycle regulation.
• Because of their vast number of polyionic GAG
chains, the aggrecans trap large volume of
water.
• Give strength, flexibility to cartilage and
tensile strength to collagen fibers.
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Glycoproteins
• Proteins that are linked covalantly to
carbohydrates through N- or 0- linkages.
• Carbohydrate content varies from 1% to more
than 85% of total weight.
• Carbohydrates include monosaccharides or
disaccharides such as those attached to
collagen or branched oligosaccharides found
on plasma glycoproteins.
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Polysaccharides
• Two broad classed viz., N-linked and O-linked.
• N-linked are attached to polypeptides by an Nglycosidic bond with the side chain amide
group of amino acid asparagine.
• O-linked are attached to polypeptides by side
chain hydroxyl group of the amino acid serine
or threonine in a polypeptide chain or the
hydroxyl group of membrane lipids.
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Polysaccharides linked to polypeptides
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Differences between Glycoproteins
and Protoglycans
• They differ from the proteoglycans (which
might be considered a special case of
glycoproteins) in that the length of the
glycoprotein's carbohydrate chain is relatively
short (usually 2–10 sugar residues in length,
although they can be longer).
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Differences between Glycoproteins
and Protoglycans ---cont
• In addition, whereas glycosaminoglycans have
diglucosyl repeat units, the carbohydrates of
glycoproteins do not have serial repeats.
• The glycoprotein carbohydrate chains are
often branched instead of linear, and may or
may not be negatively charged.
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• Glycoproteins contain highly variable amounts
of carbohydrate.
• For example, immunoglobulin IgG, contains
less than 4% of its mass as carbohydrate,
whereas human gastric glycoprotein (mucin)
contains more than 80% carbohydrate.
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Functions of glycoproteins
• Membrane-bound glycoproteins participate in a
broad range of cellular phenomena, including cell
surface recognition (by other cells, hormones
(insulin receptors), and viruses),
• cell surface antigenicity (such as the blood group
antigens),
• as components of the extracellular matrix and
• of the mucins of the gastrointestinal and
urogenital tracts, where they act as protective
biologic lubricants.
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Functions of Glycoproteins
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• Sialic acid residues are responsible for high
viscosity and luricating properties of saliva.
• Cellular adhesion eg selectins (transient cellcell interaction), intergins (cell attachment to
components of extracellular matrix) and
cadherins (calcium dependent binding of cells
to each other within tissue.
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