Download General Biochemistry Chemistry of Polysaccharides

General Biochemistry
BIOC 201
Chapter IV
Chemistry of
Polysaccharides
Objectives
1. The main objective of this chapter is getting the
student to know the polysaccharides
constituents of human food.
2. The chapter is designed to familiarize the
student with the basic biochemical classification
of polysaccharides.
3. To provide the student with the occurrence of
different polysaccharides in nature.
Most carbohydrates found in nature occur as
polysaccharide polymers of medium to high
molecular weight.
Polysaccharides, also called glycans
They differ from each other in:
the identity of their monosaccharide units
in the length of their chains
in the types of bonds linking the units
and in the degree of branching
Polysaccharides
Homopolysaccharides
Heteropolysaccharides
(same sugar units)
(sugar units + other groups)
Homopolysaccharides
(same sugar units)
Glucosans
Fructosans
Only glucose
Only fructose
Starch, Dextrin
Glycogen, Cellulose
Inulin
Heteropolysaccharides
(sugar units + other groups)
Neutral
Acidic
Some Homopolysaccharides Are Stored
Forms of Fuel
Starch and dextrin
* present only in plants.
* formed of thousands of
D-glucose units linked by
α 1-4 and α 1-6 glucosidic
linkage. (Branched)
Glycogen is the main
storage in humans (Liver and
Muscles)
Glycogen and starch ingested in the diet are
hydrolyzed by α amylases, enzymes in saliva and
intestinal secretions that break (α 1-4 ) glycosidic
bonds between glucose units.
Most animals cannot use cellulose as a fuel source,
because they lack an enzyme to hydrolyze the (β 14 ) linkages.
Dextrans are bacterial and yeast polysaccharides
made up of (α 1-6 )-linked poly-D-glucose; all have
(α 1-3) branches, and some also have (α 1-2) or
(α 1-4) branches. Dental plaque, formed by bacteria
growing on the surface of teeth, is rich in dextrans.
Some Homopolysaccharides Serve
Structural Roles
Cellulose, a fibrous, tough, water-insoluble
substance, is found in the cell walls of
plants. Cellulose constitutes much of the
mass of wood, and cotton is almost pure
cellulose.
The cellulose molecule is a linear,
unbranched homopolysaccharide, consisting
of 10,000 to 15,000 D-glucose units.
Cellulose cont.,
In cellulose the glucose residues have the
β configuration
Not digested in
our gut
Stimulate intestinal
peristalsis and prevent
constipation
Amylopectin
Cellulose breakdown by fungi
Chitin
Chitin is a linear homopolysaccharide
composed of N-acetylglucosamine residues
in β linkage
The only chemical difference from cellulose
is the replacement of the hydroxyl group at
C-2 with an acetylated amino group
Chitin
Chitin forms extended fibers similar to those
of cellulose, and like cellulose cannot be
digested by 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, next to cellulose,
in nature
Chitin
Inulin is a homopolysaccharide of fructose
(fructosan) found in tubers and roots of
artichokes.
It is readily soluble in water.
Heteropolysaccharides
Heteropolysaccharides provide extracellular
support for organisms of all kingdoms.
Rigid layer of the
bacterial cell envelope
(the peptidoglycan)
Peptidoglycan
Peptidoglycan confers mechanical support
and prevents bacteria from bursting in
response to their high internal osmotic
pressure.
Heteropolysaccharides
(sugar units + other groups)
(Mucopolysaccharide)
Neutral
Acidic
Mucopolysaccharides
Complex carbohydrates
characterized by their content of amino
sugars and uronic acids.
When attached to a protein molecule, the
result is a proteoglycan.
Proteoglycans provide the ground or
packing substance of connective tissues.
Neutral Mucopolysaccharides
Almost all the plasma proteins of humans
except albumin are glycoproteins.
A number of the blood group substances are
glycoproteins, whereas others are
glycosphingolipids.
Antigens, eg, ABO blood group substances
are Neutral Mucopolysaccharides
Acidic Mucopolysaccharides
Sulfate free (Hyaluronic
(Hyaluronic acid)
acid)
Ground substance of connective tissues
Hydrolysis = acetic acid, glucosamine & glucuronic acid
Sulfate containing Acidic Mucopolysaccharides
Chnodroitin sulfate
contributes to the tensile strength of cartilage,
tendons, ligaments, and the walls of the aorta.
(Cartilages and bones)
Hydrolysis = acetic acid
Galactose amine
Sulfuric acid
Glucuronic acid
Sulfate containing Acidic Mucopolysaccharides
Heparin is a natural anticoagulant made in mast
cells (a type of leukocyte)
Released into the blood, where it inhibits blood
coagulation by binding to the protein antithrombin
and inhibit thrombin, a protease essential to blood
clotting.
Hydrolysis = glucosamine
Sulfuric acid
Glucuronic acid