Download Notes 32 Polysaccharides

Common Modifications to
Monosaccharides
Deoxy sugars
Amino sugars
Glycosides (acetal)
Deoxy Sugar
Amino Sugar
Glucosamine
H OH
HO
HO
HO
H
H
OH
NH2
H
Formation of Glycosides Acetals
Glycoside: a carbohydrate in which the -OH of the
anomeric carbon is replaced by -OR
A monosaccharide hemiacetal can react with a
second molecule of an alcohol to form an acetal
CH3OH
O
OH
H+
O
OCH3
A „glycoside‟ bond
Glycosides
• Glycoside bond: the bond from the anomeric
carbon of the glycoside to an -OR group.
• Unlike cyclic hemiacetals, cyclic acetals are
not in equilibrium with their open chain
carbonyl-containing forms.
• Glycosides do NOT undergo mutarotation.
Naming Glycosides
• List the name of the alkyl or aryl group
attached to oxygen followed by the name of
the carbohydrate with the ending -e
replaced by -ide
– methyl -D-glucopyranoside
– methyl -D-ribofuranoside
Glucopyranoside
CH2 OH
O OCH3 ( )
H
H
OH H
H
HO
H
OH
Haworth projection
HO
HO
CH2 OH
O
OCH3 ( )
OH
Chair conformation
Methyl -D-glucopyranoside
(methyl -D-glucoside)
Disaccharides
Maltose
Lactose
Sucrose
Maltose
• From malt, the juice of sprouted barley and
other cereal grains
CH2OH
HO
HO
O
-1,4-glycoside bond
CH2OH
OH
O
HO
O
OH
OH
-maltose because
this -OH is beta
-Maltose
Lactose
The principle sugar present in milk
about 5% - 8% in human milk, 4% - 5% in cow‟s
milk
OH
-1,4-glycoside
bond
CH2OH
O
HO
OH
D-galactopyranose
CH2OH
O
HO
O
OH
OH
D-glucopyranose
-lactose because
this OH is beta
-Lactose
Sucrose
• Table sugar, obtained from the juice of
sugar cane and sugar beet
CH2OH
HO
HO
-D-glucopyranose
-2,1-glycoside
bond
O
OH
-1,2-glycoside
bond
O
OH
O
CH2OH
CH2OH
OH
-D-fructofuranose
Sucrose
N-Glycosides
• The anomeric carbon of a cyclic hemiacetal
undergoes reaction with the N-H group of an
amine to form an N-glycoside
• N-glycosides of the following purine and pyrimidine
bases are structural units of nucleic acids
O
HN
O
O
CH 3
HN
N
H
Uracil
O
NH 2
N
H
Thymine
N
O
N
H
Cytosine
N-Glycosides
O
N
HN
H 2N
N
NH2
N
N
H
Guanine
O
HOCH2
NH 2
N
N
Adenine
N
N
H
N
a -N-glycoside
bond
O
H
H
H
H
HO
OH
anomeric
carbon
Formation of N-Glycosides
(Nucleosides)
• For example, reaction between -D-ribofuranose
and cytosine produces water and uridine, one of
NH2
the structural units
of
RNA:
NH2
HOCH2
OH
O
N
+
OH
OH
-D-Ribofuranose
- H2O
O
N
H
Cytosine
N
HOCH2
OH
O
O
N
-N-glycoside bond
OH
Uridine
anomeric
carbon
Disaccharides
• Three naturally occurring glycosidic linkages:
• 1-4‟ link: The anomeric carbon is bonded
to oxygen on C4 of second sugar.
• 1-6‟ link: The anomeric carbon is bonded to
oxygen on C6 of second sugar.
• 1-1‟ link: The anomeric carbons of the two
sugars are bonded through an oxygen.
Cellobiose
• Two glucose units linked 1-4‟.
• Disaccharide of cellulose.
• A mutarotating, reducing sugar.
=>
Maltose
Two glucose units linked 1-4‟.
=>
Lactose
• Galactose + glucose linked 1-4‟.
• “Milk sugar.”
=>
Gentiobiose
• Two glucose units linked 1-6‟.
• Rare for disaccharides, but commonly seen
as branch point in carbohydrates.
=>
Sucrose
• Glucose + fructose, linked 1-1‟
• Nonreducing sugar
=>
Polysaccharides
• Polysaccharides are chains of five or more monosaccharide:
–Starch – glucose polymer that is the plant storage carbohydrate
–Glycogen – glucose polymer that is the animal storage carbohydrate
–Cellulose – glucose polymer that is a major component of the cell
wall in plants & algae.
–Agar – natural component of certain seaweed polymer of
galactose & sulfur containing carbohydrates.
–Chitin – polymer of glucosamine (an amino sugar), found
in the exoskeleton of bugs.
Starch
• Starch is used for energy storage in plants
• Two types: amylose and amylopectin. On complete
hydrolysis each type gives only D-glucose
• Amylose: is composed of continuous, unbranched
chains of up to 4000 D-glucose units joined by
a-1,4-glycoside bonds
• Amylopectin: is a highly branched polymer of D-
glucose. Chains consist of 24-30 units of Dglucose joined by -1,4-glycoside bonds and
branches created by -1,6-glycoside bonds
Amylopectin
CH2OH
O
HO
CH2OH
O
HO
O
OH
O
O
OH
CH2
O
O
HO
OH
CH2OH
O
HO
O
OH
CH2OH
O
O
HO
OH
O
Glycogen
• The reserve carbohydrate for animals
• A nonlinear polymer of D-glucose units joined
by -1,4- and -1,6-glycoside bonds bonds.
• The total amount of glycogen in the body of a
well-nourished adult is about 350 g (about 3/4
of a pound) divided almost equally between
liver and muscle.
Cellulose
• Cellulose is a linear polymer of D-glucose
units joined by -1,4-glycoside bonds.
• Average molecular weight of 400,000,
corresponds to approximately 2800
D-glucose units per molecule.
Cellulose
CH2OH
O
HO
O
OH
OH
HO
O
CH2OH
O
O
CH2OH
O
HO
O
OH
Polysaccharides Digestion
Polymers of Glucose
Starch is digestable
Cellulose is not
digestable by humans
Modification of Cellulose
• Cellulose Nitrate guncotton
•Pyroxylin Partially nitrated photographic film
•Cellulose Acetate film
Cellulose fibre - Rayon
NaOH
Cellulose
S
OH
C
Cellulose
O-Na+
S
S
Cellulose
OCS-Na+
Sodium salt of a xanthate ester
H+
Cellulose
OH
spinneret
Cellulose fibre
Biological Sugars and reactions
Membrane Carbohydrates
• Membranes of animal plasma cells have large
numbers of bound small carbohydrates to them.
•these membrane-bound carbohydrates are part of
the mechanism by which cell types recognize each
other; they act as antigenic determinants
•among the first discovered of these antigenic
determinants are the blood group substances
ABO Blood Classification
• at the cellular level, the biochemical basis for
this classification is a group of relatively small
membrane-bound carbohydrates
ABO Blood Classification
•In the ABO system, individuals are classified
according to four blood types: A, B, AB, and O
D-galactose in
type B blood
NAGal
-1,4-)
Gal
-1,3-)
NAGlu
-1-)
Cell membrane
of erythrocyte
-1,2-)
Fuc
NAGal = N-acetyl-D-galactosamine
missing in
type O blood
Gal = D-galactose
NAGlu = N-acetyl-D-glucosamine
Fuc = L-fucose
„Chemstrip Kit‟
Blood glucose test for diabetics
Based on reaction of o-toluidine with glucose
H3C
CH N
CHO
H
HO
OH
H3C
HO
H
H
OH
H
OH
CH2OH
H
H2N
OH
H
H
OH
H
OH
CH2OH
Glucose Assay
•Diabetes: A common analytical procedure in the
clinical chemistry laboratory is the determination
of glucose in blood, urine, or other biological fluid
• The o-toluidine test is applied directly to serum,
plasma, cerebrospinal fluid, and urine
samples as small as 20 L (microliters) can be used.
• glucose reacts with 2-methylaniline (o-toluidine) in the
presence of acetic acid to give an imine which has a
blue-green color
–the intensity of the absorption at 625 nm is proportional to
the glucose concentration
• Galactose, mannose, and to a lesser extent lactose and
xylose also react with o-toluidine to give colored imines
and, therefore, have the potential for false positive.
Glucose Assay
• The glucose oxidase method is completely
specific for D-glucose
HO
HO
CH2 OH
O
OH
glucose
+ O2 + H 2 O oxidase
OH
 - D-Glucopyranose
H2 O2
Hydrogen
peroxide
+
H
HO
H
H
CO2 H
OH
H
OH
OH
CH2 OH
D-Gluconic acid
Glucose Assay
• O2 is reduced to hydrogen peroxide H2O2
• the concentration of H2O2 is proportional to the
concentration of glucose in the sample
• in one procedure, hydrogen peroxide is used to oxidize
o-toluidine to a colored product, whose concentration
is determined spectrophotometrically
o-toluidine
+
H2 O2
peroxidase
colored product
+
H2 O
Vitamin C - A monosaccharide?
• Vitamin C, vital for life is a necessary part of our
diet because we cannot synthesize it. (Most
plants and animals except primates and guinea
pigs can make their own Vitamin C).
•It is needed to maintain health of dentine, cartilage,
connective tissue and bone.
•Recommended daily allowance ~45mg for adults
(60mg if pregnant, 80mg if lactating).
Ascorbic Acid (Vitamin C)
• L-Ascorbic acid (vitamin C) is synthesized both
biochemically and industrially from D-glucose
CH2OH
CH2OH
HO
HO
H
O
OH
-D-Glucopyranose
OH
O
OH
HO
L-ascorbic acid
Vitamin C
O
OH
Biosynthesis from Glucose
Glycocalyx
The outer viscous covering of fibers
extending from a bacterium
composition: The glycocalyx is usually a viscous
polysaccharide and polypeptide slime.
Glycocalyx of
Intestinal
Epithelium
Note that some
carbohydrates are
covalently attached to
membrane
components, while
others are secreted as
extracellular matrix
Fig 16, The Cell, D.W.
Fawcett (1981)
Glycocalyx of Lymphocyte
Diagram of Glycocalyx
Cellulose
• Polymer of D-glucose, found in plants.
• Mammals lack the -glycosidase enzyme.
=>
Amylose
• Soluble starch, polymer of D-glucose.
• Starch-iodide complex, deep blue.
=>
Amylopectin
Branched, insoluble fraction of starch.
Glycogen
• Glucose polymer, similar to amylopectin, but
even more highly branched.
•Energy storage in muscle tissue and liver.
•The many branched ends provide a quick
means of putting glucose into the blood.
Chitin
• Polymer of N-acetylglucosamine.
•Exoskeleton of insects.
=>
Ribonucleosides
A -D-ribofuranoside bonded to a heterocyclic
base at the anomeric carbon.
=>
Ribonucleotides
Add phosphate at 5‟ carbon.
Nucleic Acids
• Polymer of ribofuranoside rings
linked by phosphate esters.
•Each ribose is bonded to a base.
•Ribonucleic acid (RNA)
•Deoxyribonucleic acid (DNA)
=>
Structure of RNA
=>
Structure of DNA
• -D-2-deoxyribofuranose is the sugar.
•Heterocyclic bases are cytosine, thymine
(instead of uracil), adenine, and guanine.
Linked by phosphate ester groups to form
the primary structure.
Base Pairings
=>
Double Helix of DNA
•Described by Watson
and Crick, 1953.
• Two complementary
polynucleotide chains
are coiled into a helix.
DNA Replication
=>
Additional Nucleotides
• Adenosine monophosphate (AMP), a
regulatory hormone.
• Nicotinamide adenine dinucleotide
(NAD), a coenzyme.
• Adenosine triphosphate (ATP), an energy source.