Download L4 - Complex Carbohydrates

Complex Carbohydrates:
Disaccharides, Oligosaccharides and
Polysaccharides
1.Draw the Haworth Projections for the
following monosaccharides:
β-D-taltopyranose
β-L-arabinofuranose
α-D-fructofuranose
Review:
Glycoside
Glycoside Formation Cyclic
monosaccharide hemiacetals and
hemiketals react with alcohols to form
acetals and ketals, referred to as
glycosides.
The dehydration of the hemiacetal or
hemiketal forms what is called a
glycosidic linkage.
A glycosidic linkage is an ether link
between a carbohydrate (poly hydroxyl,
or glycol compound) and another carbon
molecule.
• The unstable hemiacetal is
then converted into an acetal
by the substitution of the
newly formed hydroxyl (-O-H)
group by a new ether from a
second alcohol (R-O-H), also
producing water (H2O).
Disaccharides
Two monosaccharide units can also be linked
together by acetal or ketal glycosidic linkages
forming disaccharides.
The glycosidic linkages between
monosaccharide units usually form between
the anomeric carbon and either carbon 4 or 6
on a second monosaccharide unit.
1
• The glycosidic
linkages, or bonds,
are named by the
anomer of the acetal
carbon and the bond
position on the
second sugar.
Common Glycosidic Bonds:
α(1-4)
α(1-6)
See Reference
β(1-4)
β(1-6)
• Maltose, Lactose and Cellobios are all three
reducible sugars because one of the
monosaccharide units in the disaccharide
structure is still in equilibrium with its straight
chained form.
Important Disaccharides
Maltose
•Two α glucose units linked α(1→
→4)
•Formed during the digestion of starch to glucose
Lactose
•β
β Glactose and α glucose units linked β(1→
→4)
•Found in mammalian milk
Cellobios
•Two β glucose units linked β(1→
→4)
•Derived from partial hydrolysis of cellulose
Draw Each disaccharide molecule
Important Disaccharides, Con’t
Sucrose
•β
β Fructose and α glucose units linked by a rare α
(1 → 2) glycosidic linkage involving the anomeric
carbon of both monosaccharide units
•Found in plants such as
sugar cane, sugar beets,
HO
etc…
•Not a reducing sugar due to
the inability of either
monosaccharide unit to exist
HOCH
in a straight chained form
•Produces a negative
CH OH
Benedict’s Test.
2
• All three disaccharides would therefore produce
a positive test for aldehydes with Benedict’s
reagent
2
Polysaccharides
Hydrolysis of Glycosidic Linkages
•The hydrolysis of glycosidic linkages produces
simpler carbohydrates from di- oligo- and
polysaccharides.
•It is the Reverse of Glycoside Formation
H2 O
•Specific enzymes hydrolyze specific biological
compounds and are named by adding an (-ase)
suffix to the name of the compound hydrolyzed
• long polymer chains of monosaccharide units
bonded by glycosidic linkages
• Chains between 3 and 10 units long are called
oligosaccharides.
• Chains longer than 10 units are called simply
polysaccharides
• Chains can
be branched
or
unbranched
•for example: Sucrose is hydrolyzed by enzymes
called sucrase in yeast cells.
DEMO
2
Important Polysaccharides
Amylose:
Starch
• A polymer consisting of α-linked
glucose units generally existing in two forms:
– Amylose (unbranched)
– Amylopectin (Branched)
• Major storage sugar for
plants
• Complexes with iodine to
form a dark blue color
• Hydrolyized by enzymes
produced by animals
Amylopectin:
Monosaccharides:
# of Monomer Units:
Glycosidic Linkages:
Structure:
α-D-glucopyrnose
3-3000
α(1-4)
Linear Spiral
Important Polysaccharides, Con’t.
Glycogen (animal starch)
Monosaccharides:
# of Monomer Units:
Glycosidic Linkages:
Structure:
α-D-glucopyrnose
3-3000
24-30 between branch points
α(1-4) main chain & branches
α(1-6) between chain and branches
Linear Branched
Glycogen:
•a polymer of α-linked glucose units.
•Used to store glucose, especially in the
liver and muscles.
•Structurally similar to amylopectin
•produces d-glucose upon hydrolysis in
animals for maintaining normal blood
sugar levels and for providing muscles
with energy.
Important Polysaccharides, Con’t.
Cellulose
Monosaccharides:
α-D-glucopyrnose
# of Monomer Units:
3-3000
8-12 between branch points
Glycosidic Linkages:
α(1-4) main chain & branches
α(1-6) between chain and branches
Structure:
Linear Branched
•A polymer of β-linked
glucose units
•The most important
structural polysaccharide
•Found in plant cell walls
•Can not be digested by animals, a
constituent of dietary fiber.
–herbivores contain bacteria in
their stomachs that produce the
enzymes required to hydrolyze
cellulose.
3
Important Polysaccharides, Con’t.
Cellulose:
Chitin
•A polymer of β-linked N-acetyl
glucosamine units
•A structural polysaccharide
•Major component of crustacean and
insect exoskeletons
Monosaccharides:
# of Monomer Units:
Glycosidic Linkages:
Structure:
β-D-glucopyrnose
300-3000
β(1-4)
Extened Straight Chains
Chitin:
Monosaccharides:
# of Monomer Units:
Glycosidic Linkages:
Structure:
β-N-acetylglucosamine
100-2500
β(1-4)
Ridgid Linear Chains
Table 17-4, p.554
4