Chapter 7
Carbohydrates and Glycobiology
Carbohydrates are everywhere
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Sucrose
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glazed donuts
Sucrose (table sugar)
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What are carbohydrates?
•Carbo-hydrates are a group of organic
compound that can usually represented
as (CH2O)n. Glucose, fructose, and
galactose are all carbohydrates.
a-D-Galactose
Classification of carbohydrate
–carbon number
–L- or D- isomers
–pyranose or furanose
–α or β anomers
–ketose or aldose
Carbon number
•In addition to simple hexose such as
glucose, galactose, and mannose,
there are a number of sugar
derivatives in which a hydroxyl group
in the parent compound is replaced
with another substituent, or carbon
atom is oxidized to a carboxyl group.
•Therefore, researchers must give each
carbon of a sugar a number.
H
H
HO
H
H
aldose
Carbon number
O
1
CH2OH
C1
2
H C O
C2 OH
3
HO C H
C H
3
4
C4 OH
H C OH
5
H C OH
C5 OH
6
CH2OH
CH2OH
ketose
6
L- and D- isomers
[a]25D=+8.7˚
[a]25D=-8.7˚
CHO
|
H -C- OH
|
CH2OH
CHO
|
HO -C- H
|
CH2OH
D -glyceraldehyde L -glyceraldehyde
L- and D-isomers
•Although L- and D-isomers of
glyceraldehydes are truly levorotatory (-) and dextro-rotatory (+),
L- and D- sugars are NOT. For example,
L-arabinose [a]20D=+105.1˚ and Dfructose [a]20D=-92˚
a-L-arabinose
(pyranose form)
How to determine L- and D•First, find the chiral carbon
C
H- C =O
|
H- C -OH
|
H- C -H
|
OH
How to determine L- and D•If there is more than one chiral
carbon, then…
H-C =O
|
H-C-OH
|
H-C-OH
|
H2-C-OH
D-
H2-C-OH
|
C=O
|
H-C-OH
|
HO- C-H
|
LH2-C-OH
Formation of the cyclic form
H
O
6
CH2OH
1C
5C
O HOH
H
H C OH
C OH H C1
HO C H
C
C
HOH
HO
C
H
OH
H OH
5
H C OH
anomer
CH2OH
a
b
Other sugar generate 5-member
ring
CH2OH
2C O
HO C H
H C OH
5
H C OH
CH2OH
6
HOCH2 O
CH
OH2OH
5 C H HO C2
C
C
CH
OH2OH
H
OH H
a
b anomer
(few)
The importance of a sialic acid
a sialic acid
Sialidase
(neuraminidase)
Normal protein
Asialoglycoprotein:
Will be removed by
asialoglycoprotein
receptors in the liver
Monosaccharides
•Colorless, crystalline solids
•Water soluble but not soluble in
nonpolar solvent
•Taste sweet
Monosaccharides have reducing
powers
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Disaccharide is made by joining
two monosaccharides
Lactose is only present in milk
Sucrose is also called table
sugar
Trehalose is a major
constituent of insect hemolymph
3 ≦oligosaccharide ≦20
Polysaccharides
Polysaccharides can serve as
fuels, structural elements,
and extracellular support.
Polysaccharides
•Homopolysaccharides
–Starch (amylose, amylopectin)
–Glycogen
–Chitin
•Heteropolysaccharides
–Peptidoglycan
–Agar (agarose, agaropectin)
–Glycosaminoglycans
Starch and glycogen are
polymers of a-D-glucose
•The main chain of starch and glycogen
are consisted of a-D-glucose joined
by (a14) glycosidic bonds.
The branch point of starch and
glycogen : (a16)
•Amylopectin
and
glycogen
have
branchs.
Cellulose is a polymer of b-Dglucose
amylose
cellulose
Chitin : polymer of b-N-acetylD-glucosamine
GlcNAc
GlcNAc
GlcNAc
GlcNAc
Peptidoglycan contains
heteropolysaccharides
Agarose is very important in
molecular biology applications
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Glycosaminoglycans are
heteropolysaccharides
•N-acetylglucosamine
or Nacetyllgalactosamine
+ uronic acid (Dglucuronic or Liduronic acid)
Heparin is a natural
anticoagulant
Glycoconjugates
•Proteoglycans – glycosaminoglycan
chains covalently joined to a
membrane or secreted protein
•Glycoproteins – complex
oligosaccharides covalently joined to
a protein
•Glycolipids – membrane lipids with
oligosaccharides as hydrophilic head
Proteoglycans
Site of attachment
Trisaccharide bridge
Point of attachment
Proteoglycan aggregates
Glycoproteins
•Glycoproteins are carbohydrateprotein conjugates in which the
carbohydrate moieties are smaller and
more structurally diverse than the
glycosaminoglycans of proteoglycans.
•Anomeric carbon of carbohydrate + -OH
of Ser/Thr or –NH2 of Asn
Ser1, Gly5  M
Glycophorin A
•Glycophorin A has
16 oligosaccharides
covalently attached
to it, with total
60 to 70
monosaccharide
residues.
•It is the MN
antigen of human
erythrocytes.
O- and N-linked glycosidic
linkages
Glycolipid and
lipopolysaccharides are membrane
components
Glycobiology
Carbohydrates can be served as
informational molecules
Sugar serves as “aging” marker
of proteins
a sialic acid
Sialidase
(neuraminidase)
Normal protein
Asialoglycoprotein:
Will be removed by
asialoglycoprotein
receptors in the liver
Lectins
•Lectins are proteins that bind
carbohydrates with high affinity and
specificity.
Membrane lectins of H.
pylori binds to
membrane glycoprotein
of gastric epithelial
cell
Membrane lectins (P-, E- and
L-selectins) play important
role in the movement of
immune cells