Carbohydrates and Glycobiology

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Chapter 7

• Carbohydrates and Glycobiology

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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
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Classification of carbohydrate

• carbon number
• L- or D- isomers
• pyranose or furanose
• a or ß anomers
• ketose or aldose

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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.

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Carbon number
H
O
1
CH2OH
C
1
2
C
H
O
C
H
OH
2
3
C
HO
C
HO
H
H
3
4
C
C
H
OH
H
OH
4
5
H
H
C
OH
C
OH
5
6
CH2OH
CH2OH
ketose
aldose
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L- and D- isomers
a25D8.7
a25D-8.7
CHO
CHO
-C-
-C-
OH
H
H
HO
CH2OH
CH2OH
-glyceraldehyde
L
-glyceraldehyde
D
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L- and D-isomers

• Although L- and D-isomers of glyceraldehydes are
  truly levo-rotatory (-) and dextro-rotatory (),
  L- and D- sugars are NOT. For example,
  L-arabinose a20D105.1 and D-fructose
  a20D-92

a-L-arabinose (pyranose form)
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How to determine L- and D-

• First, find the chiral carbon

C
H-
O

C
H-
-OH
C

-H
C
H-
OH
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How to determine L- and D-

• If there is more than one chiral carbon, then

H2-C-OH

H-C
O
C
O


H-C-OH
H-C-OH


H-C-OH

-H
C
HO-
H2-C-OH

L-
D-
H2-C-OH
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Formation of the cyclic form
H
O
6
CH2OH
C
1
C
O
5
OH
H
H
C
H
OH
H
OH
C
C
1
C
HO
H
C
C
H
OH
HO
C
H
OH
H
OH
H
OH
b
a
anomer
CH2OH
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Other sugar generate 5-member ring
CH2OH
6
C
O
HOCH2
2
O
OH
CH2OH
C
HO
H
HO
H
C
C
2
5
C
C
CH2OH
OH
C
H
OH
H
OH
H
H
OH
b
a
anomer
CH2OH
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(few)
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The importance of a sialic acid
a sialic acid
Sialidase (neuraminidase)
Asialo-glycoprotein Will be removed by
asialoglycoprotein receptors in the liver
Normal protein
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Monosaccharides

• Colorless, crystalline solids
• Water soluble but not soluble in nonpolar solvent
• Taste sweet

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Monosaccharides have reducing powers
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Disaccharide is made by joining two
monosaccharides
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Lactose is only present in milk
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Sucrose is also called table sugar
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Trehalose is a major constituent of insect
hemolymph
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3 ?oligosaccharide ?20
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Polysaccharides

• Polysaccharides can serve as fuels, structural
  elements, and extracellular support.

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Polysaccharides

• Homopolysaccharides
• Starch (amylose, amylopectin)
• Glycogen
• Chitin
• Heteropolysaccharides
• Peptidoglycan
• Agar (agarose, agaropectin)
• Glycosaminoglycans

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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.

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The branch point of starch and glycogen (a1?6)

• Amylopectin and glycogen have branchs.

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Cellulose is a polymer of b-D-glucose
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amylose
cellulose
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Chitin polymer of b-N-acetyl-D-glucosamine
GlcNAc
GlcNAc
GlcNAc
GlcNAc
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Peptidoglycan contains heteropolysaccharides
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Agarose is very important in molecular biology
applications
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Glycosaminoglycans are heteropolysaccharides

• N-acetylglucosamine or N-acetyllgalactosamine
  uronic acid (D-glucuronic or L-iduronic acid)

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Heparin is a natural anticoagulant
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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

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Proteoglycans
Site of attachment
Trisaccharide bridge
Point of attachment
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Proteoglycan aggregates
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Glycoproteins

• Glycoproteins are carbohydrate-protein 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

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Glycophorin A
Ser1, Gly5 ? M

• Glycophorin A has 16 oligosaccharides covalently
  attached to it, with total 60 to 70
  monosaccharide residues.
• It is the MN antigen of human erythrocytes.

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O- and N-linked glycosidic linkages
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Glycolipid and lipopolysaccharides are membrane
components
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Glycobiology

• Carbohydrates can be served as informational
  molecules

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Sugar serves as aging marker of proteins
a sialic acid
Sialidase (neuraminidase)
Asialo-glycoprotein Will be removed by
asialoglycoprotein receptors in the liver
Normal protein
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Lectins

• Lectins are proteins that bind carbohydrates with
  high affinity and specificity.

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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