Chapter 7 Carbohydrates and Glycobi ology
Chapter 7
Carbohydrates and Glycobiology
Carbohydrates are everywhere
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Sucrose
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glazed donuts
Sucrose (table sugar)
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.
-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.
Carbon numberH OC
HCC
H
CH
C
H
CH2OH
HOOH
OH
OH
CH2OH
HCC
H
CH
C
HHO
O
OH
OH
CH2OHaldose ketose
1
2
3
4
5
6
1
2
3
4
5
6
L- and D- isomers
CHO
H|
-C-|OH
CH2OH
CHO
H|
-C-|
HO
CH2OH
-glyceraldehydeD -glyceraldehydeL
[]25D=+8.7˚ []25
D=-8.7˚
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 []20
D=+105.1˚ and D-fructose []20D=-92
˚
-L-arabinose (pyranose form)
How to determine L- and D-
• First, find the chiral carbon
CH- =O|
H- -OHC
H- -HC|
|OH
C
How to determine L- and D-
• If there is more than one chiral carbon, then…
H-C
H-C-OH|
H2-C-OH
|H-C-OH
|
=OH2-C-OH
C=O
H-C-OH
H2-C-OH
HO-
|
|
|
|-HC
D- L-
C5
Formation of the cyclic form H O
HCC
H
CH
H
CH2OH
HOOH
OH
OH
C1CH
HO
OC
H
OHCHOH
C
H
OHC HOH
CH2OH
1
5
6
anomer
Other sugar generate 5-member ring
C5
HCC
H
CH
CH2OH
HOO
OH
OH
2
CH2OH
H
OC
OH
OHCHOH
CH
C
HOCH2
25
6
CH2OHOH
CH2OH
anomer
(few)
The importance of a sialic acid
a sialic acid
Normal protein Asialo-glycoprotein:Will be removed by asialoglycoprotein receptors in the liver
Sialidase(neuraminidase)
Monosaccharides• Colorless, crystalline solids• Water soluble but not soluble in nonpolar solvent
• Taste sweet
Monosaccharides have reducing powers
http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/Grafik/fehling2.gif
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 -D-glucose
• The main chain of starch and glycogen are consisted of -D-glucose joined by (14) glycosidic bonds.
The branch point of starch and glycogen : (16)
• Amylopectin and glycogen have branchs.
Cellulose is a polymer of -D-glucose
amylose
cellulose
Chitin : polymer of -N-acetyl-D-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 N-acetyllgalactosamine + uronic acid (D-glucuronic or L-iduronic 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
Point of attachment
Site of attachment
Trisaccharide bridge
Proteoglycan aggregates
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
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.
Ser1, Gly5 M
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
Normal protein Asialo-glycoprotein:Will be removed by asialoglycoprotein receptors in the liver
Sialidase(neuraminidase)
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