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

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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 -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