Carbohydrates

Structure of Carbohydrates

O

HO

CH2OH

OH

OH

O

CH2OH

OH

OH

OH

O

O

HO

CH2OH

OH

OH

O—HO

H—O

CH2OH

OH

OH

OH

H2O

Sugars OligosaccharidesPolysaccharides

Properties of Carbohydrates

• Most abundant class of organic molecules

• Source: Photosynthesis• Classification

– Monosaccharides• Stereoisomers• Aldehydes (Aldose) or Ketones (Ketose)• Number of Carbons (ie 3=triose; 6=hexose)• Combined: Aldotriose/Ketotetrose

– Polymers• Oligosaccharides (2- ~20 sugars)• Polysaccharides (> ~20 sugars)

Biological Roles of Carbohydrates

• Energy source• Energy storage• Cell walls• Recognition events

– Between proteins (targeting)– Between cells

• Signalling• Components of other biological

molecules– Antibiotics– Enzyme cofactors– Nucleic Acids

Monosaccharides(Sugars)

Classes of Monosaccharides

C

C

CH2OH

H O

H OH

CH2OH

C

CH2OH

O

D-Glyceraldehyde Dihydroxyacetone

Aldoses(Aldehydes)

Ketoses(Ketones)

Chirality

C

C

CH2OH

H O

H OH

D-Glyceraldehyde

C

C

CH2OH

H O

OH H

L-Glyceraldehyde

D- versus L- determined by chirality of highest number carbon (from aldehyde or ketone)

Figure 8-1

Aldoses

Figure 8-1

Aldoses

Figure 8-2

Ketoses

Figure 8-2

Ketoses

Epimers(stereoisomers differing by configuration

of only one of several chiral centers)

C

C

H O

H

C

C

H O

H OH

CH2OH

CHO H

CH OH

CHO H

C

C

H O

H OH

CH2OH

CHO H

CH OH

CH OH

OH

CH2OH

CHO H

CHO H

CH OH

D-Galactose (gal)D-Glucose (glc) D-Mannose (man)

Epimers(stereoisomers differing by configuration

of only one of several chiral centers)

C

C

H O

H

C

C

H O

H OH

CH2OH

CHO H

CH OH

CHO H

C

C

H O

H OH

CH2OH

CHO H

CH OH

CH OH

OH

CH2OH

CHO H

CHO H

CH OH

D-Galactose (gal)D-Glucose (glc) D-Mannose (man)

Enantiomers(mirror images)

C

C

H O

HO H

CH2OH

CH OH

CHO H

CHO H

C

C

H O

H OH

CH2OH

CHO H

CH OH

CH OH

L-GlucoseD-Glucose

Mutarotation

Creation of new chiral center

Formation of Hemiacetal

O

CR H

+

..

O

C

O

HR

R1

H

..+

Chiral

H

H R1

O

H

Formation of Hemketal

O

CR R2

O

C

O

R2R

R1

H

Chiral

..

..

+

+R1H

O

H

H

Anomeric Carbon Atom

MutarotationReversible

Creation of new asymmetric center

Cyclization of D-Glucose

Anomers

• Anomeric carbon atom– Most oxidized carbon atom– Shares electrons with 2 oxygen atoms

-configuration has -OH on opposite side of ring from CH2OH group at chiral center that designates D- or L-

Cyclization of D-Fructose(biologically relevant forms)

CH2OH

C

O

H OH

CH2OH

CHO H

CH OH

CHOCH2 CH2OH

OH

OH

OH

HOCH2 OH

CH2OH

OH

OH

O O

D-Fructose -D-Fructose-D-Fructose

Nomenclature

O

pyran

O

f uran

Pyranoses Furanoses

Examples of Nomenclature

-D-glucopyranose

-D-fructofuranoseConfiguration of anomeric carbon

Configuration of sugar

Sugarprefix

RingType*not required

Anomeric carbon modification:ose: reducingoside: non-reducing

Cyclization of D-Fructose(biologically relevant forms)

CH2OH

C

O

H OH

CH2OH

CHO H

CH OH

CHOCH2 CH2OH

OH

OH

OH

HOCH2 OH

CH2OH

OH

OH

O O

D-Fructose -D-Fructose-D-Fructose

Figure 8-5

Chair Conformations of -D-glucopyranose

Chair and Boat Forms

Equitorial and Axial Substituents

Steric Crowding: equitorial more stable

Equatorial Axial

Derivatives of Monosaccharides

Phosphate Esters

OO OH

OH

OH

HO

OH

CH2OPO32–

OH

OH

CH2OPO32–

-D-Fructose-1,6-bisphosphate (Fru-1,6-bisP)

-D-Glucose-6-P (Glc-6-P)

=O3POCH2

CH2OH

C O

CH2OPO32–

C

C OHH

OH

CH2OPO32–

C

C OHH

OH

C OHH

CH2OPO32–

D-Erythrose4-phosphate (E4P)

D-Glyceraldehyde 3-phosphate (G3P)

Dihydroxyacetone phosphate (DHAP)

Deoxy Sugars

O

OH

OH

O

OH

OH

2-Deoxyribose (dRib)

Deoxyribose 5-phosphate (dRib-5-P)[Deoxy--D-ribof uranose 5-phosphate]

HOCH22–O3POCH2

Note: 5-membered ring form is used in biological systems

Amino Sugars(e.g. GlcNAc-6-P)

O OH

NH

OH

HO

CH2OP

C

H3C O

N-Acetyl-D-glucosamine 6-phosphate

Sugar Alcohols

C

CH OH

CH2OH

CH OH

CH OH

CH2OH

CH OH

CH2OH

CH OH

CH OH

OH

D-Ribose Ribitol

CH2OH

CH OH

CH2OH

CHO H

CH OH

CH OH

C

C

H O

H OH

CH2OH

CHO H

CH OH

CH OH

D-Glucose Sorbitol

Glycosides

Structure of Glycosides

PolymerizationMonomers(Sugars)

Polymers(Disaccharides)

(Oligosaccharides)(Polysaccharides)

NOTE: linear and branched polymers

Glycosidic Linkages (glycoside)

O

CH2OH

OCH3

OH

OH

HO

Acetal

Stable: no mutarotation

Non-reducing sugar (no free anomeric C atom)

Nomenclature

O

CH2OH

OCH3

OH

OH

HO

O

CH2OH

OH

OH

OH

CH3O

OHOCH2 OCH3

CH2OH

OH

HO

Optional

[Non-reducing][Reducing][Non-reducing]

Methyl--D-Fructofuranoside

4-O-Methyl--D- Glucopyranose

O-Methyl--D-Glucopyranoside

Reducing test

• Free Aldehydes are reductants

• If free to mutarotate sugar is a reductant– Must have only –OH

at anomeric carbon

Cupric oxidebrick-red

precipitate

Cu2O

Disaccharides

Sucrose (non-reducing)

O

CH2OH

OH

OH

HOO

OHOCH2

CH2OH

OH

OH

OHOCH2 OH

CH2OH

OH

OH

-D-f ructose

-D-glucopyranosyl-(l __> 2)--D-f ructofuranoside

OR: Glc(α1 β2)Fru

Sucrose

O

CH2OH

OH

OH

HOO

OHOCH2

CH2OH

OH

OH

OHOCH2 OH

CH2OH

OH

OH

-D-f ructose

-D-glucopyranosyl-(l __> 2)--D-f ructofuranoside

OR: Glc(α1 β2)Fru

-Maltose

O

CH2OH

OH

OH

HOO

O

CH2OH

OH

OH

OH

-D-glucopyranosyl-(l __> 4)--D-glucopyranose

Glc(α14)Glc

-Lactose

Gal(β14)Glc

Nomenclature

1. Recognize individual monosaccharides2. Drop the –se and add root for rings

– 6 member: pyran– 5 member: furan

3. Attach:– ose: can mutarotate– oside: canNOT mutarotate– osyl: not terminal residue

4. Indicate carbon to carbon number linkage (##)

5. Label each residue with D or L and α or β

Oligosaccharides

• Generally complex– Heteropolymers– Branched

• Various Cellular Functions– Receptors– Antigens– Signal transduction– Trafficking

O-linked Oligosaccharides(serine/threonine)

N-linked Oligosaccharides(asparagine)

Sugar groups on glycoproteins frequently function in recognition

Polysaccharides

• Simpler structures– Homopolymers– Less branching

• Limited Cellular Functions– Structural/Protective– Energy Storage

Linear Polysaccharides

reducing end (RE)non-reducing end (NRE)

glycosidic linkages

Branched Polysaccharides

RE

NRE

glycosidic linkages

Functions of Polysaccharides

• Structural - e.g. plant cell walls, cement between cells (animals): • -linkages stable to enzymatic cleavage

• Storage - e.g. glycogen as energy reserves:• -linkages are readily cleaved

• Potential osmotic problem

• Accessibility for energy production -linkages

• Branching

Cellulose(plant cell walls)

O

CH2OH

O OH

OH

O

O

CH2OH

OH

OH

O

-(1___>4)

Chitin(1—>4)-linked homopolymer of

N-acetylglucosamine

Exoskeletin of invertebrates (e.g. crustacians, insects, and spiders)

Cell wall (most fungi and some algae)

Glycogen(storage)

Linear: α14Branches: α16

O

CH2OH

HO O

OH

OH

O

O

CH2OH

OH

OH

O

CH2OH

HO O

OH

OH

O

CH2OH

OH

OH

O

CH2

O

OH

OH

O

CH2OH

OH

OH

OHO

Starch(plants)

linear

branched (similar to glycogen, but fewer branches)