Chapter 8 (part 1)

Carbohydrates

Carbohydrates

• Most abundant class of biological molecules on Earth

• Originally produced through CO2 fixation during photosynthesis

Roles of Carbohydrates

• Energy storage (glycogen,starch)

• Structural components (cellulose,chitin)

• Cellular recognition • Carbohydrate derivatives

include DNA, RNA, co-factors, glycoproteins, glycolipids

Carbohydrates

• Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions

• Oligosaccharides = "a few" - usually 2 to 10

• Polysaccharides are polymers of the simple sugars

Monosaccharides• Polyhydroxy ketones

(ketoses) and aldehydes (aldoses)

• Aldoses and ketoses contain aldehyde and ketone functions, respectively

• Ketose named for “equivalent aldose” + “ul” inserted

• Triose, tetrose, etc. denotes number of carbons

• Empirical formula = (CH2O)n

C

C*

O

C*

C*

CH2OH

H OH

HO H

H OH

H

CH2OH

C

C*

C*

CH2OH

O

HO H

H OH

D-ribose D-ribulose

Monosaccharides are chiral

• Aldoses with 3C or more and ketoses with 4C or more are chiral

• The number of chiral carbons present in a ketose is always one less than the number found in the same length aldose

• Number of possible steroisomers = 2n (n = the number of chiral carbons)

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

H OH

H OH

H

CH2OH

C

C*

C*

C*

CH2OH

O

HO H

H OH

H OH

D-glucose D-fructose

Stereochemistry

•Enantiomers = mirror images

•Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers

•Epimers = Two sugars that differ in configuration at only one chiral center

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

HO H

H OH

HO H

HO H

H

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

HO H

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

HO H

HO H

H OH

H OH

H

C

C*

O

C*

C*

C*

CH2OH

H OH

HO H

HO H

H OH

H

D-glucoseL-glucose

Enantiomers

D-glucose D-mannose

Epimers

D-mannose D-galactose

Diastereomers

Cyclization of aldose and ketoses introduces additional

chiral center• Aldose sugars (glucose) can cyclize to

form a cyclic hemiacetal

• Ketose sugars (fructose) can cyclize to form a cyclic hemiketal

HC

R1

O

OH R2

HO

C*

OR2

R1H

H

ALCOHOL

ALDEHYDE

HEMIACETAL

NEW CHIRAL CARBON

RC

R1

O

OH R2

HO

C*

OR2

R1R

H

ALCOHOL

KETONE

HEMIKETAL

NEW CHIRAL CARBON

Haworth Projections

Anomeric carbon(most oxidized)

-OH up = beta-OH down = alpha

1

23

4

5

6

For all non-anomeric carbons, -OH groups point down in Haworth projections if pointing right in Fischer projections

C1

C2 OHH

HO

C3

C4

C5

CH2OH

HHO

OHH

OHH

Monosaccharides can cyclize to form Pyranose / Furanose

forms = 64% = 36%

= 21.5% = 58.5%

= 13.5% = 6.5%

Conformation of Monosaccharides

Pyranose sugars not planar molecules, prefer to be in either of the two chair conformations.

Reducing Sugars

• When in the uncyclized form, monosaccharides act as reducing agents.

• Free carbonyl group from aldoses or ketoses can reduce Cu2+ and Ag+ ions to insoluble products

Derivatives of Monosaccharides

Sugar Phosphates

Deoxy Acids

Amino Sugars

Sugar alcohols

Monosaccharide structures you need to

know• Glucose• Fructose• Ribulose• Glyceraldehyde• Dihydroxyacetone