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
1
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
O
H
C
CH2OH
H
C*
OH
HO
C*
H
H
C*
OH
• Ketose named for “equivalent
aldose” + “ul” inserted
• Triose, tetrose, etc. denotes
number of carbons
• Empirical formula = (CH2 O)n
C
O
HO
C*
H
H
C*
OH
CH2 OH
CH2OH
D-ribul ose
D-ri bose
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 = 2 n (n = the
number of chiral carbons)
O
H
C
CH 2OH
H
C*
OH
HO
C*
H
C
O
HO
C*
H
C*
H
OH
H
C*
H
C*
OH
OH
H
C*
OH
CH 2OH
CH 2OH
D-gl ucose
D-fruct ose
2
Stereochemistry
Enantiomers
O
H
O
C
C
HO
C*
H
H
C*
OH
HO
C*
HO
C*
Epimers
D iastereomers
O
H
O
H
C
C
C*
OH
HO
C*
H
H
C*
OH
HO
C*
H
HO
C*
H
HO
C*
H
H
C*
OH
H
C*
OH H O
H
H
C*
OH
H
C*
OH
CH 2OH
CH 2OH
D-gluc os e
H
O
H
C
H
L-glucose
O
H
H
C
H
C*
OH
HO
C*
H
H
HO
C*
H
HO
C*
H
C*
H
H
C*
OH
H
C*
OH
C*
OH
H
C*
OH
H
C*
OH
CH2OH
CH 2OH
D-mannos e
D-galactose
CH 2OH
CH 2OH
D-gluc os e
D -mannose
•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
Cyclization of aldose and ketoses
introduces additional chiral center
• Aldose sugars (glucose) can cyclize to form a
H
cyclic hemiacetal H
N EW CHIRAL
A LDEHYD E
O
O
C
H
ALC OHOL
H
R1
C*
O
R2
O
H
C ARB ON
R1
R2
HEMIAC ETAL
• Ketose sugars (fructose) can cyclize to form a
H
H
cyclic hemiketal
N EW CHIRAL
KE TON E
O
O
C ARB ON
C
R
C*
R1
R1
R
O
ALC OHOL
R2
O
H
R2
HE MIKE TAL
Glucopyranose formation
3
Fructofuranose formation
Monosaccharides can cyclize to
form Pyranose / Furanose forms
α = 64%
β = 36%
α = 21.5%
β = 58.5%
α = 13.5%
β = 6.5%
Haworth Projections
O
H
-OH up = beta
-OH down = alpha
C1
H
C2
OH
HO
C3
H
H
C4
OH
H
C5
OH
CH2 OH
6
5
4
1
3
2
Anomeric carbon
(most oxidized)
For all non-anomeric carbons, -OH groups
point down in Haworth projections if
pointing right in Fischer projections
4
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
5
Sugar Phosphates
Deoxy Acids
Amino Sugars
6
Sugar alcohols
Monosaccharide structures
you need to know
1)
2)
3)
4)
5)
6)
Glucose
Fructose
Ribose
Ribulose
Galactose
Glyceraldehyde
7