Sugars and
Polysaccharides
Voet Biochemistry:
Chapter 11, Pages 359 – 370
Voet Fundamentals:
Chapter 8, Pages 221 – 234
Biochemistry 2000
Carbohydrates






Most abundant biological molecule on the planet
Biosynthesis = structural building blocks + energy
storage
Catabolism = carbon fuels that drive biological
processes
“carbon hydrate” - reflecting the chemical
composition
poly-hydroxyl aldehyde (aldose) or
poly-hydroxyl ketone (ketose)
Each subunit composed of 3 or more carbons
Biochemistry 2000
Carbohydrates
WHY ARE CARBOHYDRATES SO DIFFICULT TO
STUDY?




Saccharide sequences are not subject to
standard genetic analysis
Carbohydrates are largely heterogeneous in size
and composition
Techniques to study of carbohydrate sequence
and composition are less advanced than for
nucleic acids and proteins
Difficult to establish functional assays as
typically carbohydrates have structural roles
Biochemistry 2000
Saccharide Classification
Monosaccharides - the basic unit of carbohydrates

synthesized from simpler substances in gluconeogenesis
or photosynthesis

major energy source & components of nucleic acids, and
lipid and protein modifications
Oligosaccharides - consist of a few covalently
linked monosaccharides (2~20)

associated with proteins and lipids

serve structural and regulatory roles
Polysaccharides - consist of ~20 or more covalently
linked monosaccharides

structural function in all organisms

nutritional reserve
Biochemistry 2000
Monosaccharides
•
Represented by Fischer projections
CHO
H
CHO
OH
C
HO
CH2OH
H
C
O
H
C
OH
CH2OH
H
C
CH2OH
H
C
O
HO
C
H
CH2OH
D-Glyceraldehyde L-Glyceraldehyde
Monosaccharide Classification
RULES:
1. Chemical nature of
carbonyl group
2. Number of C atoms
3
4
3. Stereochemistry of
chiral carbons
5
D-aldose family of
monosaccharides
6
Biochemistry 2000
Monosaccharide Classification
RULES:
1. Chemical nature of
carbonyl group
2. Number of C atoms
3
4
3. Stereochemistry of
chiral carbons
5
D-aldose family of
monosaccharides
6
Biochemistry 2000
Monosaccharide Classification
RULES:
1. Chemical nature of
carbonyl group
2. Number of C atoms
3
4
3. Stereochemistry of
chiral carbons
5
D-aldose family of
monosaccharides
6
Lecture 14
Biochemistry 2000
Monosaccharide Classification
RULES:
1. Chemical nature of
carbonyl group
2. Number of C atoms
3
4
3. Stereochemistry of
chiral carbons
5
D-aldose family of
monosaccharides
6
Biochemistry 2000
Stereochemistry
Classification of D and L carbohydrates

Based upon configuration of chiral C in
glyceraldehyde

Determined in sugars by the chiral C
furthest from carbonyl

Example of epimers
Stereochemical terminology (review)



Enantiomers: differ at all chiral
centers (mirror image molecules)
Diastereomers: differ at one or more
chiral centers (BUT NOT ALL)
Circles:
Epimeric Carbons
Squares:
D-configuration
Epimers: differ at one chiral center
Biochemistry 2000
Biochemistry 2000
Monosaccharide Classification
D-aldose family of
monosaccharides
1?
2?
4?
3?
Biochemistry 2000
Some hexoses
Most biological monosaccharides are in the D
orientation at the highest numbered chiral carbon
•
1
H
H
C
2
C
3
HO
C
H
C
H
4
5
C
6
O
H
C
O
OH
H
C
H
HO
OH
OH
CH2OH
H
C
O
CH2OH
OH
HO
C
H
C
O
C
H
HO
C
H
HO
C
H
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
CH2OH
CH2OH
D-Glucose D-Galactose D-Mannose
CH2OH
D-Fructose
Reactivity (review)


Alcohols react with aldehydes (1:1 ratio) to form hemiacetals
and (2:1 ratio) acetals
Alcohols react with ketones (1:1 ratio) to form hemiketals
and (2:1 ratio) ketals
Biochemistry 2000
Configurations

Aldoses (>4C) can react with themselves forming cyclic hemiacetals (left)

Ketoses (>4C) can react with themselves forming cyclic hemiketals (right)
d+
..
..
..
..
d-
Biochemistry 2000
Biochemistry 2000
Configurations

Saccharides forming 5 and 6 member rings are referred to as a furanose
and pyranose, respectively
Pyran = Pyranose

Furan = Furanose
Rings are dynamic structures
D-Arabinopyranose
D-Arabinose
Biochemistry 2000
D-Arabinofuranose
Biochemistry 2000
Anomers

Hemiacetal or hemiketal carbon is
referred to as the anomeric carbon
Glucose

Fructose
Cyclization can generate two different
anomers
a Anomer: anomeric carbon OH is on the
opposite side of the ring from the
CH2OH of the chiral center defining D
or L
b Anomer: anomeric carbon OH is on the
same side of the ring as the CH2OH of
the chiral center defining D or L
Learn these structures!
+ Ribose!
Biochemistry 2000
Mutarotation


a and b anomeric forms of monosaccharides (and free reducing
ends of polymerized sugars) are in equilibrium
Mutarotation: spontaneous interconversion between anomeric forms
~36 %
~1 %
~63 %
Biochemistry 2000
Cyclization of fructose
•
The hydroxyl on C5 attacks the C2 ketone
1 CH2OH
2
HO
H
6
HOCH2
5
H
H
4
OH
1
CH2OH
O
2
HO
3
OH
H
α-fructose
H
3
C
O
C
H
4C
5
OH
C
OH
6 CH OH
HOCH2
OH
O
2
H
HO
H
OH
CH2OH
H
β-fructose
Conformational Variability


4C
1
Pyranoses adopt “chair” or “boat” conformations due to the
tetrahedral configuration of carbon atoms
Boat form is only observed when bulky substituents are present
1C
4
steric crowding
eclipse
Boat conformation
Biochemistry 2000
Glycosidic Bonds

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Condensation reaction
between anomeric carbon
and any other hydroxyl
containing compound
produces a glycosidic bond
chemical formation of an
Acetal or Ketal
In disaccharides, the
glycosidic bond is typically
between C1 and C4
glycosidic bonds between
C1 & C6, C1 & C1 are also
common
Biochemistry 2000
Disaccharides
Naming of glycosidic bond:
(a/b - number of anomeric carbon, a/b if also
anomeric/number of alcohol carbon)

lactose – milk sugar:

trehalose – insect hemolymph:

sucrose – common sugar:
Learn these structures!
Biochemistry 2000
Polysaccharides

variable in length, can be composed of one or more types of
monosaccharide and can be linear or branched

Homopolysaccharides: composed of one monosaccharide type

Heteropolysaccharides: composed of two or more monosaccharide types
Biochemistry 2000
Homopolysaccharides
Cellulose: unbranched
linked
glucose; serves structural role in
plants; indigestible by mammals
Amylose: unbranched
D-glucose
linked
Glycogen: amylose with
branch every 8-12 residues
Biochemistry 2000
Homopolysaccharides
Cellulose: unbranched b1-4 linked
glucose; serves structural role in
plants; indigestible by mammals
Amylose: unbranched a1-4 linked
D-glucose
Glycogen: amylose with a1-6
branch every 8-12 residues
Biochemistry 2000