Carbohydrates
-
Saccharides (sugars)
Hydrates of carbon
Monosaccharides (CH2O)n where n  3, but usually < 9
Oligosaccharides (2-20 residues)
Polysaccharides (>20 residues)
Simplest sugars (trioses)
(An aldose)
(A ketose)
1
Most monosaccharides are chiral compounds
Reference compound: Glyceraldehyde (D- and L-enantiomers)
For sugars with more than 3 carbons, the carbon used for enantiomer
determination is the chiral carbon most far away from the C=O carbon.
In the case of glucose, it is C-5.
Most naturally occurring
sugars are D-enantiomers
2
Epimers
- Sugar isomers which differ in the configuration of 1 single chiral carbon.
- In the case of glucose, C2, C3, and C4 are chiral carbons in addition to C5.
HO
H
HO
D-Mannose
H
D-Galactose
- D-mannose is a C-2 epimer of D-glucose.
- D-galactose is a C-4 epimer of D-glucose.
- However, D-mannose and D-galactose are not epimers of each other.
3
Different aldose sugars
Which sugars are
epimers of D-mannose?
[Aldoses commonly found in living organisms are boxed]
4
Different ketose sugars
Ketotriose
Is dihydroxyacetone chiral ?
Ketotetrose
Ketopentoses
Ketohexoses
Which sugars are epimers
of D-fructose?
[Ketoses commonly found in living organisms are boxed]
5
Cyclization of monosaccharides
(For aldose with at least 5 carbons or ketose with at least 6 carbons)
6
Formation of cyclic structures
hemiacetal
(anomeric carbon)
D-Glucose
Fischer projection
α-D-Glucose
Haworth projection
hemiketal
(anomeric carbon)
D-Fructose
Fischer projection
α-D-Fructose
Haworth projection
7
Two anomeric forms of cyclic sugars
- In glucose, C1 is an anomeric carbon (i.e. chiral only in the ring structure)
- α- and β-anomers: inter-convertible forms in solution
α-D-Glucose
D-Glucose
(linear form)
β -D-Glucose
8
Haworth projections for D-sugar vs L-Sugar
H
6
H
4
HO
CH2OH
5
O
H
OH
H
3
2
OH
H
β-D-Glucose
5
OH
1
H
O
HO 6 CH OH
2
4
H HO
H
3
H
1
OH
2
OH
H
β-L-Glucose
Based on the open-chain structures, are you able to draw the Haworth
projections for the α- and β- anomers of
- D- and L-Mannose?
- D- and L-Galactose?
- D- and L-Fructose?
9
Conformational isomers of cyclic sugars
- 6-membered ring (e.g. β-D-glucose)
Chair conformations:
Boat conformation:
10
Derivatives of monosaccharides
Conformational
isomers of cyclic sugars
5-membered ring (e.g. β-D-ribose)
11
Now are you able to distinguish the following terminology
describing sugar molecules?
Enantiomers
Epimers
Anomers
Conformational isomers
12
Sugar derivatives
(1) Sugar phosphates
13
Sugar derivatives
(2) Deoxy-sugars
14
Sugar derivatives
(3) Amino sugars – commonly linked to proteins or lipids
15
Sugar derivatives
(4) Sugar alcohols
16
Sugar derivatives
(4) Sugar acids
17
Monosaccharides are reducing sugars
-
Anomeric carbon: free for reducing activity
Reduction of Cu2+ or Fe3+ ion (by the linear form only)
Fehling’s reaction (measurement of blood/urine glucose levels)
18
Maltose
-
Disaccharide of α-D-Glucose + α- or β-D-Glucose
Present in malt (a mixture from corn or grain used in malted drinks and brewing)
Released from starch digestion
Reducing sugar
(Anomeric carbon)
Non-reducing end
Anomeric C: Free for reducing activity
(Reducing end)
-1, 4 linkage
[-(1→ 4) linkage]
19
Lactose
-
Disaccharide of β-D-Galactose + α- or β-D-Glucose
Produced in lactating mammary glands
Lactose intolerance in adults
Reducing sugar
Anomeric C: Free for reducing activity
β-1, 4 linkage [β -(1→ 4) linkage]
Cellobiose
- Disaccharide of β-D-Glucose + α- or β-D-Glucose
- Repeating unit in cellulose
- Reducing sugar
Anomeric C: Free for reducing activity
β-1, 4 linkage [β -(1→ 4) linkage]
20
Sucrose
-
Table sugar
Disaccharide of α-D-Glucose + β-D-Fructose
No free anomeric carbon
Non-reducing sugar
=
6
α-1, β-2 linkage
5
4
Sugarcane
3
Chocolates and candies
- Creamy or liquid center
- Invertase + sucrose
- Fructose: twice as
sweet as sucrose
21
Artificial sweeteners
Compound
Sweetness relative to sucrose
Acesulfame
200
Aspartame
180
Saccharin
350
Sucralose
600
- How is sweetness measured?
- Many were discovered by chance
22
Aspartame
Sucralose
Saccharin
Acesulfame
23
Polysaccharides
- Homopolysaccharides (homoglycans)
- Heteropolysaccharides (heteroglycans)
24
Starch
- Polymer of α-D-glucose
- Storage and fuel molecules in plants
- a mixture of amylose and amylopectin
Amylose chain
Nonreducing
end
Amylopectin
Reducing
end
-1, 4 linkage
[-(1→ 4) linkage]
-1, 6 linkage
[-(1→ 6) linkage]
25
Glycogen
-
Storage and fuel molecule in mammals and human
α-D-Glucose polymer with the same linkages like those in starch
Shorter and more branches than starch
More soluble than starch
Glycogenin (an enzyme that initiates glycogen synthesis) remains attached to
the mature glycogen molecule at the interior ends
Glycogenin
26
Cellulose
-
A structural polysaccharide in plants
Most abundant organic compound on earth
Linear polymer of β-D-glucose residues
β-1, 4 linkages
Indigestible
Ruminants and termites: contain microorganisms
which produce cellulases for cellulose digestion
Cellulose fibers
(Inverted
orientation)
27
Intra- and inter-chain hydrogen bonding in cellulose fibers
-
Providing strength and rigidity in cellulose
Formation of bundles or fibers
Cotton: 100% cellulose
Wood: half cellulose
H- bonding
Cellulose
polymer
Cellulose
polymer
Red – O
White – H
Black - C
28
Chitin
-
Second most abundant organic compound on earth
Structural homoglycan
Exoskeleton of crustaceans and insects
Fungal and red algal cell wall
Linear polymer of β-D-N-acetylglucosamine (GlcNAc)
- β-1, 4 linkages
- Intra- and inter-chain hydrogen bonding
(Inverted
orientation)
29
Bacterial cell wall
- Gram-positive (blue)
- Gram-negative (pink)
- Peptidoglycan (thicker in gram
+ve bacteria)
Gram-staining
30
Peptidoglycan
-
Network of cross-linked polysaccharides and peptides in bacterial cell wall
Polysaccharide
- Alternating amino sugar residues
- GlcNAc (N-acetylglucosamine)
- MurNAc (N-acteylmuramic acid)
- β-1, 4 linkages
(Inverted
orientation)
For peptide
attachment
(Inverted
orientation)
For peptide
attachment
31
Structure of the peptidoglycan of
Staphylococcus aureus
amide
bond
amide bond
α-COOH
α-C
γ-COOH
amide bond
amide bond
Penicillin – blocks the bridge formation
– effective against gram +ve only
32
Glycoproteins
-
Proteins that contain covalently bound sugars
Glycosylated proteins
Carbohydrate chain: 1 to >30 residues; up to 80% of total mass
Enzymes, hormones, structural proteins, transport proteins, etc.
O-glycosidic and N-glycosidic linkages to the anomeric carbon of the sugar residue
O-glycosidic linkage:
N-glycosidic linkage:
Polypeptide
chain
Polypeptide
chain
N-Acetyl-galactosamine
(GalNAc)
N-Acetyl-glucosamine
(GlcNAc)
33
O-linked Oligosaccharides
– Sugars attached to the hydroxyl (-OH) group in the side chains of serine (Ser),
threonine (Thr), or hydroxylysine (Hyl, a modified lysine) residues in proteins.
Polypeptide
chain
(Hydroxylysine)
34
N-linked Oligosaccharides
– sugars attached to the amide nitrogen in the side chain of asparagine (Asn)
residues in proteins.
Polypeptide
chain
35
The ABO blood group system – Oligosaccharides on RBC surface
Blood type
A
B
AB
O
RBC surface oligosaccharide
Type A
Type B
Types A and B
Type O
36
Blood type
A or AB
Blood type
B or AB
Blood type O
37