XXXI. Carbohydrates
A. Overview
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Carbohydrates are the most abundant class of naturally
occurring organic compounds. They make up 50% of the
earth’s biomass and are found in animals, plants, fungi and
bacteria.
1.
The first carbohydrates had molecular formulas similar to
hydrates Cn(H2O)n
2.
D-Glucose is the most abundant carbohydrate found in
nature.
3. Biological Systems
CO2 + H2O + Energy (Photosynthesis)
Starch
Glucose
Animals
Humans
Glycogen
Glucose
Cellulose
Plants
H2O + CO2
+ Energy
B. Classification
Carbohydrates
Simple
Monosaccharides
(can not be hydrolyzed
to simpler sugars)
Complex
Disaccharides
Oligosaccharides
Polysaccharides
Starch Glycogen Cellulose
Monosaccharides
• Trioses (3-carbons)
Glyceraldehyde
• Tetroses (4-carbons)
Erythrose, Threose
• Pentoses (5-carbons)
Pentose
• Hexoses (6-carbons)
Glucose, Mannose
Galactose, Fructose
C. D and L Configurations
1. Trioses
D & L designations are based
on the configuration about
the single stereocenter in
CHO
H
glyceraldehyde.
C
Right
OH
Left
HO
C
H
CH2OH
CH2OH
D-glyceraldehyde
CHO
L-glyceraldehyde
(R)-2,3-Dihydroxypropanal
CHO
2.
Tetroses
Erythrose and Threose
H
C
OH
CH2OH
D-glyceraldehyde
CHO
HO
C
H
CH2OH
L-glyceraldehyde
3. Hexoses
For sugars with more than one
chiral center, D or L refers to the
asymmetric C farthest from the
aldehyde or keto group.
Most naturally occurring sugars
are D isomers.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
D and L sugars are enantiomers of one another.
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
The D Aldose Family
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D. Epimers
Carbohydrates that differ in configuration at only one carbon
center.
Not Epimers
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E. Cyclic Hemiacetals of Monosacchaarides
1. Cyclic Hemiacetal - Mechanism
2. Glucose
3. Anomers
• Carbohydrates that differ in configuration only at
their anomeric carbon.
• An anomeric carbon is a hemiacetal or acetal
carbon in a carbohydrate.
E. Cyclic Hemiacetals of Monosacchaarides
1. Cyclic Hemiacetal - Mechanism
2. Glucose
4. Chair
β-D-Glucopyranose
α-D-Glucopyranose
β-D-Glucose
α-D-Glucose
5. Cyclic Structure for Fructose
β-D-Fructofuranose
F. Mutarotation
1. When anomers dissolved in water undergo a
slow change in optical rotation to an
equilibrium value, this process is called
mutrotation.
2. Only sugars with hemiacetal anomeric
carbons can undergo mutarotation.
3. Sugars which undergo mutarotation are
called reducing sugars.
4. Mutarotation of Glucose
36%
α-D-Glucopyranose
0.2%
64%
β-D-Glucopyranose
H. Disaccharides
• A Disaccharide on hydrolysis is cleaved to
two monosaccharides.
• The monosaccharides are linked as a
glycoside (acetal).
1. Cellobiose
• Two glucose units linked β-1-4’.
• Disaccharide of cellulose.
• A mutarotating, reducing sugar.
2. Maltose
• Two glucose units linked α-1-4’.
• Disaccharide of starch
• A mutarotating, reducing sugar
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3. Lactose
• Galactose + glucose linked β-1-4’.
• “Milk sugar”, reducing sugar
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4. Sucrose
• Glucose + fructose, linked α,β-1,1’(2’)
• Nonreducing sugar, no mutrotation
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I. Polysaccharides
• Polysaccharides have more than ten
monosaccharides units joined by a
glycosidic linkage.
1. Starch
Amylose (20%)
H2O Soluble
Amylopectin (80%)
H2O Insoluble
2. Amylose
• Soluble starch, polymer of D-glucose.
• Starch-iodide complex, deep blue.
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Helical Structure of
Amylose
3. Amylopectin
• Branched, insoluble fraction of starch, α-1-4’, α-1-6’ every
20-30 glucose units.
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4. Cellulose
• Polymer of β-D-glucose, found in plants.
• Mammals lack the -glycosidase enzyme.
• Rigid linear structure.
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5. Glycogen
• Glucose polymer, α-1-4’, α-1-6’ every 10 glucose units.
• Similar to amylopectin, but even more highly
branched.
• Energy storage in muscle tissue and liver.
• The many branched ends provide a quick means of
putting glucose into the blood.
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CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
H
1
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Glycogen, the glucose storage polymer in animals, is similar in
structure to amylopectin. But glycogen has more a(16) branches.
The highly branched structure permits rapid release of glucose
from glycogen stores, e.g., in muscle during exercise. The ability
to rapidly mobilize glucose is more essential to animals than to
plants.
Gentiobiose
• Two glucose units linked 1-6’.
• Rare for disaccharides, but commonly seen as branch point
in carbohydrates.
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