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Chapter 20
Carbohydrates
Chapter 20
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Carbohydrates
Carbohydrate: A polyhydroxyaldehyde or
polyhydroxyketone, or a substance that gives these
compounds on hydrolysis.
Monosaccharide: A carbohydrate that cannot be
hydrolyzed to a simpler carbohydrate.
• Monosaccharides have the general formula CnH2nOn,
where n varies from 3 to 8.
• Aldose: A monosaccharide containing an aldehyde
group.
• Ketose: A monosaccharide containing a ketone group.
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Monosaccharides
•
•
•
•
•
The suffix -ose indicates that a molecule is a carbohydrate.
The prefixes tri-, tetra-, penta-, and so forth indicate the number of
carbon atoms in the chain.
Those containing an aldehyde group are classified as aldoses.
Those containing a ketone group are classified as ketoses.
There are only two trioses:
• Often aldo- and keto- are omitted and these compounds are
referred to simply as trioses.
• Although “triose” does not tell the nature of the carbonyl group, it
at least tells the number of carbons.
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Monosacharides
Figure 20-1 Glyceraldehyde, the simplest aldose, contains
one stereocenter and exists as a pair of enantiomers.
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Monosaccharides
Fischer projection: A two-dimensional representation for
showing the configuration of tetrahedral stereocenters.
• Horizontal lines represent bonds projecting forward
from the stereocenter.
• Vertical lines represent bonds projecting to the rear.
• Only the stereocenter is in the plane.
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Monosacharides
In 1891, Emil Fischer made the arbitrary assignments of
D- and L- to the enantiomers of glyceraldehyde.
• D-monosaccharide: the -OH on its penultimate carbon
is on the right in a Fischer projection.
• L-monosaccharide: the -OH on its penultimate carbon
is on the left in a Fischer projection.
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D,L-Monosaccharides
• The most common D-tetroses and D-pentoses are:
• The three most common D-hexoses are:
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Amino Sugars
Amino sugars contain an -NH2 group in place of an -OH
group.
• Only three amino sugars are common in nature:
D-glucosamine, D-mannosamine, and D-galactosamine.
N-acetyl-D-glucosamine is an acetylated derivative of
D-glucosamine.
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Cyclic Structure
• Aldehydes and ketones react with alcohols to form
hemiacetals (Chapter 17).
• Cyclic hemiacetals form readily when hydroxyl and
carbonyl groups are part of the same molecule and
their interaction can form a five- or six-membered ring.
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Haworth Projections
• Figure 20-2 D-Glucose forms these two cyclic
hemiacetals.
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Haworth Projections
• A five- or six-membered cyclic hemiacetal is
represented as a planar ring, lying roughly
perpendicular to the plane of the paper.
• Groups bonded to the carbons of the ring then lie
either above or below the plane of the ring.
• The new carbon stereocenter created in forming the
cyclic structure is called the anomeric carbon.
• Stereoisomers that differ in configuration only at the
anomeric carbon are called anomers.
• The anomeric carbon of an aldose is C-1; that of the
most common ketose is C-2.
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Haworth Projections
In the terminology of carbohydrate chemistry,
• β means that the -OH on the anomeric carbon is on the
same side of the ring as the terminal -CH2OH.
• α means that the -OH on the anomeric carbon is on the
side of the ring opposite from the terminal -CH2OH.
• A six-membered hemiacetal ring is called a pyranose,
and a five-membered hemiacetal ring is called a
furanose because these ring sizes correspond to the
heterocyclic compounds furan and pyran.
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Haworth Projections
• Aldopentoses also form cyclic hemiacetals.
• The most prevalent forms of D-ribose and other
pentoses in the biological world are furanoses.
• The prefix “deoxy” means “without oxygen.”
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Haworth Projections
D-Fructose (a 2-ketohexose) also forms a five-membered
cyclic hemiacetal.
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Chair Conformations
• For pyranoses, the six-membered ring is more accurately
represented as a strain-free chair conformation.
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Chair Conformations
• In both Haworth projections and chair conformations, the
orientations of groups on carbons 1- 5 of β-Dglucopyranose are up, down, up, down, and up and all are
equatorial.
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Mutarotation
• Mutarotation: The change in specific rotation that
accompanies the equilibration of α- and β-anomers in
aqueous solution.
• Example: When either α-D-glucose or β-D-glucose is
dissolved in water, the specific rotation of the solution
gradually changes to an equilibrium value of +52.7°,
which corresponds to 64% beta and 36% alpha forms.
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Formation of Glycosides
• Treatment of a monosaccharide, all of which exist almost
exclusively in cyclic hemiacetal forms, with an alcohol
gives an acetal.
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Formation of Glycosides
• A cyclic acetal derived from a monosaccharide is
called a glycoside.
• The bond from the anomeric carbon to the -OR group is
called a glycosidic bond.
• Mutarotation is not possible for a glycoside because an
acetal, unlike a hemiacetal, is not in equilibrium with
the open-chain carbonyl-containing compound.
• Glycosides are stable in water and aqueous base, but
like other acetals, are hydrolyzed in aqueous acid to an
alcohol and a monosaccharide.
• Glycosides are named by listing the alkyl or aryl group
bonded to oxygen followed by the name of the
carbohydrate in which the ending -e is replaced by -ide.
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Reduction to Alditols
• The carbonyl group of a monosaccharide can be reduced
to an hydroxyl group by a variety of reducing agents,
including NaBH4 and H2 in the presence of a transition
metal catalyst (H2/Pt).
• The reduction product is called an alditol.
• Alditols are named by changing the suffix -ose to -itol.
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Alditols
• Sorbitol is found in the plant world in many berries and
in cherries, plums, pears, apples, seaweed, and algae.
• It is about 60 percent as sweet as sucrose (table sugar)
and is used in the manufacture of candies and as a
sugar substitute for diabetics.
• These three alditols are also common in the biological
world. Note that only one of these is chiral.
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Oxidation to Aldonic Acids
• The aldehyde group of an aldose is oxidized under
basic conditions to a carboxylate anion.
• The oxidation product is called an aldonic acid.
• A carbohydrate that reacts with an oxidizing agent to
form an aldonic acid is classified as a reducing sugar
(it reduces the oxidizing agent).
• 2-Ketoses (e.g. D-fructose) are also reducing sugars.
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Sucrose
• Table sugar, obtained from the juice of sugar cane and
sugar beet.
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Lactose
• The principle sugar present in milk.
• About 5 - 8% in human milk, 4 - 5% in cow’s milk.
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Maltose
• From malt, the juice of sprouted barley and other cereal
grains.
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Polysaccharides
Polysaccharide: A carbohydrate consisting of large
numbers of monosaccharide units joined by glycosidic
bonds.
Starch: A polymer of D-glucose.
• Starch can be separated into amylose and amylopectin.
• Amylose is composed of unbranched chains of up to
4000 D-glucose units joined by α-1,4-glycosidic bonds.
• Amylopectin contains chains up to 10,000 D-glucose
units also joined by α-1,4-glycosidic bonds. At branch
points, new chains of 24 to 30 units are started by α1,6-glycosidic bonds.
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Polysaccharides
• Figure 20-3 Amylopectin is a branched polymer of D-
glucose units joined by α-1,4-glycosidic bonds. Branches
consist of D-glucose units that start with an α-1,6glycosidic bond.
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Polysaccharides
• Glycogen is the energy-reserve carbohydrate for animals.
• Glycogen is a branched polysaccharide of
approximately 106 glucose units joined by α-1,4- and
α-1,6-glycosidic bonds.
• The total amount of glycogen in the body of a wellnourished adult human is about 350 g, divided almost
equally between liver and muscle.
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Polysaccharides
Cellulose is a linear polysaccharide of D-glucose units
joined by β-1,4-glycosidic bonds.
• It has an average molecular weight of 400,000 g/mol,
corresponding to approximately 2200 glucose units per
molecule.
• Cellulose molecules act like stiff rods and align
themselves side by side into well-organized
water-insoluble fibers in which their -OH groups form
numerous intermolecular hydrogen bonds.
• This arrangement of parallel chains in bundles gives
cellulose fibers their high mechanical strength.
• It is also the reason why cellulose is insoluble in water.
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Polysaccharides
• Figure 20-4 Cellulose is a linear polysaccharide of D-
glucose units joined by β-1,4-glycosidic bonds.
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Polysaccharides
Cellulose (cont’d)
• Humans and other animals can not digest cellulose
because their digestive systems do not contain
β-glycosidases, enzymes that catalyze the hydrolysis
of β-glycosidic bonds.
• Termites have such bacteria in their intestines and can
use wood as their principal food.
• Ruminants (cud-chewing animals) and horses can also
digest grasses and hay.
• Humans have only α-glucosidases; hence, the
polysaccharides we use as sources of glucose are
starch and glycogen.
• Many bacteria and microorganisms have
β-glucosidases.
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Acidic Polysaccharides
Acidic polysaccharides: a group of polysaccharides that
contain carboxyl groups and/or sulfuric ester groups, and
play important roles in the structure and function of
connective tissues.
• There is no single general type of connective tissue.
• Rather, there are a large number of highly specialized
forms, such as cartilage, bone, synovial fluid, skin,
tendons, blood vessels, intervertebral disks, and
cornea.
• Most connective tissues are made up of collagen, a
structural protein, in combination with a variety of
acidic polysaccharides.
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Acidic Polysaccharides
Heparin (cont’d)
• Heparin is synthesized and stored in mast cells of
various tissues, particularly the liver, lungs, and gut.
• The best known and understood of its biological
functions is its anticoagulant activity.
• It binds strongly to antithrombin III, a plasma protein
involved in terminating the clotting process.
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Heparin
• Figure 20-5 The repeating pentasaccharide unit of
heparin.
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Chapter 20 Carbohydrates
End
Chapter 20
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