CARBOHYDRATES
Bettelheim, Brown, Campbell, Farrell, Torres, Introduction General Organic, and Biochemistry, Twelfth Edition. © 2020 Cengage.
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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 7.
• Aldose: A monosaccharide containing an aldehyde group.
• Ketose: A monosaccharide containing a ketone group.
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Monosaccharides (1 of 4)
• he 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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Monosaccharides (2 of 4)
Figure 19.1 Glyceraldehyde, the simplest aldose, contains one
stereocenter and exists as a pair of enantiomers.
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Monosaccharides (3 of 4)
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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Monosaccharides (4 of 4)
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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Important Monosaccharides
D-Glucose, D-galactose, and D-fructose are all hexoses found prominently
in our metabolism.
• D-Glucose is normally found in human blood at concentrations of 65 to 110 mg/dL.
• D-Galactose is part of the disaccharide lactose.
• D-Fructose is twice as sweet as table sugar.
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Cyclic Structure
• Aldehydes and ketones react with alcohols to form
hemiacetals
• 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 (1 of 5)
• Figure 19.4 D-Glucose forms these two cyclic
hemiacetals.
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Haworth Projections (2 of 5)
• 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 carbon 1; that of most common
ketoses is carbon 2.
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Haworth Projections (3 of 5)
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 (4 of 5)
• 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 (5 of 5)
D-Fructose (a 2-ketohexose) also forms a five-membered
cyclic hemiacetal.
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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 (1 of 2)
• 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 (2 of 2)
• 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.
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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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Uronic Acids
• Enzyme-catalyzed oxidation of the primary alcohol at carbon 6 of a
hexose yields a uronic acid.
• D-Glucuronic acid is widely distributed in both the plant and animal
worlds.
• D-Glucuronic acid is an important component of the acidic
polysaccharides of connective tissues.
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Sucrose
• Table sugar, obtained from the juice of sugar cane and
sugar beet.
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Lactose
• The principal 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 (1 of 6)
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 α-1,6-glycosidic bonds.
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Polysaccharides (2 of 6)
• Figure 19.5 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-1,6-glycosidic bond.
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Polysaccharides (3 of 6)
• 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 well-nourished
adult human is about 350 g, divided almost equally between
liver and muscle.
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Polysaccharides (4 of 6)
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 (5 of 6)
• Figure 19.6 Cellulose is a linear polysaccharide of
D-glucose units joined by β-1,4-glycosidic bonds.
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Polysaccharides (6 of 6)
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 (1 of 2)
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 (2 of 2)
Heparin
• 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 19.7 The repeating pentasaccharide unit of
heparin.
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End
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