Transcript Chapter 12-carbohydrates

Chapter 12
Carbohydrates
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.
Monosaccharides
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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:
Monosaccharides

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.
Monosaccharide

Monosaccharides with
◦ three carbons: trioses
◦ Five carbons: pentose
◦ Six carbons: hexose
◦ And so on …
Monosacharides
Figure 12.1 Glyceraldehyde, the simplest aldose, contains one
stereocenter and exists as a pair of enantiomers.
Enantiomers

Enantiomers: a molecule has a nonsuperimposable mirror
image
◦ Chiral molecule – has four different groups
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.
(R)-Glyceraldehyde
(3-D representation)
(R)-Glyceraldehyde
(Fisher projection)
Monosacharides
In 1891, Emil Fischer made the arbitrary assignments of D- and Lto the enantiomers of glyceraldehyde.
•
D-monosaccharide: the –OH is attached to the bottom-most
assymetric center (the carbon that is second from the bottom) is on
the right in a Fischer projection.
Monosacharides
• L-monosaccharide: the -OH is on the left in a Fischer
projection.
Table 12.1
Table 20-1 p532
Table 12.2
Table 20-2 p532
Examples

Draw Fisher projections for all 2-ketopentoses. Which are D2-ketopentoses, which are L-2-ketopentoses? Prefer to table
12.2 (your textbook) to write their names
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.
Cyclic Structure
•
Aldehydes and ketones react with alcohols to form hemiacetals
• Cyclic hemiacetals form readily when the hydroxyl and carbonyl
groups are part of the same molecule and their interaction can
form a five- or six-membered ring.
Epimers

Diastereomers that differ in configuration at only on
asymmetric center
Haworth Projections
•
Figure 12.2 D-Glucose forms these two cyclic hemiacetals.
Same side
D-glucose
Β-D-Glucopyranose
β-D-Glucose
-D-Glucopyranose
-D-glucose
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.
Haworth Projections
In the terminology of carbohydrate chemistry,
◦ A six-membered hemiacetal ring is called a pyranose, and a fivemembered hemiacetal ring is called a furanose because these ring
sizes correspond to the heterocyclic compounds furan and pyran.
Haworth Projections
◦ Aldopentoses also form cyclic hemiacetals.
◦ The most prevalent forms of D-ribose and other pentoses in the
biological world are furanoses.
-D-Ribofuranose
-D-Ribose
β-2-Deoxy-D-ribofuranose
Β-2-Deoxy-D-ribose
◦ The prefix “deoxy” means “without oxygen.” at C2
Haworth Projections
D-Fructose (a 2-ketohexose) also forms a five-membered cyclic
hemiacetal.
-D-Fructofuranose
-D-Fructose
D-Fructose
β-D-Fructofuranose
β-D-Fructose
Examples

Give structure of the cyclic hemiacetal formed by
◦ 4-hydroxybutanal
◦ 5-hydroxypentanal
Chair Conformations
•
For pyranoses, the six-membered ring is more accurately
represented as a strain-free chair conformation.
β-D-Glucopyranose
D-Glucose
-D-Glucopyranose
Chair Conformations
•
In both Haworth projections and chair conformations, the
orientations of groups on carbons 1- 5 of b-D-glucopyranose are up,
down, up, down, and up.
Chair Conformations
Examples

Which OH groups are in the axial position in
β-D-mannopyranose
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β-D-idopyranose
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Mutarotation
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Mutarotation: The change in specific rotation that accompanies the
equilibration of a- and b-anomers in aqueous solution.
◦ Example: When either a-D-glucose or b-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.
β-D-Glucopyranose
β-D-Glucopyranose
D-Glucose
-D-Glucopyranose
Formation of Glycosides
•
Treatment of a monosaccharide, all of which exist almost exclusively
in cyclic hemiacetal forms, with an alcohol gives an acetal.
Glycosidic
bond
β-D-Glucopyranose
β-D-Glucose
Glycosidic
bond
Methyl β-D-glucopyranoside Methyl -D-glucopyranoside
Methyl -D-glucoside
Methyl β-D-glucoside
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.
Formation of Glycosides
• 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.
Examples

Draw a Haworth projection and a chair conformation for
methyl -D-mannopyranoside. Label the anomeric
carbon and glycosidic bond
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.
• The reduction product is called an alditol.
• Alditols are named by changing the suffix -ose to -itol
Alditols

The product formed when the CHO group of monosaccharide
is reduced to CH2OH group
•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.
Alditols

These three alditols are also common in the biological world.
Note that only one of these is chiral.
Erythritol
D-Mannitol
Xylitol
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).
• Itself is being oxidized
Oxidation to Aldonic Acids
• 2-Ketoses (e.g. D-fructose) are also reducing sugars.
Oxidation to Aldonic Acids
β-D-Glucopyranose
D-Glucose
D-Gluconate
an aldonic acid
Oxidation to Aldonic Acids
• The body uses glucuronic acid to detoxify foreign alcohols and
phenols.
• These compounds are converted in the liver to glycosides of
glucuronic acid and then excreted in the urine.
• The intravenous anesthetic propofol is converted to the
following water-soluble glucuronide and excreted.
Formation of Phosphoric esters
What are Disaccharides and
Oligosaccharides?
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Disaccharide: A carbohydrate containing two
monosaccharide units joined by a glycosidic bond
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Oligosaccharide: A carbohydrate containing from six to ten
monosaccharide units, each joined to the next by glycosidic
bond
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Polysaccharide: A carbohydrate consisting of large
numbers of monosaccharide units joined by glycosidic bonds.
Sucrose
•
Table sugar, obtained from the juice of sugar cane and sugar beet.
-1,2Glycosidic
bond
Sucrose
Lactose

The principle sugar present in milk.
◦ About 5 - 8% in human milk, 4 - 5% in cow’s milk.
◦ Has no sweetness
β-1,4-Glycosidic
bond
β-1,4-Glycosidic
bond
Lactose
Maltose
•
From malt, the juice of sprouted barley and other cereal grains.
-1,4Glycosidic bond
Maltose
Polysaccharides
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 a-1,4-glycosidic bonds.
• Amylopectin contains chains up to 10,000 D-glucose
units also joined by a-1,4-glycosidic bonds; at branch
points, new chains of 24 to 30 units are started by a1,6-glycosidic bonds.
Polysaccharides
•
Figure 12.3 Amylopectin is a branched polymer of D-glucose
units joined by a-1,4-glycosidic bonds. Branches consist of Dglucose units that start with an a-1,6-glycosidic bond.
-1,6-Glycosidic bond
-1,4-Glycosidic bonds
Polysaccharides
•
Glycogen is the energy-reserve carbohydrate for animals.
• Glycogen is a branched polysaccharide of approximately 106
glucose units joined by a-1,4- and a-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.
Polysaccharides
Cellulose is a linear polysaccharide of D-glucose units joined
by b-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 the 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.
Polysaccharides
•
Figure 12.4 Cellulose is a linear polysaccharide of D-glucose
units joined by b-1,4-glycosidic bonds.
β-1,4-Glycosidic bonds
Polysaccharides
Cellulose (cont’d)
◦ Humans and other animals can not digest cellulose because
their digestive systems do not contain b-glycosidases,
enzymes that catalyze the hydrolysis of b-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 a-glucosidases; hence, the
polysaccharides we use as sources of glucose are starch and
glycogen.
◦ Many bacteria and microorganisms have b-glucosidases.
Example

Draw a chair conformation for a disaccharide in which two
units of D-glucopyranose are joined by a β -1,3-glycosidic
bond
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.
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.
Heparin
•
Figure 12.5 The repeating pentasaccharide unit of heparin.
p546