Transcript Chem 150 Unit 9 - Biological Molecules II Carbohydrates

Chem 150
Unit 9 - Biological Molecules II
Carbohydrates
Carbohydrates play many important roles in biological
systems. They represent the major form of chemical energy
for both plants and animals. In plants they represent the end
product of photosynthesis, and therefore connect all living
systems to the sun’s sustaining light energy. Our discussion of
carbohydrates will also introduce us to biopolymers, of which
proteins and nucleic acids also belong. One of these
polymers, the structural polysaccharide cellulose, ties more of
the earth’s organic carbon than any other molecule.
Introduction
Polymers are large molecules that are made by stringing
together, like beads on a string, smaller units called monomers.
• Poly- is the Greek prefix meaning “many”.
The names of may polymers describe what they are made from
• Polyethylene is made by stringing together many ethylene
units.
• Ethylene (ethene) is the monomer
• Polypropylene is made by stringing together many propylene
units.
• Propylene (propene) is the monomer.
• Polysaccharides are made by stringing together many
monosaccharides.
• Monosaccharides (simple sugars) are the monomers.
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Introduction
Carbohydrates are placed in to one of three catagories,
depending on the number of monosaccharide units, or
residues, they contain.
•
Monosaccharides, contain a single monosaccharide
residue.
•
Oligosaccharides, contain 2 to 10 monosaccharide
residues.
• These include the disaccharides, which contain 2 monosaccharide
residues.
•
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Polysaccharides, which contain more than 10
monosaccharide residues.
• These can contain thousands of monosaccharide residues.
Monosaccharides
•
•
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Monosaccharides are polyhydroxy aldehydes or ketones.
Monosaccharides contain 3 to 7 carbon atoms.
Monosaccharides
Monosaccharides are
classified according to
the number of carbons
and whether they
contain an aldehyde or
ketone.
•
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The “-ose” ending is
used to designate
carbohydrates.
Question (Clickers)
The monosaccharide shown below is a member of which type
of monosaccharide?
A)
Aldotetrose
B)
Aldopentose
C)
Ketopentose
D)
Aldohexose
CH2 OH
C
O
HO
C
OH
H
C
OH
H
C
OH
CH2 OH
E)
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Ketohexose
Question (Clickers)
The monosaccharide shown below is a member of which type
of monosaccharide?
A)
Aldotetrose
O
B)
C)
Ketopentose
D)
Aldohexose
E)
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Aldopentose
Ketohexose
H
C
H
C
OH
HO
C
OH
H
C
OH
CH2 OH
Monosaccharides
The physical properties of
monosaccharides are heavily
influenced by the large number of
hydroxy groups that they contain.
• There ability to participate in
numerous hydrogen bonds gives
them high melting points and
high solubilities in water.
• Each hydroxyl group has two
•
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hydrogen bonding acceptor sites
and one hydogen bonding donor
site.
Each carbonyl group has two
acceptor sites.
O
H
H
O
H
O
H
H
H
O
H
C
H
C
O
H
H
H
O
H
O
H
Monosaccharides
Monosaccharides contain chiral carbon atoms.
• This is what accounts for the large number of different
monosacchides.
• For each chiral carbon, n, a monosacharide has 2n
stereoisomers.
• These will be divided among 2n/2 pairs of enantiomers.
•Glucose contains 4 chiral carbons
•Glucose has 24 = 16 stereoisomers
•These stereoisomers can be grouped
into 16/2 = 8 pairs of enantiomers.
O
H
C
H
C
OH
HO
C
OH
H
C
OH
H
C
OH
H
C
OH
H
D-glucose
http://www.preparatorychemistry.com/Bishop_Jmol_carbohydrates.htm
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Monosaccharides
Fischer projections are
used to distinguish the
different stereoisomers.
• The letters D and L are
used to distinguish
between the members
of a pair of
enantiomers.
• The D or L designation is
based on the chiral
carbon furthest from the
carbonyl carbon.
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Fischer projection:
Question (Clickers)
How many stereoisomers does the the monosaccharide
shown below have?
A)
5
B)
32
C)
16
D)
8
CH2 OH
C
O
HO
C
OH
H
C
OH
H
C
OH
CH2 OH
E)
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3
Question (Clickers)
Is the monosaccharide shown below a D- or and L- isomer?
A)
D-isomer
B)
L-isomer
CH2 OH
C
O
HO
C
OH
H
C
OH
HO
C
H
CH2 OH
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Monosaccharides
Diasteriomers are stereoisomers that are not enantiomers.
• Diasteriomers have have different names
• Enatiomers have the same name and are distinguished by
a D or L.
enantiomers
diastereomers
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Monosaccharides
Important monosaccharides.
• pentose and hexoses are the most abundant
Pentoses
• D-ribose and D-2-deoxyribose are found in DNA, RNA
and nucleotides such as FADH2 and NADH
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Monosaccharides
Important monosaccharides.
• pentose and hexoses are the most
abundant
Hexoses
• D-glucose (dextrose or blood surgar) major metabolite and strorage form of
chemical energy.
• D-galactose - combines with glucose
to produce lactose (milk sugar)
• D-fructose (fruit sugar) - major
metabolite and sweetest tasting natural
sugar.
• fructose is a ketose
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Monosaccharides
Monosaccharide derivatives
• Deoxy sugars.
• One or more -OH’s are replaced with -H’s
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Monosaccharides
Monosaccharide derivatives
• Amino sugars.
• One or more -OH’s are replaced with -NH2’s
• Often these are acetylated to form amides.
Arthritis relief??
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Monosaccharides
Monosaccharide derivatives
• Alcohol sugars.
• The ketone or aldehyde is reduced to an alcohol are reduced to
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Monosaccharides
Monosaccharide derivatives
• Carboxylic acid sugars.
• The ketone, aldehyde, or primary alcohol is oxidized to a carboxylic
acid.
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Question
The monosaccharide D-xylose is
shown below. Draw the derivative
describe:
A)
B)
C)
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D-2-deoxyxylose
xylitol
D-xylonic acids (carbon 1 is
oxidized to a carboxylic acid)
O
H
C
H
C
OH
HO
C
H
H
C
OH
CH2
D-xylose
OH
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Reactions of Monosaccharides
Reduction of the carbonyl
• In the lab this can be done with H2 and a platinum catalyst.
• In the cell, NADH + H+ is used.
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Reduction of Aldehydes and Ketones (Unit 8)
The same reaction can also be used to reduce aldehydes
and ketones to alcohols:
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Reduction of Aldehydes and Ketones (Unit 8)
In biochemistry, NADH + H+ is used instead of H2
• The reduction of a ketone containing steroid by the enzyme
Hydroxsteroid dehydrogenase.
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Reactions of Monosaccharides
Oxidation of sugars
• Oxidation with Cu+ The Benedict’s test
• Sugars that are capable of producing a positve Benedicts
test are called reducing sugars.
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Oxidation of Aldehydes (Unit 8)
Aldehydes can also be oxidized with the copper(II) ion (Cu2+)
• This reaction oxidizes aldehydes, but not alcohols.
• The Cu2+ ion forms a clear blue solution
• The Cu+ that is produced in the reaction forms an orange/red
precipitate.
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Oxidation of Aldehydes (Unit 8)
Aldehydes can also be
oxidized with the copper(II)
ion (Cu2+)
• The reaction is called the
Benedict’s reaction, and
has been used for years
in a clinical setting to test
O
H
for the presence
of
C
glucoseH inC the
urine.
OH
HO
C
H
H
C
OH
H
C
OH
CH2
OH
glucose
Cu2+
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Cu2+ + Cu+
Cu+
Reactions of Monosaccharides
Oxidation of sugars
• Even though ketones should not give a positive Benedict’s
test, ketoses do.
• This is because under the basic conditions of the test, the
ketoses can isomerize to form aldoses, which the react.
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Reactions of Monosaccharides
Reactions with alcohols to form hemiacetals and hemiketals
• Since monosaccharides contain both hydroxyl groups
along with either aldehyde or ketone groups, they can react
to form hemiacetals and hemiketals.
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Reactions of Alcohols with Aldehydes and
Ketones (Unit 7)
The first reaction, which is similar to the reduction of
aldehydes and ketones, involves adding an alcohol across
the carbonyl to form a hemiacetal (from aldehydes) or a
hemiketal (from ketones).
O
H3C
CH2 C
H
+
O
CH2 CH3
Propanal
(Aldehyde)
Ethanol
(Alcohol)
O
H
+
O
CH2 CH3
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CH2 C
O
CH2 CH3
(Hemiacetal)
O
H
CH3 C
O
CH3
CH3
Propanone
(Ketone)
H
H
H
CH3 C
H3C
O
Ethanol
(Alcohol)
(Hemiketal)
CH2 CH3
Reactions of Alcohols with Aldehydes and
Ketones (Unit 7)
Hemiacetal and hemiketal formation is catalyzed by acids.
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Reactions of Alcohols with Aldehydes and
Ketones (Unit 7)
As we will see with the carbohydrates, the carbonyl group
and the alchohol that react can come from the same
molecule.
• This will produce a ring molecule.
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Cyclic Form of Monosaccharides
Monosaccharides form rings by reacting one of the hydroxyls
with the carbonyl to form a hemiacetal or hemiketal:
http://www.preparatorychemistry.com/Bishop_Jmol_carbohydrates.htm
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Cyclic Form of Monosaccharides
Usually these are drawn using a Haworth project:
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•
The OH’s that were on the right in the Fisher projection are
placed in the down position on the Haworth projection
•
The OH’s that were on the left in the Fisher projection are
placed in the up position on the Haworth projection
•
The CH2OH on the number 5 carbon points up for D
sugars and down for L sugars.
Cyclic Form of Monosaccharides
The hemiacetal or hemiketal carbon that forms in the ring is
called the anomeric carbon.
• The anomeric carbon is also chiral, which increases the
number of chiral carbons by 1 and increases the doubles
the number of stereoisomers.
• The two forms of the anomeric carbon are designated as α
or β.
• The β-anomer has the -OH pointing up in the ring form.
• The α-anomer has the -OH pointing down in the ring form.
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Cyclic Form of Monosaccharides
The ring formation is a dynamic equilibrium reaction.
• The open form can switch back and forth between the two
ring forms.
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Cyclic Form of Monosaccharides
When naming the ring forms of monosaccharides, the
endings -pyranose and -furanose to designate the sixmember and five-member rings, respectively.
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Cyclic Form of Monosaccharides
In general, the -OH on the chiral carbon furthest from the the
carbonyl is the one that reacts to from the pyranose or
furanose ring. This means that
• Aldohexoses will form pyranose rings:
D-glucose
(aldohexose)
•
Aldopentoses and ketohexoses will form furanose rings:
D-ribose
(aldopentose)
D-fructose
(ketohexose)
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Question (Clicker)
Shown to the below is the
Fischer projection
CH2 OH for sorbose
C O
Is the structure
shown
H C OH
A) D-sorbose
HO
B)
C
H
CH2OH
OH
Draw and name the α and β
ring forms for sorbose
H
H
L-sorbose
H C OH
CH2 OH
CH3OH
O
OH
OH
H
α-D-sorbofuranose
CH2OH
OH
OH
O
H
H
H
OH
CH2OH
β-D-sorbofuranose
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Question (Clicker)
Shown to the below is the
Fischer projection
for galactose
O
H
H
C
Is the structure
HO C Hshown
H C OH
A)
D-galactose
H
B)
C
OH
L-galactose
HO C H
CH2 OH
Draw and name the α and β
ring forms for sorbose
O
H
H
CH2OH
OH
H
OH
OH
OH
H
α-L-galactopyranose
H
O
H
OH
CH2OH
OH
H
H
OH
OH
H
β-L-galactopyranose
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Oligosaccharides
Monosaccharides are connected to one another to form
oligosaccharides and polysaccharides by reacting the
anomeric (hemiacetal or hemiketal) hydroxyl group on one
sugar in its ring form, with a hydroxyl group from another
sugar.
• We saw in Unit 8 how this leads to the formation of acetals
and ketals.
• The bond that forms between the two monosaccharides is
called a glycosidic bond.
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Reactions of Alcohols with Aldehydes and
Ketones (Unit 7)
A hemiacetal or hemiketal can react with a second alcohol to
form an acetal or ketal.
• This is a substitution reaction and produces an water
molecule:
O
H3C
CH2 C
H +
O
H
H +
O
H
O
CH2 CH3
CH2 CH3
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CH2 CH3
CH2 C
O
Ethanol
(Alcohol)
CH2 CH3 +
H
O
H
H
O
H
H
(Acetal)
O
CH3 C
CH2 CH3
O
CH3
CH3
(Hemiketal)
O
H3C
Ethanol
(Alcohol)
(Hemiacetal)
CH3 C
CH2 CH3
CH2 CH3
H
O
O
(Ketal)
CH2 CH3 +
Oligosaccharides
The disaccharide D-maltose forms when the anomeric
carbon on a D-glucopyranose molecule in the α form reacts
with the hydroxyl group on the forth carbon of a second Dglucopyranose molecule:
• The bond that forms is called an α(1→4) glycosidic bond
Maltose is produced
from the breakdown
of the
polysaccharides
starch and glycogen
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Oligosaccharides
Maltose is still able to
reduce Cu+ in a
Benedict’s test, though it
is only 1/2 as reactive.
• Like monosaccharides,
maltose is considered a
reducing sugar.
• This is because the one
monosaccharide is still
able to open to expose
an aldehyde.
nonreducing end
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reducing end
Oligosaccharides
The disaccharide D-cellobiose forms when the anomeric
carbon on a D-glucopyranose molecule in the β form reacts
with the hydroxyl group on the forth carbon of a second Dglucopyranose molecule:
• The bond that forms is called an β(1→4) glycosidic bond
Cellobiose is
produced from the
breakdown of the
polysaccharids
cellulose.
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Unlike the α(1→4)
glycosidic bond in
maltose, most
organisms are
unable to cleave the
β(1→4) glycosidic
bond
Oligosaccharides
The disaccharide D-lactose forms when the anomeric
carbon on a D-galactopyranose molecule in the β form reacts
with the hydroxyl group on the forth carbon of a Dglucopyranose molecule:
• The bond that forms is called an β(1→4) glycosidic bond
Lactose is milk
sugar.
By the age of 5,
some people
become unable to
break the β(1→4)
glycosidic bond in
lactose, resulting in
lactose intolerance.
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Oligosaccharides
The disaccharide D-sucrose forms when the anomeric
carbon on a D-glucopyranose molecule in the α form reacts
with the hydroxyl group of the anomeric carbon of Dfructofuranose in the β form:
• The bond that forms is called an α,β(1↔2) glycosidic bond
Sucrose is table
sugar.
Because both
anomeric carbons
are involved in
forming the
glycosidic bond,
sucrose is not a
reducing sugar.
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Oligosaccharides
There are also oligosaccharides with 3 or more
monosaccharides
• The blood group antigens are oligosaccharides that are
attached to lipids and proteins found on cell surfaces.
A: N-Acetyl-D-galactosamine
(as shown)
B: D-galactose
O: none
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Polysaccharides
Polysaccharides are polymers of 10 or more monosaccharide
units
• Homopolysaccharides contain a single type of
monosaccharide unit.
• Heteropolysaccharides contain more than one typee of
monosaccharide unit.
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Polysaccharides
The polysaccharide cellulose is a structural polymer
produced by plants:
• It is a linear, unbranched polyer, with D-glucopyranose
units connected by β(1→4) glycosidic bonds
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Polysaccharides
The polysaccharide cellulose is a structural polymer
produced by plants:
• Cellulose forms a very insoluble, fibrous network
• Most organism are unable to digest cellulose because they
lac the enzymes needed to break the β(1→4) glycosidic
bonds
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Polysaccharides
The polysaccharide starch is a polymer produced by plants
for glucose storage:
• It is a linear, or branched polymer, with D-glucopyranose units
connected by α(1→4) glycosidic bonds
amylose
amylopectin
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Polysaccharides
The polysaccharide starch is a polymer produced by plants
for glucose storage:
• Unlike cellulose, starch has a very open and soluble
structure.
• Animals also produce a storage form of glucose called
glycogen, which has a structure similar to amylopectin.
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Polysaccharides
Heteropolymers
• Hyaluronic acid
• Found in lubricating fluid that surrounds joints and in the vitreous
humor of the eye.
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Polysaccharides
Heteropolymers
• Chondroitin Sulfate
• Present in connective tissue
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The End