Transcript 21.5 Some Important Monosaccharides

Outline
21.1 An Introduction to Carbohydrates
21.2 Handedness of Carbohydrates
21.3 The D and L Families of Sugars: Drawing Sugar
Molecules
21.4 Structure of Glucose and Other Monosaccharides
21.5 Some Important Monosaccharides
21.6 Reactions of Monosaccharides
21.7 Disaccharides
21.8 Variations on the Carbohydrate Theme
21.9 Some Important Polysaccharides
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Goals
1. What are the different kinds of
carbohydrates?
Be able to define monosaccharides,
disaccharides, and polysaccharides, and
recognize examples.
2. Why are monosaccharides chiral, and how
does this influence the numbers and types
of their isomers?
Be able to identify the chiral carbon atoms in
monosaccharides, predict the number of
isomers for different monosaccharides, and
identify pairs of enantiomers.
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Goals, Continued
3. What are the structures of monosaccharides,
and how are they represented in written
formulas?
Be able to explain relationships among open-chain
and cyclic monosaccharide structures, describe the
isomers of monosaccharides, and show how they
are represented by Fischer projections and cyclic
structural formulas.
4. How do monosaccharides react with oxidizing
agents and alcohols?
Be able to identify reducing sugars and the
products of their oxidation, recognize acetals of
monosaccharides, and describe glycosidic linkages
in disaccharides.
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Goals, Continued
5. What are the structures of some important
disaccharides?
Be able to identify the monosaccharides combined in
maltose, lactose, and sucrose, and describe the types of
linkages between the monosaccharides.
6. What are the functions of some important
carbohydrates that contain modified monosaccharide
structures?
Be able to identify the functions of chitin, connective-tissue
polysaccharides, heparin, and glycoproteins.
7. What are the functions of some important
carbohydrates that contain modified monosaccharide
structures?
Be able to describe the monosaccharides and linkages in
these polysaccharides, their uses and fates in metabolism.
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21.1 An Introduction to Carbohydrates
• Carbohydrates are a large class of
naturally occurring polyhydroxy aldehydes
and ketones.
• Monosaccharides, or simple sugars,
have from three to seven carbon atoms,
and one aldehyde or one ketone group.
– If the sugar has an aldehyde group, it is an
aldose.
– If it has a ketone group, the sugar is classified
as a ketose.
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21.1 An Introduction to Carbohydrates
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21.1 An Introduction to Carbohydrates
• Monosaccharides undergo a variety of
structural changes and chemical reactions.
• They form disaccharides and
polysaccharides (complex
carbohydrates), which are polymers of
monosaccharides.
• Functional groups are involved in
reactions with alcohols, lipids, or proteins
to form biomolecules.
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21.2 Handedness of Carbohydrates
• Chiral compounds lack a plane of
symmetry and exist as a pair of
enantiomers in either a “right-handed”
D- form or a “left-handed” L- form.
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21.2 Handedness of Carbohydrates
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21.2 Handedness of Carbohydrates
• Aldotetroses have two chiral carbon atoms
and can exist in four isomeric forms.
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21.2 Handedness of Carbohydrates
• By convention the carbonyl group and the
terminal CH2OH are drawn pointing to the right.
• In general, a compound with n chiral carbon
atoms has a maximum of 2n possible
stereoisomers and half that many pairs of
enantiomers.
• In some cases, fewer than the maximum
predicted number of stereoisomers exist
because some of the molecules have symmetry
planes that make them identical to their mirror
images.
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21.3 The D and L Families of Sugars:
Drawing Sugar Molecules
• In a Fischer projection, the aldehyde or ketone
group is always placed at the top. The result is
that —H and —OH groups projecting above the
page are on the left and right of the chiral
carbons, and groups projecting behind the page
are above and below the chiral carbons.
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21.3 The D and L Families of Sugars:
Drawing Sugar Molecules
• Monosaccharides are divided into D sugars and
L sugars based on their structural relationships
to glyceraldehyde.
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21.3 The D and L Families of Sugars:
Drawing Sugar Molecules
• In the D form, the —OH group on carbon 2
comes out of the plane of the paper and points
to the right; in the L form, the —OH group at
carbon 2 comes out of the plane of the paper
and points to the left.
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21.3 The D and L Families of Sugars:
Drawing Sugar Molecules
• D and L derive from the Latin dextro for “right” and levo
for “left.” In Fischer projections, the D form of a
monosaccharide has the hydroxyl group on the chiral
carbon atom farthest from the carbonyl group pointing
toward the right, whereas the L form has the hydroxyl
group on this carbon pointing toward the left.
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21.3 The D and L Families of Sugars:
Drawing Sugar Molecules
Chirality and Drugs
• The goal of modern drug development is creation of a drug
molecule that binds with a specific hormone, enzyme, or cellular
receptor.
• Because most biomolecules are chiral, a chiral drug molecule
(single isomer) is likely to meet the need most effectively.
• The route to a chiral drug molecule begins with chiral reactants
and an enzyme, or with chemical synthesis of a pair of
enantiomers that are then separated from each other.
• Enantiomers can have different effects. the active ingredient in
the pain killer and anti-inflammatory Aleve, is sold as a single
enantiomer. The other enantiomer causes liver damage.
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21.4 Structure of Glucose and Other
Monosaccharides
• Monosaccharides with five or six carbon
atoms exist primarily in cyclic forms when
in solution.
• Aldehydes and ketones react reversibly
with alcohols to yield hemiacetals.
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21.4 Structure of Glucose and Other
Monosaccharides
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21.4 Structure of Glucose and Other
Monosaccharides
• D-Glucose can exist as an open-chain hydroxy
aldehyde or as a pair of cyclic hemiacetals.
• The cyclic forms differ only at C1, where the —OH
group is either on the opposite side of the sixmembered ring from the CH2OH (a) or on the same
side (b).
• The —CH2OH group in D sugars is always above
the plane of the ring.
• Cyclic monosaccharides that differ only in the
positions of substituents at carbon 1 are known as
anomers, and carbon 1 is said to be an anomeric
carbon atom.
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21.4 Structure of Glucose and Other
Monosaccharides
• Although the structural difference between anomers
appears small, it has enormous biological
consequences.
• Crystalline glucose is entirely in the cyclic a form. In
water, equilibrium is established among the open-chain
form and the two anomers through mutarotation.
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21.4 Structure of Glucose and Other
Monosaccharides
Monosaccharide Structures—Summary
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Monosaccharides are polyhydroxy aldehydes or ketones.
Monosaccharides have three to seven carbon atoms, and a maximum of 2n
possible stereoisomers, where n is the number of chiral carbon atoms.
D and L enantiomers differ in the orientation of the —OH group on the chiral
carbon atom farthest from the carbonyl. In Fischer projections, D sugars
have the —OH on the right and L sugars have the —OH on the left.
D-Glucose (and other 6-carbon aldoses) forms cyclic hemiacetals
conventionally represented so that —OH groups on chiral carbons on the left
in Fischer projections point up and those on the right in Fischer projections
point down.
In glucose, the hemiacetal carbon (the anomeric carbon) is chiral, and a and
b anomers differ in the orientation of the —OH groups on this carbon. The
a anomer has the —OH on the opposite side from the —CH2OH and the
b anomer has the —OH on the same side as the —CH2OH.
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21.5 Some Important Monosaccharides
• Monosaccharides can form multiple hydrogen
bonds, and are generally high-melting, white,
crystalline solids that are soluble in water and
insoluble in nonpolar solvents.
• Most monosaccharides and disaccharides are
sweet-tasting, digestible, and nontoxic.
• Except for glyceraldehyde (an aldotriose) and
fructose (a ketohexose), the carbohydrates of
interest in human biochemistry are all
aldohexoses or aldopentoses.
• Most are in the D family.
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21.5 Some Important Monosaccharides
Glucose
• Glucose is the most important simple carbohydrate in
human metabolism.
• It is the final product of complex carbohydrate digestion
and provides acetyl groups for entry into the citric acid
cycle as acetyl-CoA.
• Maintenance of an appropriate blood glucose level is
essential to human health.
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21.5 Some Important Monosaccharides
Galactose
• D-Galactose is widely distributed in plant gums and
pectins, and is a component of the disaccharide lactose.
• Galactose is an aldohexose; it differs from glucose only
in the spatial orientation of the —OH group at carbon 4.
• In the body, galactose is converted to glucose to provide
energy and is synthesized from glucose as needed.
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21.5 Some Important Monosaccharides
Fructose
• D-Fructose, often called levulose or fruit sugar, occurs in
honey and many fruits.
• It is one of the two monosaccharides combined in the
disaccharide sucrose.
• It is a ketohexose rather than an aldohexose. In solution,
fructose forms five-membered rings:
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21.5 Some Important Monosaccharides
Ribose and 2-Deoxyribose
• Ribose and its relative 2-deoxyribose are both 5-carbon
aldehyde sugars. These two sugars are most important
as parts of larger biomolecules.
• Ribose is a constituent of coenzyme A, ATP, oxidizing
and reducing agent coenzymes and cyclic AMP.
• 2-deoxyribose differs from ribose by the absence of one
oxygen atom, that in the —OH group at C2.
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21.5 Some Important Monosaccharides
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CELL-SURFACE CARBOHYDRATES AND BLOOD TYPE
Human blood can be classified into four blood group types,
called A, B, AB, and O, based on the presence of three
different oligosaccharide units, designated A, B, and O.
Among the targets of antibodies are cell-surface molecules
that not present on the individual’s own cells, thus “foreign
blood cells.”
Type O cell-surface oligosaccharides are similar in
composition to those of types A and B. People with blood
types A, B, and AB all lack antibodies to type O cells making
individuals with type O blood “universal donors.”
People with type AB blood have both A and B molecules on
their red cells. Their blood contains no antibodies to A, B, or
O, and they can, if necessary, receive blood of all types.
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21.6 Reactions of Monosaccharides
Reaction with Oxidizing Agents: Reducing Sugars
• Aldehydes can be oxidized to carboxylic acids (RCHO
→ RCOOH).
• As the open-chain aldehyde is oxidized, its equilibrium
with the cyclic form is displaced, so that the open-chain
form continues to be produced.
• Carbohydrates that react with mild oxidizing agents are
classified as reducing sugars.
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21.6 Reactions of Monosaccharides
Reaction with Oxidizing Agents: Reducing Sugars
• In basic solution, ketones can undergo rearrangement
and will also act as reducing sugars.
• Keto–enol tautomerism is an equilibrium that results
from a shift in position of a hydrogen atom and a double
bond. It is possible whenever there is a hydrogen atom
on a carbon adjacent to a carbonyl carbon.
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21.6 Reactions of Monosaccharides
Reaction with Alcohols: Glycoside and Disaccharide
Formation
• Hemiacetals react with alcohols with the loss of water
to yield acetals, compounds with two —OR groups
bonded to the same carbon.
• Because glucose and other monosaccharides are
cyclic hemiacetals, they also react with alcohols to
form acetals, which are called glycosides.
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21.6 Reactions of Monosaccharides
• In larger molecules monosaccharides are connected
to each other by glycosidic bonds.
• The reverse of this reaction is a hydrolysis.
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21.6 Reactions of Monosaccharides
Formation of Phosphate Esters of Alcohols
• Phosphate esters of alcohols contain a —PO3–2 group
bonded to the oxygen atom of an —OH group.
• The —OH groups of sugars can add —PO3–2 groups
to form phosphate esters in the same manner.
• Phosphate esters of monosaccharides appear as
reactants and products throughout the metabolism of
carbohydrates.
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21.7 Disaccharides
Disaccharide Structure
• The two monosaccharides in a disaccharide are connected
by a glycosidic bond. The bond may be a or b as in cyclic
monosaccharides.
• The structures include glycosidic bonds that create a 1,4
link between C1 of one monosaccharide and C4 of the
second monosaccharide.
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21.7 Disaccharides
Maltose
• Maltose can be prepared by enzyme-catalyzed
degradation of starch.
• Two a-D-glucose molecules are joined in maltose by an
a-1,4 link.
• It is both an acetal (at C1 in the glucose on the left) and
a hemiacetal (at C1 in the glucose on the right), making
maltose a reducing sugar.
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21.7 Disaccharides
Lactose
• Lactose, or milk sugar, is the major carbohydrate in
mammalian milk. Human milk, for example, is about 7%
lactose.
• Structurally, lactose is composed of D-galactose and
D-glucose. The two monosaccharides are connected by
a b-1,4 link. Like maltose, lactose is a reducing sugar
because the glucose ring (on the right in the following
structure) is a hemiacetal at C1.
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21.7 Disaccharides
Sucrose
• Hydrolysis of sucrose yields one
molecule of D-glucose and one
molecule of D-fructose. The 50:50
mixture of glucose and fructose that
results, invert sugar, is sweeter than
sucrose.
• Sucrose has no hemiacetal group
because a 1,2 link joins both
anomeric carbon atoms (C1 on
glucose, C2 on fructose). The
absence of a hemiacetal group
means that sucrose is not a reducing
sugar.
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21.7 Disaccharides
Carbohydrates and Fiber in the Diet
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The major monosaccharides in our diets are fructose and glucose from fruits
and honey. The major disaccharides are sucrose and lactose.
Our diets contain large amounts of the complex carbohydrate starch and the
indigestible polysaccharide cellulose.
Cellulose and all other indigestible carbohydrates are collectively known as
dietary fiber.
Consumption of the more easily digested carbohydrates results in rapid
elevation of blood glucose levels followed by lower-than-desired levels a few
hours later.
Carbohydrates that are digested and absorbed more slowly are associated
with healthier blood sugar responses.
The Nutrition Facts labels on packaged foods give percentages based on a
recommended 300 g per day of total carbohydrate and 25 g per day of
dietary fiber.
Pectins and vegetable gums comprise the “soluble” portion of dietary fiber.
Hemicellulose and lignin are the major components of “insoluble” fiber.
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21.8 Variations on the Carbohydrate Theme
• Variations on carbohydrates often
incorporate modified glucose molecules.
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21.8 Variations on the Carbohydrate Theme
Chitin
• The shells of lobsters, beetles, and spiders are
made of chitin, the second most abundant
polysaccharide in the natural world. (Cellulose is
the most abundant.)
• Chitin is a hard, structural polymer. It is
composed of N-acetyl-D-glucosamine rather
than glucose but is otherwise identical to
cellulose.
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21.8 Variations on the Carbohydrate Theme
Connective Tissue and Polysaccharides
• Connective tissues are composed of protein fibers in a matrix
that contains unbranched mucopolysaccharides.
• The gel-like mixtures of these polysaccharides with water
serve as lubricants and shock absorbers.
• Hyaluronate molecules contain up to 25,000 disaccharide
units. It forms the synovial fluid that lubricates joints and is
present within the eye.
• Chondroitin 6-sulfate (also the 4-sulfate) is present in tendons
and cartilage, where it is linked to proteins.
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21.8 Variations on the Carbohydrate Theme
Heparin
• Heparin is valuable medically as an anticoagulant
(an agent that prevents or retards the clotting of
blood).
• Heparin is composed of a variety of different
monosaccharides, many of them containing sulfate
groups.
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21.8 Variations on the Carbohydrate Theme
Glycoproteins
• Proteins that contain short
carbohydrate chains
(oligosaccharide chains) are
glycoproteins.
• The carbohydrate is connected to
the protein by a glycosidic bond
between an anomeric carbon and
a side chain of the protein.
• The oligosaccharide chains
function as receptors or, in one
case, antifreeze.
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21.9 Some Important Polysaccharides
• Polysaccharides are polymers of tens,
hundreds, or even many thousands of
monosaccharides linked together through
glycosidic bonds.
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21.9 Some Important Polysaccharides
Cellulose
• Cellulose provides structure in plants.
• Each molecule consists of several thousand
D-glucose units joined in a long, straight chain by
b-1,4 links.
• Because of the tetrahedral bonding at each carbon
atom, the carbohydrate rings bent up at one end
and down at the other in what is known as the chair
conformation.
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21.9 Some Important Polysaccharides
• The hydrogen bonds within chains and between chains (shown in
red) contribute to the rigidity and toughness of cellulose fibers.
• Grazing animals, termites, and moths are able to digest cellulose
because microorganisms colonizing their digestive tracts produce
enzymes that hydrolyze b-glycosidic bonds.
• Humans neither produce such enzymes nor harbor such organisms,
and therefore cannot hydrolyze cellulose.
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21.9 Some Important Polysaccharides
Starch
• In starch, individual glucose units are joined by
a-1,4 links rather than by the b-1,4 links of cellulose.
• Starch is fully digestible and is an essential part of
the human diet. It is present only in plant material;
our major sources are beans, wheat and rice, and
potatoes.
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21.9 Some Important Polysaccharides
Starch
• Amylose, accounts for about 20% of starch. It is
somewhat soluble in hot water and consists of
several hundred to a thousand D-glucose units
linked in long chains.
• Amylose tends to coil into helices.
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21.9 Some Important Polysaccharides
Starch
• Amylopectin accounts for about 80% of starch. It
has up to 100,000 glucose units per molecule
and a-1,6 branches approximately every 25 units
along its chain. It is not soluble.
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21.9 Some Important Polysaccharides
Glycogen
• Glycogen stores energy in animals in the liver and
muscles.
• Structurally, glycogen is similar to amylopectin in being a
long polymer of D-glucose with the same type of branch
points in its chain.
• Glycogen has many more branches and contains up to
one million glucose units per molecule.
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21.9 Some Important Polysaccharides
Cell Walls: Rigid Defense Systems
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Bacteria and higher plants surround the plasma membrane with a rigid
cell wall. The rigidity of the wall prevents the cell from bursting due to
osmotic pressure, gives shape to the cell, and protects it from
pathogens.
Plant cell walls are composed of fibrils of cellulose in a polymer matrix
of pectins, lignin, and hemicellulose.
Bacterial cell walls provide a rigid platform for the attachment of flagella
and pilli. They do not contain cellulose. A majority of bacterial cell walls
are composed of a polymer of peptidoglycan.
Animals have developed natural defenses that can control many
bacteria. For example, lysozyme—an enzyme found naturally in tears,
saliva, and egg white—hydrolyzes the cell wall of pathogenic bacteria.
Penicillin contains a beta-lactam ring that acts as a “suicide inhibitor” of
the enzymes that synthesize the peptidoglycan cross-linking peptide
chain. Unfortunately, many bacteria have developed enzymes that
destroy the ring, granting resistance to penicillin.
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Chapter Summary
1. What are the different kinds of
carbohydrates?
• Monosaccharides are compounds with
three to seven carbons, an aldehyde group
on carbon 1 (an aldose) or a ketone group
on carbon 2 (a ketose), and hydroxyl groups
on all other carbons.
• Disaccharides consist of two
monosaccharides.
• Polysaccharides are polymers composed of
up to thousands of monosaccharides.
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Chapter Summary, Continued
2. Why are monosaccharides chiral, and how
does this influence the numbers and types
of their isomers?
• Monosaccharides can contain several chiral
carbon atoms, each bonded to one —H one
—OH and two other carbon atoms in the carbon
chain. A monosaccharide with n chiral carbon
atoms may have 2n stereoisomers and half
that number of pairs of enantiomers. The
members of different enantiomeric pairs are
diastereomers; they are not mirror images of
each other.
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Chapter Summary, Continued
3. What are the structures of monosaccharides, and
how are they represented in written formulas?
• Fischer projection formulas represent the open-chain
structures of monosaccharides. They are interpreted
as shown below, with the D and L enantiomers in a pair
identified by having the —OH group on the chiral
carbon farthest from the carbonyl group on the right
(the D isomer) or the left (the L isomer).
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Chapter Summary, Continued
3. Continued…
• In solution, open-chain monosaccharides with five or
six carbons establish equilibria with cyclic forms that
are hemiacetals. The hemiacetal carbon (bonded to
two O atoms) is referred to as the anomeric carbon,
and this carbon is chiral. Two isomers of the cyclic
form of a D or L monosaccharide, known as anomers,
are possible because the —OH on the anomeric
carbon may lie above or below the plane of the ring.
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Chapter Summary, Continued
4.
How do monosaccharides react with oxidizing agents
and alcohols?
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Oxidation of a monosaccharide can result in a carboxyl group on
the first carbon atom (C1 in the Fischer projection).
Ketoses, as well as aldoses, are reducing sugars because the
ketose is in equilibrium with an aldose form (via an enediol) that
can be oxidized.
Reaction of a hemiacetal with an alcohol produces an acetal.
For a cyclic monosaccharide, reaction with an alcohol converts
the —OH group on the anomeric carbon to an —OR group. The
bond to the —OR group, known as a glycosidic bond, is a or b to
the ring as was the —OH group.
Disaccharides result from glycosidic bond formation between two
monosaccharides.
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Chapter Summary, Continued
5. What are the structures of some important
disaccharides?
• In maltose, two D-glucose molecules are joined by an
a-glycosidic bond that connects C1 (the anomeric
carbon) of one molecule to C4 of the other—an a-1,4
link.
• In lactose, D-galactose and D-glucose are joined by a
b-1,4 link.
• In sucrose, D-fructose and D-glucose are joined by a
glycosidic bond between the two anomeric carbons, a
1,2 link.
• Unlike maltose and lactose, sucrose is not a reducing
sugar because it has no hemiacetal that can establish
equilibrium with an aldehyde.
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Chapter Summary, Continued
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What are the functions of some important
carbohydrates that contain modified monosaccharide
structures?
Chitin is a hard structural polysaccharide found in the shells
of lobsters and insects.
Joints and intracellular spaces are lubricated by
polysaccharides like hyaluronate and chondroitin 6-sulfate,
which have ionic functional groups and form gel-like
mixtures with water.
Heparin, a polysaccharide with many ionized sulfate groups,
binds to a clotting factor in the blood and thus, acts as an
anticoagulant.
Glycoproteins have short carbohydrate chains bonded to
proteins; the carbohydrate segments (oligosaccharides)
function as receptors at cell surfaces.
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Chapter Summary, Continued
7. What are the structures and functions of cellulose,
starch, and glycogen?
• Cellulose is a straight-chain polymer of b-D-glucose with
b-1,4 links; it provides structure in plants. Cellulose is not
digestible by humans, but is digestible by animals whose
digestive tract contains bacteria that provide enzymes to
hydrolyze the b-glycosidic bonds.
• Starch is a polymer of a-D-glucose connected by a-1,4
links in straight-chain (amylose) and branched-chain
(amylopectin) forms. Starch is a storage form of glucose
for plants and is digestible by humans. Glycogen is a
storage form of glucose for animals, including humans. It
is structurally similar to amylopectin, but is more highly
branched. Glycogen from meat in the diet is also
digestible.
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