chapter 12 carbohydrates. carbohydrates carbohydrate: carbohydrate: a polyhydroxyaldehyde or...
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Chapter 12 Chapter 12 CarbohydratesCarbohydrates
CarbohydratesCarbohydrates
Carbohydrate:Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.
Monosaccharide:Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.• Monosaccharides have the general formula
CCnnHH2n2nOOnn, where nn varies from 3 to 8.
• AldoseAldose:: A monosaccharide containing an aldehyde group.
• KetoseKetose:: A monosaccharide containing a ketone group.
MonosaccharidesMonosaccharidesThe suffix -ose-ose indicates that a molecule is a
carbohydrate.The prefixes tri-tri-, tetratetra, pentapenta, and so forth
indicate the number of carbon atoms in the chain.
Those containing an aldehyde group are classified as aldosesaldoses. .
Those containing a ketone group are classified as ketosesketoses..
There are only two trioses:
MonosaccharidesMonosaccharides 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.
HC
HC
H2C OH
O
Glyceraldehydean aldotrioses
OH C
H2C
H2C OH
OH
O
Dihydroxyacetonea ketotrioses
MonosaccharideMonosaccharide Monosaccharides with
◦ three carbons: trioses◦ Five carbons: pentose◦ Six carbons: hexose ◦ And so on …
M M M M M M
Polysaccharide
hydrolysisn M
monosaccharide
MonosacharidesMonosacharides
Figure 12.1 Glyceraldehyde, the simplest aldose, contains one stereocenter and exists as a pair of enantiomers.
EnantiomersEnantiomers Enantiomers: a molecule has a nonsuperimposable
mirror image◦ Chiral molecule – has four different groups
MonosaccharidesMonosaccharidesFischer projection: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)
MonosacharidesMonosacharidesIn 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.
• D-monosaccharide: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.
HC
HC
H2C OH
O
OH
achiral carbon
D-Glyceraldehyde
MonosacharidesMonosacharides
• L-monosaccharide:L-monosaccharide: the -OH is on the left in a Fischer projection.
CH
HC
H2C OH
O
HO
achiral carbon
L-Glyceraldehyde
Table 20-1 p532
Table 12.1
Table 20-2 p532
Table 12.2
ExamplesExamples Draw Fisher projections for all 2-ketopentoses.
Which are D-2-ketopentoses, which are L-2-ketopentoses? Prefer to table 12.2 (your textbook) to write their names
Amino SugarsAmino SugarsAmino 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 StructureCyclic Structure• Aldehydes and ketones react with alcohols to form
hemiacetalshemiacetals • 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.
EpimersEpimers Diastereomers that differ in configuration at only on
asymmetric center
HC O
OHH
OHH
OH
CH2OH
H
HC O
HHO
OHH
OH
CH2OH
H
D-ribose D-arabinose
1
2
3
4
5
1
2
3
4
5
C2-epimers*dif ferent conf iguration at C2
HC O
HHO
OHH
HHO
HC O
HHO
HHO
OHHO
CH2OH CH2OH
H OH OHH
C3-epimers*dif ferent conf iguration at C3
D-iodose D-talose
Haworth ProjectionsHaworth Projections• Figure 12.2 D-Glucose forms these two cyclic hemiacetals.
D-glucose
Β-D-Glucopyranoseβ-D-Glucose
-D-Glucopyranose-D-glucose
Same side
Haworth ProjectionsHaworth 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 carbonanomeric carbon.
• Stereoisomers that differ in configuration only at the anomeric carbon are called anomersanomers.
• The anomeric carbon of an aldose is C-1; that of the most common ketose is C-2.
Haworth ProjectionsHaworth Projections
In the terminology of carbohydrate chemistry,
◦ A six-membered hemiacetal ring is called a pyranosepyranose, and a five-membered hemiacetal ring is called a furanosefuranose because these ring sizes correspond to the heterocyclic compounds furan and pyran.
Haworth ProjectionsHaworth Projections◦ Aldopentoses also form cyclic hemiacetals.◦ The most prevalent forms of D-ribose and other pentoses
in the biological world are furanoses.
◦ The prefix “deoxydeoxy” means “without oxygen.” at C2
-D-Ribofuranose-D-Ribose
β-2-Deoxy-D-ribofuranose
Β-2-Deoxy-D-ribose
Haworth ProjectionsHaworth ProjectionsD-Fructose (a 2-ketohexose) also forms a five-membered cyclic hemiacetal.
-D-Fructofuranose-D-Fructose
D-Fructose β-D-Fructofuranoseβ-D-Fructose
ExamplesExamples Give structure of the cyclic hemiacetal formed by
◦ 4-hydroxybutanal
◦ 5-hydroxypentanal
Chair ConformationsChair Conformations• For pyranoses, the six-membered ring is more accurately
represented as a strain-free chair conformationstrain-free chair conformation.
β-D-Glucopyranose
D-Glucose
-D-Glucopyranose
Chair ConformationsChair Conformations• In both Haworth projections and chair conformations, the
orientations of groups on carbons 1- 5 of -D-glucopyranose are up, down, up, down, and up.
O
CH2OH
OH
OH
OH
1
23
4
5
6
OH
HOH2C
HOHO
OH123
45
6
-D-glucose
OH
same = cis
Chair ConformationsChair Conformations
O
OH
CH2OH
OH
OH
OH
HOH2C
1
23
4
5
6
HOHO
OHOH
123
45
6
D-glucose
opposite = trans
ExamplesExamples Which OH groups are in the axial position in β-D-mannopyranose
β-D-idopyranose
MutarotationMutarotation Mutarotation: Mutarotation: The change in specific rotation that
accompanies the equilibration of - and -anomers in aqueous solution.◦ Example: When either -D-glucose or -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 GlycosidesFormation of Glycosides• Treatment of a monosaccharide, all of which exist almost
exclusively in cyclic hemiacetal forms, with an alcohol gives an acetal.
Glycosidic bond
Glycosidic bond
β-D-Glucopyranose
β-D-Glucose
Methyl β-D-glucopyranoside
Methyl β-D-glucoside
Methyl -D-glucopyranoside
Methyl -D-glucoside
Formation of GlycosidesFormation of Glycosides
• A cyclic acetal derived from a monosaccharide is called a glycosideglycoside.
• The bond from the anomeric carbon to the -OR group is called a glycosidic bondglycosidic 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 GlycosidesFormation 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 -ee is replaced by -ide-ide.
ExamplesExamples Draw a Haworth projection and a chair conformation
for methyl -D-mannopyranoside. Label the anomeric carbon and glycosidic bond
Reduction to AlditolsReduction 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 alditolalditol.• Alditols are named by changing the suffix -ose-ose to -itol-itol
AlditolsAlditols The product formed when the CHO group of
monosaccharide is reduced to CH2OH group
OHO
HO
HOH2C
OH
OH
CHO
OHH
HHO
OHH
OHH
CH2OH
CH2OH
OHH
HHO
OHH
OHH
CH2OH
D-Glucopyranose
D-Glucose D-GlucitolD-Sorbitol
NaBH4
•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.
AlditolsAlditols 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 AcidsOxidation 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 acidaldonic acid.• A carbohydrate that reacts with an oxidizing agent to
form an aldonic acid is classified as a reducing sugarreducing sugar (it reduces the oxidizing agent).• Itself is being oxidized
H
C
C O
R
OHH
2-Ketose
C
C
HO H
R OH
C
C
O HC
C
O O
R
OHOHH
R
H
-OH -OH
enediol aldose aldonate
Oxidizingagent
• 2-Ketoses (e.g. D-fructose) are also reducing sugars.
Oxidation to Aldonic AcidsOxidation to Aldonic Acids
Oxidation to Aldonic AcidsOxidation to Aldonic Acids
β-D-Glucopyranose
D-GlucoseD-Gluconatean aldonic
acid
Oxidation to Aldonic AcidsOxidation 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 Formation of Phosphoric estersesters
CHO
OHH
HHO
OHH
OHH
CH2OH
D-Glucose
Enzyme-catalyzed phosphorylation
CHO
OHH
HHO
OHH
OHH
CH2O
D-Glucose 6-phosphate
P
O
O
O
OHO
HO
H2C
OHOH
O
P
O
OO
-D-Glucose 6-phosphate
What are Disaccharides and What are Disaccharides and Oligosaccharides?Oligosaccharides? Disaccharide: A carbohydrate containing two
monosaccharide units joined by a glycosidic bond
Oligosaccharide: A carbohydrate containing from six to ten monosaccharide units, each joined to the next by glycosidic bond
Polysaccharide:Polysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds.
SucroseSucrose• Table sugar, obtained from the juice of sugar cane and
sugar beet.
-1,2-Glycosidic
bond
Sucrose
LactoseLactose 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
MaltoseMaltose• From malt, the juice of sprouted barley and other
cereal grains.-1,4-
Glycosidic bond
Maltose
PolysaccharidesPolysaccharides
Starch: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 -1,4-glycosidic bonds.
• Amylopectin contains chains up to 10,000 D-glucose units also joined by -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by -1,6-glycosidic bonds.
PolysaccharidesPolysaccharides• Figure 12.3 Amylopectin is a branched polymer of D-
glucose units joined by -1,4-glycosidic bonds. Branches consist of D-glucose units that start with an -1,6-glycosidic bond.
-1,6-Glycosidic bond
-1,4-Glycosidic bonds
PolysaccharidesPolysaccharides
• GlycogenGlycogen is the energy-reserve carbohydrate for animals.• Glycogen is a branched polysaccharide of approximately
106 glucose units joined by -1,4- and -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.
PolysaccharidesPolysaccharidesCelluloseCellulose is a linear polysaccharide of D-glucose units joined by -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.
PolysaccharidesPolysaccharides• Figure 12.4 Cellulose is a linear polysaccharide of D-
glucose units joined by -1,4-glycosidic bonds.
β-1,4-Glycosidic bonds
PolysaccharidesPolysaccharides
Cellulose (cont’d)◦ Humans and other animals can not digest cellulose
because their digestive systems do not contain -glycosidases, enzymes that catalyze the hydrolysis of -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 -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen.
◦ Many bacteria and microorganisms have -glucosidases.
ExampleExample Draw a chair conformation for a disaccharide in
which two units of D-glucopyranose are joined by a β -1,3-glycosidic bond
Acidic PolysaccharidesAcidic PolysaccharidesAcidic 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 PolysaccharidesAcidic PolysaccharidesHeparin (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.
HeparinHeparin• Figure 12.5 The repeating pentasaccharide unit of
heparin.
p546
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