Download - Chapter 12 Carbohydrates
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Chapter 12 Chapter 12 CarbohydratesCarbohydrates
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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.
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Reduction to AlditolsReduction to AlditolsAlditols are named by changing
the suffix -ose-ose to -itol-itol
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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.
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AlditolsAlditols These three alditols are also common in the biological
world. Note that only one of these is chiral.
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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).
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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
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Oxidation to Aldonic AcidsOxidation to Aldonic Acids
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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.
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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
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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.
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SucroseSucrose• Table sugar, obtained from the juice of sugar cane and
sugar beet.
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LactoseLactose The principle sugar present in milk.
◦ About 5 - 8% in human milk, 4 - 5% in cow’s milk.◦ Has no sweetness
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MaltoseMaltose• From malt, the juice of sprouted barley and other cereal
grains.
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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.
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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.
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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.
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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.
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PolysaccharidesPolysaccharides• Figure 12.4 Cellulose is a linear polysaccharide of D-
glucose units joined by -1,4-glycosidic bonds.
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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.
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ExampleExample Draw a chair conformation for a disaccharide in which
two units of D-glucopyranose are joined by a β -1,3-glycosidic bond
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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.
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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.
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HeparinHeparin• Figure 12.5 The repeating pentasaccharide unit of
heparin.