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19
General, Organic, and
Biochemistry, 7e
Bettelheim,
Brown, and March
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19-1
19 Chapter 19
Carbohydrates
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19-2
19 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
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19-3
19 Monosaccharides
• Monosaccharides are classified by their number
of carbon atoms
Name
Formula
Triose
Tetrose
Pentose
C3 H6 O3
C4 H8 O4
Hexose
Heptose
Octose
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C5 H1 0 O 5
C6 H1 2 O 6
C7 H1 4 O 7
C8 H1 6 O 8
19-4
19 Monosaccharides
• There are only two trioses
CHO
CH2 OH
CHOH
C= O
CH2 OH
Glyceraldehyde
(an aldotriose)
CH2 OH
D ihydroxyacetone
(a ketotriose)
• often aldo- and keto- are omitted and these compounds
are referred to simply as trioses
• although this designation does not tell the nature of the
carbonyl group, it at least tells the number of carbons
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19-5
19 Monosaccharides
• Glyceraldehyde, the simplest aldose, contains a
stereocenter and exists as a pair of enantiomers
CHO
CHO
H
C
OH
CH2 OH
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HO
C
H
CH2 OH
19-6
19 Monosaccharides
• Fischer projection: a two dimensional
representation for showing the configuration of
tetrahedral stereocenters
• horizontal lines represent bonds projecting forward
• vertical lines represent bonds projecting to the rear
CHO
H
C
OH
CH2 OH
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con vert to
a Fischer
projection
CHO
H
OH
CH2 OH
19-7
19 D,L Monosaccharides
• In 1891, Emil Fischer made the arbitrary
assignments of D- and L- to the enantiomers of
glyceraldehyde
CHO
H
OH
CHO
HO
H
CH2 OH
CH2 OH
D-Glyceraldehyde
L-Glyceraldehyde
[]25 = +13.5°
[]25 = -13.5°
D
D
• D-monosaccharide: the -OH on its penultimate carbon
is on the right
• L-monosaccharide: the -OH on its penultimate carbon
is on the left
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19-8
19 D,L Monosaccharides
• the most common D-tetroses and D-pentoses
CHO
H
OH
H
OH
CH2 OH
D -Erythros e
CHO
HO
H
H OH
CHO
H
OH
H
OH
H
OH
CH2 OH
D -Threose
CH2 OH
D-Rib os e
CHO
H
H
H
OH
H
OH
CH2 OH
2-Deoxy-D -rib os e
• the three common D-hexoses
H
HO
H
H
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CHO
OH
H
OH
OH
CH2 OH
D-Gl u co s e
H
HO
HO
H
CHO
OH
H
H
OH
CH2 OH
D-Gal acto se
CH2 OH
C=O
HO
H
H OH
H OH
CH2 OH
D -Fru c tos e
19-9
19 Amino Sugars
• Amino sugars contain an -NH2 group in place of
an -OH group
• only three amino sugars are common in nature: Dglucosamine, D-mannosamine, and D-galactosamine
CHO
H NH2
HO H
H OH
H OH
CH2 OH
CHO
H2 N 2 H
HO H
H OH
H OH
CH2 OH
CHO
H NH2
HO H
HO 4 H
H OH
CH2 OH
H
HO
H
H
CHO O
NHCCH3
H
OH
OH
CH2 OH
D -Glucosamine D -Man nosamine D -Galactosamine N-Acetyl-D (C-2 stereoisomer (C-4 stereois omer glu cosamine
of D -glu cosamine of D -glucos amin e)
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19-10
19 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
O
4
1
H
red raw to show
-OH an d -CHO
clos e to each oth er
O-H
4-Hyd roxypentanal
1
4
O
H
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C
H
O
H
O-H
O
A cyclic hemiacetal
19-11
19 Haworth Projections
• D-Glucose forms these cyclic hemiacetals
1
CHO
H
OH
HO
H
H
H
red raw to sh ow th e -OH
on carbon-5 close to the
aldeh yd e on carbon-1
OH
5
OH
H
CH2 OH
D -Glucose
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CH 2 OH
5
OH
H
O
H
OH H C1
HO
H
CH2 OH
O OH (  )
H
H
OH H
HO
H
H OH
-D -Glucopyranose
(-D -Glucose)
OH
CH2 OH anomeric
carb on
OH
H
H
+
OH H
HO
OH(  )
H OH
-D -Glucopyranose
( -D -Glucos e )
19-12
19 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 an 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 ketoses is C-2
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19-13
19 Haworth Projections
•
In the terminology of carbohydrate chemistry,
•  means that the -OH on the anomeric carbon is on the
same side of the ring as the terminal -CH2OH
•  means that the -OH on the anomeric carbon is on the
side of the ring opposite from the terminal -CH2OH
• a six-membered hemiacetal ring is called a pyranose,
and a five-membered hemiacetal ring is called a
furanose
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O
O
Furan
Pyran
19-14
19 Haworth Projections
• aldopentoses also form cyclic hemiacetals
• the most prevalent forms of D-ribose and other
pentoses in the biological world are furanoses
HOCH2
H
H
O
H
HOCH2
H
OH ()
O
H
H
OH ()
H
H
OH
OH
OH
H
-2-D eoxy-D -ribofuranose
-D -Ribofuranose
(-2-D eoxy-D -rib os e)
(-D -Rib os e)
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19-15
19 Haworth Projections
• D-fructose also forms a five-membered cyclic
hemiacetal
HOCH2
5
1
O
H HO
CH2 OH
2
OH( )
H
HO
H
 -D -Fructofuranose
( - D -Fructos e)
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1
2
CH2 OH
C=O
HO
H
H
OH
H 5 OH
CH2 OH
D -Fru ctose
HOCH2
5
O
H HO
H
OH ( )
2
CH2 OH
HO
H
1
 - D -Fru ctofu ran os e
(- D -Fructose)
19-16
19 Chair Conformations
• For pyranoses, the six-membered ring is more
accurately represented as a chair conformation
HO
HO
CH2 OH
O
anomeric
carbon
OH()
OH
 -D -Glu copyran os e
( - D -Glucos e)
HO
HO
CH2 OH
OH
O
C
OH H
D -Glucos e
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HO
HO
CH2 OH
O
HO
OH( )
- D -Glu copyran os e
(  - D -Glucose)
19-17
19 Chair Conformations
• in both a Haworth projection and a chair conformation,
the orientations of groups on carbons 1- 5 of -Dglucopyranose are up, down, up, down, and up
6
CH2 OH
5
O OH()
H
H
4 OH
1
H
HO
H
3
2
H OH
-D -Glucop yranose
(Haw orth p rojection)
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6
CH2 OH
4
HO
HO
O
5
3
2
OH 1
OH( )
 - D -Glucopyranose
(ch air con formation)
19-18
19 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
HO
HO
CH2 OH
O
OH
OH
-D -Glucopyranose
[] D 2 5 = + 18.7°
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HO
HO
CH2 OH
OH
O
C
HO
H
Open-chain form
HO
HO
CH2 OH
O
HO
OH
-D -Glucopyranose
[] D 2 5 = +112°
19-19
19 Physical Properties
• Monosaccharides are colorless crystalline solids,
very soluble in water, but only slightly soluble in
ethanol
• sweetness relative to sucrose:
S w eetness
Relative to
Carbohydrate
S ucrose
fructos e
1.74
sucrose (tab le sugar) 1.00
honey
0.97
glu cose
0.74
maltose
0.33
galactos e
0.32
lactose (milk su gar)
0.16
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S w eetness
Relative to
Artificial
Sw eetener
S ucrose
saccharin
450
acesu lfame-K
200
aspartame
180
19-20
19 Formation of Glycosides
• Treatment of a monosaccharide, all of which exist
almost exclusively in a cyclic hemiacetal form,
with an alcohol gives an acetal
anomeric
carbon
CH2 OH
O OH
H
+
H
H
+ CH3 OH
OH H
-H2 O
HO
H
glycos idic
H OH
CH2 OH
bond
-D -Glu copyran os e
O OCH3
H
(-D -Glu cose)
H
+
OH H
H
HO
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CH2 OH
OH
H
H
OH H
HO
OCH3
H OH
H OH
Methyl -D -glu copyran os ide Methyl -D -glu copyran os ide
(Methyl -D -glu coside)
(Methyl -D -glucos ide)
19-21
19 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 in a glycoside because an
acetal, unlike a hemiacetal, is not in equilibrium with
the open-chain carbonyl-containing compound
• 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
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19-22
19 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
HO
HO
CH2 OH
O
OH
OH
-D -Glucop yranose
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CHO
H OH
HO H
NaBH4
H OH
H OH
CH2 OH
D -Glu cose
CH2 OH
H OH
HO H
H OH
H OH
CH2 OH
D -Glucitol
(D -Sorbitol)
19-23
19 Reduction to Alditols
• 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
• these three alditols are also common in the biological
world
CH2 OH
H
OH
H
OH
CH2 OH
Erythritol
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CH2 OH
HO
H
HO
H
H
OH
H
OH
CH2 OH
D -Mannitol
CH2 OH
H
OH
HO
H
H
OH
CH2 OH
Xylitol
19-24
19 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
• any carbohydrate that reacts with an oxidizing agent to
form an aldonic acid is classified as a reducing sugar
(it reduces the oxidizing agent)
H
O
C
HO
CH2 OH
O
HO
OH
OH
 - D -Glucopyranose
( - D -Glucos e)
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O-
O
C
H
HO
H
H
OH oxidizin g
H
OH
agent
H
HO
H
OH
b asic
H
OH
OH solution
H
OH
CH2 OH
CH2 OH
D -Glu cose
D -Glu conate
19-25
19 Oxidation to Uronic Acids
• Enzyme-catalyzed oxidation of the primary
alcohol at C-6 of a hexose yields a uronic acid
• enzyme-catalyzed oxidation of D-glucose, for example,
yields D-glucuronic acid
CHO
enzymeH
OH
catalyzed
HO
H
oxidation
H
OH
H
OH
CH2 OH
D -Glu cose
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H
HO
H
H
CHO
OH
H
OH
OH
COOH
COOH
HO
HO
O
OH
OH
D -Glucu ronic acid
(a u ronic acid )
19-26
19 D-Glucuronic Acid
• D-glucuronic acid is widely distributed in the plant and
animal world
• in humans, it is an important component of the acidic
polysaccharides of connective tissues
• it is used by the body to detoxify foreign phenols and
alcohols; in the liver, these compounds are converted
to glycosides of glucuronic acid and excreted in the
urine
COOHO
HO
HO
O
O
OH
Propofol
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A u rin e-s olu ble glucuronide
19-27
19 Phosphate Esters
• Mono- and diphosphoric esters are intermediates
in the metabolism of monosaccharides
• for example, the first step in glycolysis is conversion of
D-glucose to -D-glucose 6-phosphate
• note that at the pH of cellular and intercellular fluids,
both acidic protons of a phosphoric ester are ionized,
giving it a charge of -2
H
HO
H
D -Glucos e 6-phosp hate
H
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CHO
OH
H
OH
OH O
CH2 O-P-O
O
O
O P O
O
CH2
HO
HO
O
HO
OH
19-28
19 Disaccharides
• Sucrose (table sugar)
• sucrose is the most abundant disaccharide in the
biological world; it is obtained principally from the juice
of sugar cane and sugar beets
• sucrose is a nonreducing sugar
CH2 OH
OH
1
HO
HO
OH
HO
OH
O
O
HO 2
CH2 OH
1
OH
HOCH2
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a unit of -D glu copyran os e
CH2 OH
O
HOCH2
O
HO
OH
1
O
a unit of -D fructofuranose
2
1
CH2 OH
19-29
19 Disaccharides
• Lactose
• lactose is the principal sugar present in milk; it makes
up about 5 to 8 percent of human milk and 4 to 6
percent of cow's milk
• it consists of D-galactopyranose bonded by a -1,4glycosidic bond to carbon 4 of D-glucopyranose
• lactose is a reducing sugar
CH2 OH
OH
O
CH2 OH
O
OH
4
1
OH
CH2 OH
-1,4-glycosid ic bond
O
4
O
OH
OH
OH
HO
1
OH
O
HO
CH2 OH
O
OH
OH
OH
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19-30
19 Disaccharides
• Maltose
• present in malt, the juice from sprouted barley and
other cereal grains
• maltose consists of two units of D-glucopyranose
joined by an -1,4-glycosidic bond
• maltose is a reducing sugar
1
HOCH2 O
HO
OH
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CH2 OH
4
O
OH
OH
HO
O OH
HO
HO
-1,4-glycosid ic
b on d
CH2 OH
O
1
OH 4 CH2 OH
O
O
HO
OH
OH
19-31
19 Polysaccharides
• Polysaccharide: a carbohydrate consisting of
large numbers of monosaccharide units joined by
glycosidic bonds
• 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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19-32
19 Polysaccharides
• Glycogen 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 wellnourished adult human is about 350 g, divided almost
equally between liver and muscle
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19-33
19 Polysaccharides
• Cellulose 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 waterinsoluble 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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19-34
19 Polysaccharides
• Cellulose (cont’d)
• humans and other animals cannot use cellulose as
food because our digestive systems do not contain glucosidases, enzymes that catalyze hydrolysis of glucosidic bonds
• instead, we have only -glucosidases; hence, the
polysaccharides we use as sources of glucose are
starch and glycogen
• many bacteria and microorganisms have glucosidases and can digest cellulose
• termites have such bacteria in their intestines and can
use wood as their principal food
• ruminants (cud-chewing animals) and horses can also
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digest grasses and hay
19-35
19 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
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19-36
19 Acidic Polysaccharides
• Hyaluronic acid
• contains from 300 to 100,000 repeating units
• it is most abundant in embryonic tissues and in
specialized connective tissues such as synovial fluid,
the lubricant of joints in the body, and the vitreous of
the eye where it provides a clear, elastic gel that
maintains the retina in its proper position
D -glucu ronic acid
N-Acetyl-D -glu cosamine
-
4
HO
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COO
4
O HO
O
1
CH2 OH
O
1
NH
C
H3 C
O
The rep eating unit of h yalu ronic acid
3
OH
O
3
19-37
19 Acidic Polysaccharides
• Heparin: a heterogeneous mixture of variably
sulfonated polysaccharide chains, ranging in
molecular weight from 6,000 to 30,000 g/mol
N -acetyl-D -glu cos amin e
OSO3
CH2
O
HO
D -glucuronic acid
-
D -glucosamine
O
NH
O C O
HO
CH3
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OH
CH2
-
COO
O
O
OH - O S
3
L-id uronic acid
D -glucosamine
O
O
NH
O
SO3 - HO
O
O
HO
COO OSO 3
OSO3 CH2
O
NH
OSO3
19-38
19 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
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19-39
19 Carbohydrates
End
Chapter 19
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19-40