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Principles of
BIOCHEMISTRY
Third Edition
HORTON
MORAN
Prentice Hall c2002
OCHS
Chapter 8
RAWN
SCRIMGEOUR
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Chapter 8 - Carbohydrates
• Carbohydrates (“hydrate of carbon”) have
empirical formulas of (CH2O)n , where n ≥ 3
• Monosaccharides one monomeric unit
• Oligosaccharides ~2-20 monosaccharides
• Polysaccharides > 20 monosaccharides
• Glycoconjugates linked to proteins or lipids
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8.1 Most Monosaccharides are
Chiral Compounds
• Aldoses - polyhydroxy aldehydes
• Ketoses - polyhydroxy ketones
• Most oxidized carbon: aldoses C-1, ketoses
usually C-2
• Trioses (3 carbon sugars) are the smallest
monsaccharides
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Aldoses and ketoses
• Aldehyde C-1 is drawn at the top of a Fischer
projection
• Glyceraldehyde (aldotriose) is chiral (C-2 carbon
has 4 different groups attached to it)
• Dihydroxyacetone (ketotriose) does not have an
asymmetric or chiral carbon and is not a chiral
compound
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Fig 8.1 Fischer projections of: (a) L- and Dglyceraldehyde, (b) dihydroxyacetone
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Fig 8.2 Stereo view of L- and D-glyceraldehyde
(L)
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(D)
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Fig 8.3 Fisher projections of
3 to 6 carbon D-aldoses
• D-sugars have the same configuration as
D-glyceraldehyde in their chiral carbon
most distant from the carbonyl carbon
• Aldoses shown in blue (next slide) are most
important in biochemistry
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Fig. 8.3
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Fig. 8.3 (continued)
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Fig 8.3 (continued)
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Enantiomers and epimers
• D-Sugars predominate in nature
• Enantiomers - pairs of D-sugars and L-sugars
• Epimers - sugars that differ at only one of
several chiral centers
• Example: D-galactose is an epimer of
D-glucose at C-4
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Fig 8.4 Fisher projections of L- and D-glucose
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Fig 8.5 Fisher projections of the 3 to 6 carbon
D-ketoses (blue structures are most common)
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Fig. 8.5 (continued)
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Fig 8.5 (continued)
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8.2 Cyclization of Aldoses and Ketoses
Fig. 8.6 Reaction of
an alcohol with:
(a) An aldehyde to
form a hemiacetal
(b) A ketone to form a
hemiketal
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Fig 8.7 (a) Pyran and (b) furan ring systems
• (a) Six-membered sugar
ring is a “pyranose”
• (b) Five-membered sugar
ring is a “furanose”
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Fig 8.8 Cyclization of D-glucose
to form glycopyranose
• Fischer projection
(top left)
• Threedimensional figure
(top right)
• C-5 hydroxyl close
to aldehylde
group (lower left)
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Fig. 8.8 (continued)
• Reaction of C-5
hydroxyl with one
side of C-1 gives a,
reaction with the
other side gives b
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Fig 8.9 Cyclization of D-ribose to form a- and
b-D-ribopyranose and a- and b-D-ribofuranose
Continued on next slide
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Fig. 8.9 (continued)
Continued next slide
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Fig 8.9 (continued)
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8.3 Conformations of Monosaccharides
Fig. 8.10 Conformations of b-D-ribofuranose
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Fig 8.11 Conformations of b-D-glucopyranose
Haworth projection
Chair
conformation
Boat
conformation
(b) Stereo
view of
chair (left),
boat (right)
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Fig 8.12 Conformations of b-D-glucopyranose
• Top conformer is more
stable because it has
the bulky hydroxyl
substituents in
equatorial positions
(less steric strain)
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8.4 Derivatives of Monosaccharides
• Many sugar derivatives are found in
biological systems
• Some are part of monosaccharides,
oligosaccharides or polysaccharides
• These include sugar phosphates, deoxy and
amino sugars, sugar alcohols and acids
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Table 8.1
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A. Sugar Phosphates
Fig 8.13 Some important sugar phosphates
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B. Deoxy Sugars
• In deoxy sugars an H replaces an OH
Fig 8.14 Deoxy sugars
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C. Amino Sugars
• An amino group replaces a monosaccharide OH
• Amino group is sometimes acetylated
• Amino sugars of glucose and galactose occur
commonly in glycoconjugates
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Fig 8.15 Several amino sugars
• Amino and acetylamino groups are shown in red
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Fig. 8.15 (continued)
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D. Sugar Alcohols (polyhydroxy alcohols)
• Sugar alcohols: carbonyl oxygen is reduced
Fig 8.16 Several sugar alcohols
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E. Sugar Acids
• Sugar acids are carboxylic acids
• Produced from aldoses by:
(1) Oxidation of C-1 to yield an aldonic acid
(2) Oxidation of the highest-numbered carbon
to an alduronic acid
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Fig 8.17 Sugar acids derived from glucose
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Fig. 8.17 (continued)
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F. Ascorbic Acid (Vitamin C)
• L-Ascorbic acid is derived from D-glucuronate
Fig 8.18 L-Ascorbic acid
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8.5 Disaccharides and Other Glycosides
• Glycosidic bond - primary structural linkage in
all polymers of monosaccharides
• An acetal linkage - the anomeric sugar carbon is
condensed with an alcohol, amine or thiol
• Glucosides - glucose provides the anomeric
carbon
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Fig 8.19 Glucopyranose + methanol
yields a glycoside
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A. Structures of Disaccharides
Fig 8.20 Structures of (a) maltose, (b) cellobiose
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Fig. 8.20 (continued)
Structures of (c) lactose, (d) sucrose
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B. Reducing and Nonreducing Sugars
• Monosaccharides and most disaccharides are
hemiacetals (contain a reactive carbonyl group)
• Called reducing sugars because they can reduce
metal ions (Cu2+, Ag+)
• Examples: glucose, maltose, cellobiose, lactose
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C. Nucleosides and Other Glycosides
• Anomeric carbons of sugars can form glycosidic
linkages with alcohols, amines and thiols
• Aglycones are the groups attached to the
anomeric sugar carbon
• N-Glycosides - nucleosides attached via a ring
nitrogen in a glycosidic linkage
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Fig 8.21 Structures of three glycosides
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8.6 Polysaccharides
• Homoglycans - homopolysaccharides
containing only one type of monosaccharide
• Heteroglycans - heteropolysaccharides
containing residues of more than one type of
monosaccharide
• Lengths and compositions of a polysaccharide
may vary within a population of these molecules
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A. Starch and Glycogen
• D-Glucose is stored intracellularly in polymeric
forms
• Plants and fungi - starch
• Animals - glycogen
• Starch is a mixture of amylose (unbranched)
and amylopectin (branched)
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Fig 8.22 Structure of amylose
(a) Amylose is
a linear
polymer
(b) Assumes a
left-handed
helical
conformation
in water
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Fig 8.23 Structure of amylopectin
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Fig 8.24 Action of a- and b-amylase
on amylopectin
• a-Amylase
cleaves random
internal a-(1-4)
glucosidic bonds
• b-Amylase acts
on nonreducing
ends
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B. Cellulose and Chitin
Fig 8.25 Structure of cellulose
(a) Chair
conformation
(b) Haworth
projection
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Fig 8.26 Stereo view of cellulose fibrils
• Intra- and interchain H-bonding gives strength
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Fig 8.27 Structure of chitin
• Repeating units of b-(1-4)GlcNAc residues
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8.7 Glycoconjugates
• Heteroglycans appear in three types of
glycoconjugates:
Proteoglycans
Peptidoglycans
Glycoproteins
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A. Proteoglycans
• Proteoglycans - glycosaminoglycan-protein
complexes
• Glycosaminoglycans - unbranched
heteroglycans of repeating disaccharides
(many sulfated hydroxyl and amino groups)
• Disaccharide components include: (1) amino
sugar (D-galactosamine or D-glucosamine),
(2) an alduronic acid
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Fig 8.28 Repeating disaccharide
of hyaluronic acid
• GlcUA =
D-glucuronate
• GlcNAc=
N-acetylglucosamine
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Fig 8.29 Proteoglycan aggregate of cartilage
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B. Peptidoglycans
• Peptidoglycans - heteroglycan chains linked to
peptides
• Major component of bacterial cell walls
• Heteroglycan composed of alternating GlcNAc
and N-acetylmuramic acid (MurNAc)
• b-(1
4) linkages connect the units
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Fig 8.30 Glycan moiety of peptidoglycan
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Fig 8.31 Structure of the peptidoglycan
of S. aureus
(a) Repeating disaccharide unit, (b) Cross-linking of
the peptidoglycan macromolecule
(to tetrapeptide, next slide)
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Fig. 8.31 (continued)
(to disaccharide, previous slide)
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Penicillin inhibits a transpeptidase
involved in bacterial cell wall formation
• Fig 8.32 Structures of
penicillin and
-D-Ala-D-Ala
• Penicillin structure
resembling -D-AlaD-Ala is shown in red
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C. Glycoproteins
• Proteins that contain covalently-bound
oligosaccharides
• O-Glycosidic and N-glycosidic linkages
• Oligosaccharide chains exhibit great variability
in sugar sequence and composition
• Glycoforms - proteins with identical amino acid
sequences but different oligosaccharide chain
composition
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Four subclasses of O-glycosidic linkages
(1) GalNAc-Ser/Thr (most common)
(2) 5-Hydroxylysine (Hyl) to D-galactose
(unique to collagen)
(3) Gal-Gal-Xyl-Ser-core protein
(4) GlcNAc to a single serine or threonine
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Fig. 8.33 O-Glycosidic and
N-glycosidic linkages
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Fig 8.34 Four subclasses of
O-glycosidic linkages
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Fig 8.35 Structures of N-linked
oligosaccharides
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Fig. 8.35 (continued)
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