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Chem 1152: Ch. 17
Carbohydrates
Introduction
Biomolecules: Organic compounds produced by living organisms
• Carbohydrates
• Lipids
• Proteins
• Nucleic acids
Functions of carbohydrates
1. Provide energy through their oxidation
2. Supply carbon for synthesis of cell components
3. Serve as stored form of chemical energy
4. Form structural elements of some cells and
tissues
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Classes of Carbohydrates (saccharides)
Hydrates of carbon Cm(H2O)n characterized by having multiple functional groups
• Hydroxyl (alcohols)
• Carbonyl (aldehydes or ketones)
Mono- and disaccharides are simple sugars
glucose
fructose
galactose
ribose
deoxyribose
Fructo-oligosaccharides
galactooligosaccharides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011;
http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/sugar.htm
sucrose
lactose
starch
glycogen
cellulose
chitin
Stereochemistry of Carbohydrates
Stereoisomerism: Isomeric molecules can have the same molecular formula and
sequence of bonded atoms (constitution), but that differ in the threedimensional orientations of their atoms in space.
Enantiomers: Stereoisomers that are mirror images of each other.
Chirality: “Handedness”. Refers to compounds that cannot be superimposed on
mirror image.
-Defined relative to central, chiral atom (carbon)
enantiomers
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011; http://web.fccj.org/~ethall/stereo/stereo.htm
Stereochemistry of Carbohydrates
Chiral Carbon: Carbon in organic compound that has four different groups
attached to it.
Chirality: “Handedness”. Refers to compounds that cannot be superimposed on
mirror image.
Chiral carbon
H
CHO indicates
aldehyde
OHC
C
H
OH
HO
CH2OH
OHC
C
OH
CH2OH
enantiomers
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
C
CHO
CH2OH
HO
CHO
C
CH2OH
Identify Chiral Carbons
OHCCHOH
CH2OH
CH3CCH2CH3
O
CH3CHCH3
OH
CH3CHCH2CH3
OH
HO
O
H
OHC
*C
OH
CH2OH
Identify chiral carbons in glucose
H
O
1C
H
HO
H
H
2
3
4
5
C
OH
C
H
C
OH
C
OH
6 CH2
glucose
OH
chiral carbons in glucose
Max # of possible stereoisomers = 2n
where n = number of chiral carbon atoms
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
p. 528
Fischer Projections
 Chiral Carbons represented by intersection of two lines
 For fischer projections the prefixes (L-) and (D-) do not indicate Levorotatory
(L-) and Dextrorotatory (D-) related to rotation of polarized light, but
represent orientation of functional groups when compared to glyceraldehyde
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
 When carbonyl is up:
Fischer Projections
 D-family: OH (or NH2) group of chiral C most distant from anomeric center projects to right
 L-family: OH (or NH2) group of chiral C most distant from anomeric center projects to left
 Anomeric center: Carbonyl (aldehyde or ketone) carbon
 Biological systems can only utilize the D- isomers.
*
*
*
*
*
*
*
*
*
*
*
*
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Monosaccharides
 Contain single polyhydroxy aldehyde or ketone unit
 Further classified based on number of C atoms
 Aldehydes contain prefix aldo-, ketones have prefix keto-
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Monosaccharides: D- aldoses
Chiral C’s?
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
stereoisomers
1
2
2
4
3
8
4
16
Monosaccharides: Chemical Properties
 Because aldehydes and ketones react with alcohols to form hemiacetals and
hemiketals, all monosaccharides with at least 5 carbon atoms exist
predominantly in cyclic forms.
 These are depicted using Haworth structures
 Ring is drawn with oxygen to the back, and anomeric carbon to the right.
 Furanose ring: A 5-member ring containing an oxygen atom.
 Pyranose ring: A 6-member ring containing an oxygen atom.
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Glucose (Blood Sugar)
 Primary source of energy in cells
 Precursor for Vitamin C synthesis
 Modified subunits can form long polymer chains
 starch, cellulose, glycogen
H
0.02%
O
1C
H
HO
H
H
2
3
4
5
C
OH
C
H
C
OH
C
OH
6 CH2
64%
36%
OH
Hemiacetal (on C1)
 Beta-hydroxy group: OH attached to anomeric carbon above ring.
 Alpha-hydroxy group: OH attached to anomeric carbon below ring.
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Fructose
 Fruit sugar
 Absorbed directly into bloodstream during digestion (like glucose and galactose)
 Anomeric carbon is C2
1
2
3
22%
4
5
6
70%
Hemiketal (on C2)
http://upload.wikimedia.org/wikipedia/commons/1/1e/Fructose-isomers.jpg
Chemical Reactions of Sugars
 Open chain forms of monosaccharides (aldehydes, hydroxyketones) can be readily
oxidized.
 Ex. Use of Benedict’s reagent to oxidize aldehydes and ketones with hydroxy group
on adjacent carbon.
 At the same time, the cyclic forms are converted to open-chain forms and also react.
 Reducing sugars: Monosaccharides that can be oxidized
 Oxidation of carbohydrates to CO2 and H2O very important at cellular level, serves as
source of heat and energy.
Reducing sugar +
Cu(II) 
oxidized cmpd
Deep blue
solution
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
+ Cu2O
Red-orange
precipitate
Other Important Monosaccharides
Ribose and Deoxyribose
 Used to synthesize RNA and DNA
 Used in protein synthesis
Phosphodiester
bond
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011:
http://www.istpace.org/Web_Final_Report/WP_4_chem_subsystems/descr_chem_subsystems/pna_intro/pna_intro.html
Other Important Monosaccharides
Galactose
 Synthesized in mammary gland, incorporated into milk lactose
 Component of the antigens present on blood cells that determine blood type
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Joining Monosaccharides
 Monosaccharide units can be joined together by glycosidic linkages.
 Glycosidic linkages are same as adding alcohol to hemi- intermediates
H
O
O
R
+
H
O
R
O
R
H
+
H
R
H
O
OH
R
R
Hemiacetal
intermediate
acetal
CH 2OH
CH 2OH
CH 2OH
O
+
H-O-H
O
O
CH 2OH
HO
R
H+, RO-H
O
O
+
H O
OH
HO
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
O
OH
H2O
Polymerization of Monosaccharide Subunits
•Through glycosidic linkages at the hemiacetal carbons, many
monosaccharide subunits can be put together to form long,
branching chains via 1,4 or 1,6 linkages.
•These can be between 2 α, 2 β, or between an α and a β.
Disaccharides
Maltose
Lactose
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Sucrose
Properties of Polysaccharides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011
Plant starches
•Components of plant starch built from glucose residues.
Amylose (10-20%)
No branching
Amylopectin (80-90%)
Branching 24-30 residues
Structure of glycogen
•Polymer built from glucose subunits.
•Glucose in Glycogen are connected via α-1,4 or α-1,6 linkage.
•α-1,4 linkage makes a linear chain, α-1,6 linkage makes a branch
(~every 10 residues).
•The end glucose residues without open 1’-OH is called
nonreducing ends.
•Branches provide more non-reducing ends for rapid degradation.
What is glycogen and why do we need it?
•Long-term energy reservoir for glucose in animal and fungal
cells.
•Found primarily in muscle and liver cells.
•Liver produces glycogen for needs of organism, while muscle
takes care of only itself.
•Glycogen not as energy rich as fatty acids, and is used
differently.
•Controlled breakdown of glycogen and glucose release
maintain blood-glucose levels.
•Glucose is the primary fuel used by the brain.
•Unlike fatty acids, glucose from glycogen is readily mobilized
and can provide energy for sudden, strenuous activity.
Cellulose
•Most important structural polysaccharide and single most
abundant organic compound on earth.
• Provides strength and rigidity to plant cell walls.
•Wood is ~50% cellulose.
•Contains 300-3000 glucose subunits.
•Form extended straight chains that hydrogen bond with parallel
chains, creating long, rigid fibers.
•Undigestible by humans.
Intramolecular addition of alcohols to aldehyde
H
O
1C
6
*
H
C
2
HO
C
H
C
OH
C
OH
H
H
3
4
5
6 CH2
glucose
H
OH
4
5
C
OH3
CH2OH
OH
C
H
OH
O
C
C2 H
H
OH
H
C1
OH
Intramolecular
hemiacetal
 C1 is hemiacetal carbon.
 Attached to it you will find: H, OH, OR and R, just like non-cyclical compounds.