CHAPTER 17: Carbohydrates
FUNCTION
• Describe the 4 major functions of
carbohydrates in living systems
• Sources of energy
• Classify carbohydrates as monosaccharides
(aldoses or ketoses), disaccharides, or
polysaccharides
• Means of energy storage
• Recognize molecules possessing chiral carbon
atoms
• Sources of carbon for biomolecule synthesis
• Structure (Plants)
• Identify major chemical reactions and
predict the resulting products
– Oxidation
– Glycoside formation
– Di- and polysaccharide hydrolysis
17.1 CLASSES of CARBOHYDRATES
CARBOHYDRATES
CARBOHYDRATES
Compounds
Compounds with
with the
the general
general formula
formula CCXX(H
(H22O)
O)XX
that
that are
are polyhydroxy
polyhydroxy aldehydes
aldehydes or
or ketones
ketones or
or
compounds
compounds that
that yield
yield polyhydroxy
polyhydroxy aldehydes
aldehydes or
or
ketones
ketones
NO CHANGE
• May also include other functional groups:
–NH2 (bacterial cell walls)
–PO4 (metabolic intermediates)
–SO4 (connective tissue)
+
H
H22O
O
H+,, H
HYDROLYZE
MONOMONOSACCHARIDES
SACCHARIDES
DI-,
DI-, TRI-,
TRI-, POLYPOLYSACCHARIDES
SACCHARIDES
SIMPLE SUGARS
BREAK DOWN TO YIELD
SIMPLE SUGARS
17.2 STEREOISOMERISM
GENERAL NOMENCLATURE
Glyceraldehyde, the simplest monosaccharide,
can exist in 2 forms:
• MONOSACCHARIDE
– CX(H2O)X unit (X = 3-8)
• DISACCHARIDE
– 2 monosaccharide units
HOH2C
• POLYSACCHARIDE
– 1000+ monosaccharide units
CHO
OHC
C
H
H
OH
L-Glyceraldehyde
Page 17-1
C
HO
CH2OH
D-Glyceraldehyde
D- and L-glyceraldehyde are examples of
ENANTIOMERS
ENANTIOMERS
Nonsuperimposable
Nonsuperimposable mirror-image
mirror-image stereoisomers
stereoisomers
STEREOISOMERS
STEREOISOMERS
Molecules
Molecules that
that have
have the
the same
same composition,
composition,
the
the same
same number
number and
and types
types of
of bonds,
bonds, but
but
different
different arrangements
arrangements of
of the
the atoms
atoms in
in
space
space
L
HOH2C
Cis- and trans- isomers are stereoisomers
(Chapters 11, 12), but D- and L- isomers
differ in a specific way:
D
THEY ARE MIRROR IMAGES OF EACH
OTHER
HOH2C
CHO
OHC
C
H
H
OH
C
HO
D
CH2OH
CHO
C
OH
ASYMMETRIC (CHIRAL) CARBON
4 different groups attached
H
17.3 FISCHER PROJECTIONS
CHO
CHO
HOH2C
D- Glyceraldehyde
L-Glyceraldehyde
C
H
⇒
HO C
OH
H
CH2OH
L- Glyceraldehyde
CHO
(rotated)
HO
H
CH2OH
HOH2C
CHO
OHC
C
H
H
OH
L-Glyceraldehyde
HO
H
CH2OH
IDENTICAL except:
• Mirror-image crystals (sometimes)
• Rotate plane of polarized light in different
directions: (+) = right (−) = left
CH2OH
D-Glyceraldehyde
CHO
HO
C
PHYSICAL PROPERTIES OF
ENANTIOMERS
CHO
H
POLARIZER
ENANTIOMER SOLUTION
OH
CH2OH
OPTICAL
OPTICAL ISOMERS
ISOMERS
Page 17-2
EYEBALL
17.4 MONOSACCHARIDES
1
CHO
H
• ALDOSES
“Aldo” + multiplier + “ose”
HO
CHO
H
(CHOH)n-2
CH2OH
H
6
TOTAL Nº of CARBONS = n
• KETOSES
“Keto” + multiplier + “ose”
CHO
H
H
OH
OH
H
OH
OH
H
1 CH
2OH
2OH
ALDOPENTOSE
C O
O
C
OH
5 CH
CH2OH
ALDOHEXOSE
CH2OH
1
OH
HO
H
H
OH
H
OH
2OH
6 CH
(CHOH)n-3
CH2OH
KETOHEXOSE
17.5b CHEMICAL PROPERTIES of
MONOSACCHARIDES
17.5a PHYSICAL PROPERTIES
• CYCLIZATION
• Sweetness
CHO
• Extreme H2O solubility (many H-bonds)
H
• Optical activity
HO
CH2OH
OH
H
H
OH
H
OH
≡
CH2OH
CH2OH
OH
OH
OH
OH CHO
HO
HO
OH
OH
OH
• Monosaccharides contain both OH and C=O
groups
• Monosaccharides can form intramolecular
hemiacetals
In solution:
CH2OH
OH
HO
CH2OH
O
• OXIDATION
OH
OH CHO
OH
OH
α-D-(+)-GLUCOSE
(36%)
ANOMERS
HO
(<0.02%)
HO
OH
CH2OH
O OH
OH
HO
β-D-(+)-GLUCOSE
COOH
CHO
H
H
H
OH
H
OH
Benedict’s solution
H
H
or enzyme
OH
H
OH
CH2OH
enzyme
CHO
H
HO
H
OH
OH
HO
OH
CH2OH
MUTAROTATION:
MUTAROTATION:
interconversion
interconversion of
of
anomers
anomers
H
H
OH
H
OH
OH
COOH
(64%)
Page 17-3
• GLYCOSIDE FORMATION
• Carbohydrates that react with Benedict’s
or
Fehling’s solutions are REDUCING SUGARS
CH2OH
OH
• Ketoses react after rearranging to aldoses:
CHO
CH2OH
C
C
O
HO
OH
CH2OH
O
ROH
H+
OH
OH
OH
HO
hemiacetal
ALL
ALL MONOSACCHARIDES
MONOSACCHARIDES
ARE
ARE REDUCING
REDUCING SUGARS
SUGARS
O
GLYCOSIDIC
LINKAGE
OR
OH
acetal
Glycosidic
Glycosidic bond
bond locks
locks pyranose
pyranose ring
ring shut,
shut, so
so
there
there is
is no
no more
more reducing
reducing sugar
sugar function
function
17.6 IMPORTANT MONOSACCHARIDES
• D-(+)-GLUCOSE
6
CH2OH
OH
4
1
HO
CHO
H
O
1
C
O
HO
2OH
CH2OH
CH2OH
H
H
OH
OH
H
OH
2OH
6 CH
α-furanose
aldohexose
O
HO
OH
OH
6
1
5
OH
H
α-pyranose
6
H
H
OH
OH
1 CH
HOH2C
OH
HO
• D-(-)-FRUCTOSE
ketohexose
Also forms a pyranose
17.7 DISACCHARIDES
• D-(-)-RIBOSE
HOH2C
O
OH
OH
OH
β-furanose
• β-MALTOSE: α(1→4) linkage
CHO
H
OH
H
OH
H
CH2OH
2
OH
CH2OH
aldopentose
4
OH
HO
O
CH2OH
1
OH
OH
OH
O
1
CH2OH
4
O
HO
OH
OH
O
OH
OH
Contains
Contains aa hemiacetal:
hemiacetal:
Reducing
Reducing sugar
sugar
Page 17-4
• β-LACTOSE: β(1→4) linkage
CH2OH
HO
OOH
OH
+
• SUCROSE: α(1→2) linkage
CH2OH
CH2OH
OH
HO
O
OH
OH
OH
OH
CH2OH
HO
O
Galactose
Reducing
Reducing
sugar
sugar
CH2OH
OH
HO
O O
OH
OH
OH
OH
OH
OH
HO
STARCH: glucose storage molecule of plants
GLYCOGEN: glucose storage molecule of
animals
HOH2C
+
OH
OH
O
O
OH
OH
17.8 POLYSACCHARIDES
H2O
CH2OH
OH
CH2OH
O
HO
O
CH2OH
H+
CH2OH
O
OH
Invert
sugar
O
OH
O
GLYCOSIDIC BOND
CH2OH
O
HO
O
CH2OH
OH OH
CH2OH
CH2OH
HO
OH
HOH2C
Polymers of α-D-glucose:
—Glc—Glc—Glc—Glc—Glc—Glc—Glc—Glc—
O
HO
β-D-Fructose
No
No
mutarotation
mutarotation HO
• HYDROLYSIS
OH
+
OH
OH
O
HO
Contains
Contains only
only an
an acetal:
acetal:
Nonreducing
Nonreducing sugar
sugar
CHEMICAL PROPERTIES
HOH2C
HOH2C
O
O
HO
POLYMER
POLYMER
AA large
large molecule
molecule made
made up
up of
of small
small molecule
molecule
subunits
subunits (monomers).
(monomers).
CH2OH
HO
OH
Linear form
(in starch)
O
CH2OH
OH
OH
Linear
O
STARCH
O
Branched form
(in starch and GLYCOGEN)
OH
CH2OH
OH
O
CH2
OH
OH
GLYCOGEN
O
O
AA glycogen
glycogen molecule
molecule is
is
6
made
made up
up of
of about
about 10
106
glucose
units.
glucose units.
(plants)
(animals)
20%
(amylose)
Branched
Glucose
Units
80%
~1,000 linear
~100,000
(amylopectin)
100%
branched
~1,000,000
O
OH
STORAGE
STORAGE POLYSACCHARIDES
POLYSACCHARIDES
Page 17-5
• ENZYMATIC HYDROLYSIS & ISOMERIZATION
CH2OH
OH
O
CH2OH
OH
O
OH
glucoamylase
O
H2O
O
CH2OH
O
OH
HO
OH
OH
OH
cornstarch
high-fructose
corn syrup
CH2OH
O
OH
HO
(50%)
OH
OH
HOH2C
+
xylose
isomerase
O
HO
HO
CH2OH
OH
(45%)
Page 17-6