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Bio 98 - Lecture 11
Carbohydrates
a.k.a. Sugars…
I. Definition of a carbohydrate
General formula: (CH20)n, "hydrated carbon"
Example: C6H1206 is glucose
Many carbohydrates have more complex formulas &
contain amino, phosphate, sulfate & other groups
II. Functions
1. Fundamental source of metabolic energy for most
life forms.
2. Components of many important biomolecules.
1. Carbohydrates as an energy source
sunlight
^—N
plants
M m—^Z
photosynthesis
2
'
carbohydrate + 02
AA
2'
CO, + H>0
animals, plants
respiration
ATP
ADP
2. Biological molecules
Penicillin
Cellulose - wood, plant fiber, etc. Chitin exoskeleton of arthropods Cell walls of
bacteria & yeast Glycoproteins, glycolipids
- cell membranes DNA, RNA deoxyribose and ribose
Carbohydrates are often polymers
Monosaccharides: glucose, ribose, fructose, etc.
Oligosaccharides: di-, tri-, tetra- etc. Sucrose is a
disaccharide: glucose + fructose Polysaccharides: can
be linear or branched (i.e. starch)
Monosaccharide nomenclature
1. Carbon number: triose, .., pentose, hexose, .., octose
2. Aldoses and ketoses
■VO
0
H
C#
1
2
3
4
5
6
D-glucose
HC=0
<=^ HC-OH
HO-CH
HC-OH
HC-OH
H2C-OH
D-fructose
H2C-OH i
C=0
HO-CH
HC-OH
HC-OH
H2C-OH
C
an aldohexose
a ketohexose
Asymmetric (chiral) carbons allow many distinct
monosaccharides; compare mannose, glucose, and
galactose, which are all aldohexoses.
^HO
^HO
^HO
HO—C—H
3I
HO—C—H
4I
H —C—OH
H—C—OH
3I
HO—C—H
4I
H—C—OH
H —C—OH
3I
HO—C—H
4I
HO—C—H
si
si
si
H —C—OH
6
CH2OH
D-Mannose
(epimer at C-2)
H—C—OH
6
CH2OH
D-Glucose
H—C—OH
6
CH2OH
D-Galactose
(epimer at C-4)
How many chiral carbons are in glucose?
O-chem terms relevant to monosaccharide structure
1. diastereomers - identical structures except for
configuration (chirality) at one or more carbons; e.g., all
aldohexoses are diastereomers of each other.
2. epimers - differ in chirality at only one carbon. Glucose
and galactose are epimers at carbon 4.
3. enantiomers - mirror images; designated as D- & L-;
does not constitute a name change.
How many aldohexose names are possible?
2 configuration choices at each of 4 asymmetric carbons;
however (1.), half of these represent enantiomers (3.).
Number of unique names = 24/ 2 = 8.
Ring structures
Most pentoses and hexoses
spontaneously form ring
(cyclized) structures in solution.
^
rlu-U
HC-OH
i
5-member ring: furanose
u
■
6-member ring: pyranose
--------
When forming a ring, a
new asymmetric center
is created, giving rise to
2 possible anomers.
HO-CH
"i
HC-OH
i ••
HC-OH
i
2
D-glucose
(linear form)
V° i
i
l
H—C—OH C
n
H
HO—C—H 2
4I
H—C—OH D-Glucose
O
si
H—C—OH H
Linear form
11
6
CH2OH
OH
H
H
n
4/-
l\
0H
f
H
/
HO
fl
Ha worth
projection
a and (3
anomers
OH
a-D-glucopyranose
hemiacetal
a-D-Glucopyranose /J-D-Glucopyranose
In solution rapid mutarotation occurs
(3-D-glucopyranose
Linear
D-glucose
(1%)
^
(~66%)
a-D-glucopyranose
(~33%)
Disaccharides
1.  Mainly found in plants
2.  Three common disaccharides
• sucrose - sugar cane, sugar beets
• lactose - milk sugar
• maltose - malted (germinating) barley, wheat
3.  2 monosaccharides joined covalently by an
O-glycosidic bond
Formation of hemiacetal and acetals
Glycosidic
bond
1
HO —R3
OH
0
#
R —C
2
+HO —R
\
Aldehyde
w
1
2
^^ R—C—OR
I
Alcohol
H
Hemiacetal
^________ ,
N
. ----^
^
HO —R
3
OR3
1
R —C —OR
I
H
Acetal
+ H20
Common disaccharides are produced by
enzyme-catalyzed condensation/dehydration
reactions
OH
H
OH
a-D-Glucose
H
OH
/3-D-Glucose
hydrolysis
condensation
H->0^ ^HoO
6
CH2OH
acetal
\J
2
OH
\
hemiacetal
OH^
H
H
Can be either
a or (3 due to
muta rotation
Maltose
a-D-glucopyranosyl-(1^4)-D-glucopyranose
Notice:
there is no
hemiacetal
Polysaccharides - aka Glycans
1.  homopolysaccharides vs
heteropolysaccharides
2.  can be branched or unbranched
3.  used by animals and plants as a
compact storage form of CHOs
4.  common examples
• starch - plants, roots and seeds
• glycogen - liver of mammals
• cellulose - plant fiber, wood
Starch = amylose + amylopectin
found in corn, rice, potato, wheat and barley
a linear polysaccharide of glucose units
Amylopectin - a branched form of amylose
Structure of starch
glycogen (animal starch) is like starch, but more highly branched
High-fructose corn syrup
High-fructose corn syrup is produced by milling corn to produce
corn starch, then processing that starch to yield corn syrup,
which is almost entirely glucose, and then adding enzymes that
change most of the glucose into fructose.
1.  Corn starch is treated with alpha-amylase to produce shorter
chains of sugars called oligosaccharides. 2.  Glucoamylase which is produced by Aspergillus, a fungus, in
a fermentation vat — breaks the sugar chains down even
further to yield the simple sugar glucose. 3.  The enzyme
xylose isomerase (aka glucose isomerase) then
converts some of the glucose to a mixture of about 42%
fructose and 50–52% glucose with some other sugars mixed
in.
http://en.wikipedia.org/wiki/High-fructose_corn_syrup
Protein glycosylation: A
post-translational modification
Protein
chain
• Sugars covalently attached to the polypeptide as oligosaccharide
chains containing
CH2OH 4 to 15
sugars
• Sugars frequently comprise 50% or
more of the total molecular weight of a
H
NHCOCH,
H
glycoprotein
• Most glycosylated proteins are either
secreted or remain membrane-bound
• Glycosylation is the most abundant
form of post-translational modification
• Glycosylation confers resistance to
protease digestion by steric protection
• Important in cell-cell recognition
Asparagine
CH2OH
H
NHCOCH
Protein
chain
Blood group antigens on erythrocyte surface
•  The O substance is a
tetrasaccharide which is missing the 5th
residue and does not elicit an antibody
response (non-antigenic).
•  The A antigen and B antigen are
pentasaccharides which differ in
composition of the 5th sugar residue