138
chapter 9
Simple Carbohydrates
6
CH2OH
H /f
°\H
J / h
1
\ ? H
HOU
\ l
y C,
Y / r
J/OH
Ï
3
|
2
H
OH
a-o-Glucopyranose
6
CH2OH
4 - O H
H /|
J / h
i\ ? H
H o \l
u CHO
Ï / 1
Î 3
|
2
H
OH
o-GIucose
(a)
H
OH
p-o-Glucopyranose
Equilibrium mixture
a-p-GIucose •=* containing a-, B-, and = p-D-Glucose
open-chain forms
Specific
rotation
[a] ^
+112.2'
+52.7'
+18.7'
p-form: 63%
a-form: 36%
open form: <
1
% .
(b)
F IG U R E 9-9
(a) Interconversion of the anomeric forms of D-glucopyranose via the intermediate open-chain form, (b) Mutarotation
of D-glucose.
enzyme-substrate complexes (e.g., during the hydrolysis
of polysaccharides, catalyzed by the enzyme lysozyme;
see Chapter 11 ).
D-Fructose, a ketohexose, can potentially form either a
five-membered (furanose) or a six-membered (pyranose)
ring involving formation of an internal hemiketal link-
age between C
2
(the anomeric carbon atom) and the C
5
or
C(,
hydroxyl group, respectively. The hemiketal link-
age introduces a new asymmetrical center at the C
2
po-
sition. Thus, two anomeric forms of each of the fructo-
furanose and fructopyranose ring structures are possible
(Figure 9-10). In aqueous solution at equilibrium, fructose
is present predominantly in the /
1
-fructopyranose form.
However, when fructose is linked with itself or with other
sugars or when it is phosphorylated, it assumes the fu-
ranose form. Fructose 1,
6
-bisphosphate is present in the
/3-fructofuranose form, with a 4:1 ratio of
fi
-to a-anomeric
forms.
Fructose is a major constituent (38%) of honey ; the other
constituents are glucose (31%), water (17%), maltose (a
glucose disaccharide, 7%), sucrose (a glucose-fructose
disaccharide, 1
%), and polysaccharide (1 %). The variabil-
ity of these sugars in honey from different sources is quite
large.
Sugars that contain a carbonyl functional group are
reducing agents. Therefore, an oxidizing agent (e.g.,
CH2OH
2I
c = o
3I
HO— —H
H— C — OH
5 I
H— c — OH
s|
CH2OH
D -F ructose (0.01% )
(o p e n -c h a in form )
a -o -F ru c to p y ra n o se (3% )
sC H Æ tU 0 ^
1CH
2
0H
OH
a-D -F ru cto fu ran o se (9% )
h o \ i
h ? / c h ,o h
p -o -F ru cto p y ran o se (5 7 ±6% )
6CH20 H
' ° \
OH
(CH2OH
p -D -F ru cto fu ran o se (31 ±3% )
F IG U R E 9 -1 0
Structures of open-chain and Haworth projections of D-fructose and the approximate percentage of each
at equilibrium in aqueous solution.
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