section 15.3
Alternative Pathways of Glucose Metabolism and Hexose Interconversions
299
s
----------- G ly c o g en ---------
G lu co se 1-p h o sp h ate
3
G lu c o se — ►
G lu co se
6
-p h o sp h ate
F ru cto se
6
-p h o sp h ate
G lu co sam in e -
A T P A D P
\ ^ L
•
A m in o tran sferase
-G lu tam in e
©
^ — G lu tam ate
G lu co sam in e
6
-p h o sp h a te
A cetyl C oA
♦G lycolysis
P h o sp h o g lu co m u tase
G lu co sam in e
ace ty ltran sfe ra se
♦CoASH
G lu co sam in e 1-p h o sp h a te
N -A cetylgiucosam ine
6
-p h o sp h ate
------------
J
'--------------.
N -A cetylgiucosam ine
p h o sp h o m u ta se
N -A cetylglucosam ine 1-p h o sp h a te
-U T P
U D P -glucosam ine
p y ro p h o sp h o ry lase ^
A cetylglucosam ine-
2
-e p im e ra se
N -A cetylm annosam ine
6
-p h o sp h ate
•U T P
PP.
U D P -glucosam ine
U D P-acetylglucosam ine
p y ro p h o sp h o ry lase
►
PP,
P h o s p h o e n o y K
pyruvate
'
R-
G ly co sam in o g ly can s
N -A cety ln eu ram in ate-9 -p h o sp h ate sy n th a s e
U D P -N -acetylglucosam ine
N -A cetylneuram inic acid 9 -p h o sp h ate
G lycoproteins,
g lycosam inogiycans
\U D P -N -acetyl-
k glucosam ine-
4-epim erase
Sialic acid s,
gly co p ro tein s
U D P -N -acetylgalactosam ine
l
G ly co sam in o g iy can s
FIGURE 15-18
Summary of some interconversions and synthetic reactions in which amino sugars participate. Substrates for the
pathway can be derived from glucose, glycogen, and gluconeogenesis.
are summarized in Figure 15-19, in which stoichiometry
is ignored.
FIGURE 15-19
Summary of the pentose phosphate pathway. This diagram is intended to
show the two major parts of the pathway: oxidation and decarboxylation of
glucose-6-phosphate to ribulose 5-phosphate, and resynthesis of the former
from the latter. The stoichiometry of the pathway is ignored.
The pentose phosphate pathway can be thought of as
two separate pathways:
1. The oxidative conversion of glucose-
6
-phosphate to
ribulose 5-phosphate and CO
2
and
2. Resynthesis of glucose-
6
-phosphate from ribulose
5-phosphate. Because the pathway begins and ends
with glucose-
6
-phosphate, it is a cycle, or shunt. For
every six molecules of glucose-
6
-phosphate that enter
the pathway, five molecules of glucose-
6
-phosphate
+
6
CO
2
are produced. Thus, there is a net loss of one
carbon from each glucose-
6
-phosphate that enters the
cycle.
Interpreting the pathway as a means of oxidizing glu-
cose to ribulose 5-phosphate and recycling the ribulose
5-phosphate back to glucose-
6
-phosphate is too narrow.
The pathway performs a variety of functions, the least
important of which seems to be that of an alternative
pathway for glucose metabolism. Production of ribose
5-phosphate for nucleotide synthesis by
de novo
and
