26
Chapter 2
2.5), NADH is consumed mainly within the catabolic reaction pathways, e.g., by oxidation with
free oxygen in respiration (see Section 2.1.3.2). Most sugars are converted to glucose-
6
-phosphate
(G
6
P) or fructose-
6
-phosphate (F
6
P) before being metabolized. The intracellular isomerization of
G
6
P to F
6
P is normally in equilibrium, and G
6
P can therefore be considered a common starting
point in many catabolic pathways. In some microorganisms formation of G
6
P from glucose occurs
in the transport process (see Section 2.1.1.3), but in others this compound is formed from
intracellular glucose in a reaction coupled with the hydrolysis of ATP. The catabolism of sugars
from G
6
P is traditionally divided into glycolysis and pyruvate metabolism. Glycolysis is defined as
the sum of all pathways by which glucose (or G
6
P) is converted to pyruvate. The two major
pathways are the Embden-Meyerhof-Pamas (EMP) pathway and the pentose phosphate (PP)
pathway (see Fig. 2.4).
Glucose
ATP)l
(
1
)
GI u cose-6-phosphate
$ (
2
)
Fructose-6-phosphate
ATPt7 i
O)
(
1 1
)
ADP
Fructose-1,6bisphasphate
J4)
NADP*NADPH
NADP+NADPH
V/
1
/
~
6-Phosphogluconate
» Ribulose-5-phosphate + COa
Ribose-5-phosphate
^
Xylulose-5-phosphate
Dl hydroxyaceton e
phosphate
.
Glyceraktehyde-3
t
□hoop halo
? ! (
6
)
NADH
K
>
1,3 Diphosphogtycerate
<7 >
3 Phosphogly cerate
i
(
8
}
2 Phosphoglycerate
i O)
Pti osphoenol pyruvate
JS*
oo)
Pyruvate
G lycera Idehy de-3-p hosphate
$---------------
(16)
Se doheptu lose -7 -ph osp hate
---------------
1
F ructose-6-p hosphate
t
Erythrose-4-p hosphate
(15)___________
J
______
Figure 2.4 Embden-Meyerhof-Pamas (EMP) pathway and the pentose phosphate (PP) pathway. The
scheme is without a PTS transport system. The enzymes are: (1) hexokinase; (2) phosphohexoseisomerase;
(3) phosphofructokinase; (4) aldolase; (5) triosephosphate isomerase); (
6
) 3-phosphoglyceraldehyde
dehydrogenase; (7) 3-phosphoglycerate kinase; (
8
) phosphoglycerate mutase; (9) enolase; (10) pyruvate
kinase;
(11)
gIucose-
6
-phosphate
dehydrogenase;
(12)
6
-phosphogluconate
dehydrogenase;
(13)
ribulosephosphate-3-epimerase; (14) ribosephosphate isomerase; (15) transketolase; (16) transaldolase,
Fructose-
6
-phosphate and glyceraldehyde-3-phosphate takes part in both pathways and hereby allows
recycling of carbon from the PP pathway back to the EMP pathway. Transaldolase can catalyze two
reactions: 1) the interconversion of xylulose-5-phosphate and ribose-5-phosphate to glyceraldehydes-3-
phosphate and sedoheptulose-7-phosphate, and 2) the interconversion of xylulose-5-phosphate and
erythrose^-phosphate to glyceraldehyde-3-phosphate and fructose-
6
-phosphate.
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