1. PENTOSE PHOSPHATE CYCLE
11
fate of this intermediate further down the line of the metabolic pathway.
A prime example of this pitfall is the presence of 6-phosphogluconate
arising from glucose-6-phosphate, say, in extracts of certain bacteria. The
point of departure from the oxidative pentose phosphate cycle is in the
step where the phosphogluconate is dehydrated to 2-keto-3-deoxy-6phosphogluconate. The sequence of reactions is shown in Fig. 3 and is
GLUCOSE
coo©
I
HCOH
I
HOCH
I
HCOH
I
HCOH
HgCO®
6-PH0SPH0GLUCONATE
6-PG
DEHYDRASE
SPLITTING
• ENZYME
COO©
I
C=0
I
CH 2
HCOH
HCOH
H 2 CO®
2-KET0-3-DE0XY6-PH0SPH0GLUC0NATE
COO©
I
C=0
I
CH 3
PYRUVATE
+
CHO
HCOH
H 2 C0®
GLYCERALDEHYDE3-PHOSPHATE
FIG. 3. The Entner-Doudoroff Pathway.
known as the Entner-Doudoroff pathway (62, 63). This pathway is
limited to bacteria.
Even the most stalwart indicators of the pentose phosphate cycle,
such as the pentose phosphates themselves, or sedoheptulose-7-phosphate, may arise from the nonoxidative portion of the oxidative pentose
phosphate cycle (cf., e.g., the counterclockwise sequence of Fig. 1).
Brenneman et al. showed that in Alcaligenes faecalis, ribose-5-phosphate
is synthesized from fructose-6-phosphate not via the oxidative shunt
mechanism, but from the action of transketolase, transaldolase, isomerase,
and epimerase. Both glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are absent in extracts of this organism (64).
Even triose phosphate need not be present for the fructose-6-phosphate
to be transformed to sedoheptulose-7-phosphate; all that is required is
the simultaneous action of both transaldolase and transketolase (65).
The mechanism involves the coupled action of these two enzymes on two
moles of fructose phosphate, yielding one pentose phosphate and one
sedoheptulose monophosphate.
C. PENTOSE PHOSPHATE PATHWAY INTERMEDIATES AS
SUBSTRATES FOR RESPIRATION
A decision can be made as to whether a compound is an intermediate
of the pentose phosphate cycle by feeding it to a tissue and observing
whether it is rapidly metabolized to C0 2 or whether it stimulates oxygen
uptake. A number of examples of the former case (i.e., rapid formation
of C0 2 ) is cited in the review article by Axelrod and Beevers (5). A
11
fate of this intermediate further down the line of the metabolic pathway.
A prime example of this pitfall is the presence of 6-phosphogluconate
arising from glucose-6-phosphate, say, in extracts of certain bacteria. The
point of departure from the oxidative pentose phosphate cycle is in the
step where the phosphogluconate is dehydrated to 2-keto-3-deoxy-6phosphogluconate. The sequence of reactions is shown in Fig. 3 and is
GLUCOSE
coo©
I
HCOH
I
HOCH
I
HCOH
I
HCOH
HgCO®
6-PH0SPH0GLUCONATE
6-PG
DEHYDRASE
SPLITTING
• ENZYME
COO©
I
C=0
I
CH 2
HCOH
HCOH
H 2 CO®
2-KET0-3-DE0XY6-PH0SPH0GLUC0NATE
COO©
I
C=0
I
CH 3
PYRUVATE
+
CHO
HCOH
H 2 C0®
GLYCERALDEHYDE3-PHOSPHATE
FIG. 3. The Entner-Doudoroff Pathway.
known as the Entner-Doudoroff pathway (62, 63). This pathway is
limited to bacteria.
Even the most stalwart indicators of the pentose phosphate cycle,
such as the pentose phosphates themselves, or sedoheptulose-7-phosphate, may arise from the nonoxidative portion of the oxidative pentose
phosphate cycle (cf., e.g., the counterclockwise sequence of Fig. 1).
Brenneman et al. showed that in Alcaligenes faecalis, ribose-5-phosphate
is synthesized from fructose-6-phosphate not via the oxidative shunt
mechanism, but from the action of transketolase, transaldolase, isomerase,
and epimerase. Both glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are absent in extracts of this organism (64).
Even triose phosphate need not be present for the fructose-6-phosphate
to be transformed to sedoheptulose-7-phosphate; all that is required is
the simultaneous action of both transaldolase and transketolase (65).
The mechanism involves the coupled action of these two enzymes on two
moles of fructose phosphate, yielding one pentose phosphate and one
sedoheptulose monophosphate.
C. PENTOSE PHOSPHATE PATHWAY INTERMEDIATES AS
SUBSTRATES FOR RESPIRATION
A decision can be made as to whether a compound is an intermediate
of the pentose phosphate cycle by feeding it to a tissue and observing
whether it is rapidly metabolized to C0 2 or whether it stimulates oxygen
uptake. A number of examples of the former case (i.e., rapid formation
of C0 2 ) is cited in the review article by Axelrod and Beevers (5). A