9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
451
C-4 of glucose are converted to C0 2 extensively at the same rate. This
is to be expected from operation of the Entner-Doudoroff pathway.
(b) A. suboxydans is one of the few microorganisms in which glucose
appears to be catabolized exclusively via the pentose cycle. As with Z.
mobilis above, the TCA cycle is reported not to be operative in this
organism (144). As shown in Fig. 7B, one finds that the conversion of
glucose carbon atoms to C0 2 follows the order of C-l > C-2 > C-3,4
> C-6. It is noteworthy that although the over-all rates of C
14 0 2 production from the labeled carbon atoms are essentially alike, the total
recovery of each of the labeled carbon atoms in C0 2 is by no means
the same. This indicates that, despite the fact that experiments were
carried out under resting conditions, there was drainage of intermediates
from the pentose cycle either for biosynthetic functions or as fermentation products. Extensive recycling of some of the glucose carbon atoms
is therefore depressed.
(c) The radiorespirometric patterns for the utilization of glucose by
Bacillus subtilis (145) is typical for organisms that seem to rely mainly
on glycolysis (in conjunction with the Krebs cycle) and less significantly
on phosphogluconate cleavage for carbohydrate catabolism (Fig. 7C).
One can readily recognize two distinct phases displayed on the pattern,
with the dividing line located approximately at 1 RTU (relative time
unit). During the first phase, assimilation of substrate takes place predominantly. Glycolysis is clearly indicated by the high recovery of C-4
and C-3 of glucose in C0 2 . Meanwhile, the occurrence of an alternate
pathway is revealed by the observation that complete equivalence in
metabolic behavior between C-3 and C-4 or C-l and C-6 was not
realized as called for by the exclusive operation of glycolysis (Fig. 8).
The preferential conversion of C-l over that of C-6 to C0 2 suggested
that the alternate pathway involved is phosphogluconate decarboxylation.
However, the pentose phosphate so formed evidently did not participate
extensively in respiratory functions via the pentose phosphate cycle,
otherwise C
14 0 2 production from C-2 should approach or equal that
of C-l. This is not surprising, inasmuch as cell proliferation occurred
in this experiment, which would necessitate drainage of various intermediates for biosynthetic functions.
In the second phase of the radiorespirometric pattern [after exhaustion of administered glucose (or after 1 RTU)] one finds that the substrate carbon atoms, particularly C-6, C-2, and to some extent C-l, which
were previously retained in the medium or incorporated into the cellular
constituents, are converted to respiratory C0 2 at a significantly enhanced
rate for a period of some hours. The order of C
14 0 2 production during
this phase, i.e., C-2 > C-6 > C-l > C-3 or C-4, is in line with the con-
451
C-4 of glucose are converted to C0 2 extensively at the same rate. This
is to be expected from operation of the Entner-Doudoroff pathway.
(b) A. suboxydans is one of the few microorganisms in which glucose
appears to be catabolized exclusively via the pentose cycle. As with Z.
mobilis above, the TCA cycle is reported not to be operative in this
organism (144). As shown in Fig. 7B, one finds that the conversion of
glucose carbon atoms to C0 2 follows the order of C-l > C-2 > C-3,4
> C-6. It is noteworthy that although the over-all rates of C
14 0 2 production from the labeled carbon atoms are essentially alike, the total
recovery of each of the labeled carbon atoms in C0 2 is by no means
the same. This indicates that, despite the fact that experiments were
carried out under resting conditions, there was drainage of intermediates
from the pentose cycle either for biosynthetic functions or as fermentation products. Extensive recycling of some of the glucose carbon atoms
is therefore depressed.
(c) The radiorespirometric patterns for the utilization of glucose by
Bacillus subtilis (145) is typical for organisms that seem to rely mainly
on glycolysis (in conjunction with the Krebs cycle) and less significantly
on phosphogluconate cleavage for carbohydrate catabolism (Fig. 7C).
One can readily recognize two distinct phases displayed on the pattern,
with the dividing line located approximately at 1 RTU (relative time
unit). During the first phase, assimilation of substrate takes place predominantly. Glycolysis is clearly indicated by the high recovery of C-4
and C-3 of glucose in C0 2 . Meanwhile, the occurrence of an alternate
pathway is revealed by the observation that complete equivalence in
metabolic behavior between C-3 and C-4 or C-l and C-6 was not
realized as called for by the exclusive operation of glycolysis (Fig. 8).
The preferential conversion of C-l over that of C-6 to C0 2 suggested
that the alternate pathway involved is phosphogluconate decarboxylation.
However, the pentose phosphate so formed evidently did not participate
extensively in respiratory functions via the pentose phosphate cycle,
otherwise C
14 0 2 production from C-2 should approach or equal that
of C-l. This is not surprising, inasmuch as cell proliferation occurred
in this experiment, which would necessitate drainage of various intermediates for biosynthetic functions.
In the second phase of the radiorespirometric pattern [after exhaustion of administered glucose (or after 1 RTU)] one finds that the substrate carbon atoms, particularly C-6, C-2, and to some extent C-l, which
were previously retained in the medium or incorporated into the cellular
constituents, are converted to respiratory C0 2 at a significantly enhanced
rate for a period of some hours. The order of C
14 0 2 production during
this phase, i.e., C-2 > C-6 > C-l > C-3 or C-4, is in line with the con-