Biochemical Reaction Networks
163
5.4.2 Use of Labeled Substrates
If cells are fed with a '’C-labeled substrate, e.g., glucose that is enriched with "C in the first
carbon position, then, when this substrate is catabolized, there will be enrichment in l?C of some
of the carbon atoms present in the different intermediates of the metabolism. The enrichment of
the individual carbon atoms of the intermediary metabolites of a pathway will depend on the
activity of the pathway. To illustrate this consider the catabolism of glucose to pyruvate by three
different pathways:
the Embden Meyerhof Pamas pathway (EMP pathway), the pentose
phosphate pathway and the Entner Doudoroff pathway. If glucose is labeled specifically in the
first position, there will be a different labeling of pyruvate depending on which pathway that
operates (see Fig. 5.6).
If one measures the fractional enrichment of pyruvate (note 5.6) compared with that of glucose it
is possible not only to identify which of the three pathways is operative but also to calculate the
relative activity of the different pathways. Consider a simple case where we feed with 100%
EMP pathway
G3P(=Pyr)
C
C
C +C —
C* C
G6P
C*
Ç
c
- c
c
c
Glucose
ED pathway
G3P
C
1 (
C*
c
c
Pyr
6-Phospho-
gluconate
Ç*
Ç
C
G6P
Ç*
Ç
-Ç -
Ç
Ç
c
Pentose Phopshate
pathway
6-Phospho
gluconate
Glucose
G6P
Ç*
Ç
Ç
_
Ç
Ç
C
Ç
Ç
l
t
Ç
Ç -
c
Ç*
c
c
c
c
c
Pyr
c
- c
c
Glucose
Figure 5.6 O verview o f carbon transitions in three different catabolic pathw ays: EM P pathw ay, ED
pathway and pentose phosphate pathw ay. T he ' ’C -labelled carbon in glucose is m arked w ith *. In the
EMP pathw ay there is labeling at the third carbon position o f pyruvate. In the ED pathw ay there is
enrichment at the first carbon position o f pyruvate. In the pentose phosphate pathw ay there is no
C
enrichment o f the pyruvate since the first carbon atom o f glucose escapes as carbon dioxide.
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