Biochemical Reaction Networks
165
oxaloacetate —-y-—
pyruvate
GXIXIXS
I
0-GEHI]
H4D
DAP
GXIKIXiHIHIHT]
>
1-Y
DAP
DAP
GXIKIXiHiHZHI]
[IHIHIKi)-GKI)<D
i '
Y)/2
L-lysine
GKIXIKiHIHZ]
L-lysine
GKIKIKiKTHi]
L-lysine
lIH IH IK iK IK I)
Figure 5.7 Overview of carbon transitions in two different pathways to lysine from oxaloacetate and
pyruvate. The four-step succinylase pathway with the flux (1-Y)vlys proceeds via diaminopimelate, which
is an intermediate in cell wall biosynthesis in certain bacteria. The four-step pathway contains the
enzymatic
sequence A-succinyl-2,6-ketopimelate
synthase,
A-succinylaminoketopimelateiglutamate
amino transferase, A'-succinyldiaminopimelate desuccinylase, and diaminopimelate epimerase. The one-
step pathway with the flux Yviys involves the action of meso-DAP dehydrogenase.
Thus, through measurements of the l3C-enrichment in several different carbon positions one has
redundant information and it may be possible to obtain a robust estimate of the flux through the two
different pathways. It is generally the case that measurement of the l3C-enrichment combined with
balances for the individual carbon atoms supplies redundant information, and that robust flux estimates
are obtained.
Sonntag
et al.
(1993) used the approach of above for quantification of the fluxes through the two
different pathways at different growth conditions. The l3C-enrichment of secreted lysine was measured
by NMR, and this information was used to quantify the flux through the two branches at different
ammonia concentrations (Fig. 5.8). It was found that the relative flux through the direct pathway
increased for increasing ammonia concentration and at the same time the lysine secretion was enhanced.
This pointed to an important role of the direct pathway for over-production of lysine.
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