Biochemical Reaction Networks
the spectrum. C om bining 13C -N M R w ith p roton-N M R it is possible to resolve peaks for m any o f the
carbon atoms, and hereby it is possible to obtain relatively detailed inform ation about the isotopom ers.
However, in m ost cases only a fraction o f the isotopom ers can be identified. W ith proton-N M R it is
possible to identify the specific resonance b etw een protons and l3C, and hereby it can be resolved
whether a given carbon is i3C or '"C. It is, how ever, not possible to tell w hether the l3C is connected to a
l3C or a l3C. T hus, only inform ation about the so-called
can be obtained (see Fig.
5.9B). Inform ation about the fractional en richm ent can be used directly in balances for the individual
carbon atom s as illustrated in E xam ple 5.9. T he fractional enrichm ent can, how ever, alw ays be
calculated if the distribution o f isotopom ers is know n.
Using MS it is possible to m easure the m ass o f the individual m etabolites, since l3C has a higher m ass
than '"C. Thus, using M S it is possible to m easure the so-called
(see Fig. 5.9C). It is
somewhat m ore com plicated to use this inform ation directly for flux analysis, but as w ill be show n later,
information about 13C enrichm ents is rarely used directly for flux quantification. N otice that also the
mass isotopom ers can be calculated if the d istribution o f isotopom ers is know n.
Exam ple 5.10 A nalysis o f a sim ple netw ork
To illustrate a more formal approach to the use o f labeled substrates for flux quantification we
will consider the simple network shown below.
• o o
909 099 909 090 990
090 990 990 099 009 909
P Y R ( l)
P Y R (2 )
Fig. 5.9 O verview o f different labeling patterns p ossible for a m olecule w ith three carbon atom s, e.g.
A. All possible isotopom ers.
B. Grouping o f isotopom ers to indicate the different isotopom ers that are included w hen the fractional
enrichment is m easured.
C. Grouping o f isotopom ers into different m ass isotopom ers. T he m ass o f the non-labeled m etabolite is
___ ____ ______