124
Chapter 5
state metabolite concentration c,
(unit: moles (g DW)'1) must stay constant, and the balance reduces to:
0 =
rme, —
MCmtt
(
2
)
rma
combines all the rates of formation and all the rates of consumption of the intracellular metabolite:
0
= Z
r» « . M -
Z
w » «
(3)
To keep the metabolite concentration constant when the cell mass increases by growth the total rate of
formation must be larger than the total rate of consumption by other pathway reactions. The dilution term
is, however, normally negligible, as illustrated below for two different types of intracellular compounds:
intermediates in the glycolytic pathway and ATP.
In aerobically grown
S. cerevisiae,
the intracellular level of metabolites in the glycolytic pathway
has been measured in continuous cultures (dilution rate = 0.1 h'1) to be in the range of 0.05-1.0 pmol
(g DW)'1
(Theobald
et al.,
1997). At these growth conditions, the flux through the EMP pathway is about
1.1 mmol (g DW)"1
h'1, and it is therefore seen that the flux through a metabolite pool, i.e., each of the
two first terms in Eq. (3), is much higher than the dilution term of Eq. (2), which is about 0.005-0.1 pmol
(g DW) 1
h'1. Thus, even for much lower glycolytic fluxes the dilution term is negligible.
The ATP pool in
S. cerevisiae
is about 8.0 pmol (g DW)"1
. Because ATP is involved in many reactions, it
is difficult to assess the flux through the pool. However, the total ATP requirements for cell growth is
about 7.0 mmol (g DW )1
h'1
at a specific growth rate of 0.1 h'1
(see Section 5.2). Thus, the flux through
the pool is a factor of 10,000 higher than the dilution term, and this term obviously can be neglected in a
balance for ATP. Even for cells that have a higher pool of ATP,
e.g.,
lactic acid bacteria, this conclusion
still holds. A similar conclusion can be drawn for other cofactors like NADH and NADPH.
For intracellular metabolites present at relatively high concentrations (or rather with relatively low turn-
over frequency), e.g., some amino acids, the dilution term may account for up to 10% of the total fluxes
leaving the pool (the sum of the consumption terms in Eq. (3) and the dilution term). Here the dilution
term may be included in the model simply by including an additional reaction consuming the metabolite.
The product of the specific growth rate and the (experimentally determined or estimated) concentration
of the metabolite determines the flux of this reaction.____________________________________________
5.2 Growth Energetics
In order to harness the free energy produced by catabolic processes in terms of high-energy
phosphate bonds - in particular in the form o f ATP - for subsequent use in the biosynthesis of
biomass constituents in the anabolism the cellular content of ATP (and ADP) must be controlled
quite rigorously. As discussed in Section 2.1.4 the turnover time of ATP is low (see Fig. 2.7), and
there must be tight balancing of the energy-forming reactions (catabolism) and the energy-utilizing
reactions (the anabolism) inside the cell. This can be formulated as:
(5.2)
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