128
Chapter 5
One may also calculate theoretical values for TxATP from the metabolic network. In Table 2.5 a value
of 41 mmoles of ATP per gram dry weight was given for cell synthesis on a minimal medium. If
we add 6 mmoles of ATP per gram dry weight used in the transport processes (Stouthamer, 1979),
we obtain a theoretical value for
YtAT?
of 47 mmol of ATP per gram dry weight for
E. coli.
By
comparison it is seen that the experimental value of
YxATr
for
E. coli
in Table 5.1 is 2.0-2.5 times
larger than the theoretical value. This is a general observation made also for other microorganisms.
The reason is that energy used in the maintenance processes is included in TxATP, as discussed in
Note 5.2.
Note 5.2 Calculation of the total ATP consumption for maintenance
We now want to evaluate the total ATP consumption for maintenance reactions in
E. coli
with growth on a
minimal medium. As discussed in the text there is a substantial deviation between the value of F<ATP for
E.
coli
in Table 5.1 and the value of 41 mmoles (g DW)'1
for synthesis of an
E. coli
cell found in Table 2.5.
Even if transport of substrates is considered there is a large deviation as mentioned above. The difference
between the “theoretically” calculated value and the experimentally determined value must be due to the
three types of maintenance processes.
Many of the cellular macromolecules are very stable, and it is mainly enzymes and mRNA that are
degraded and resynthesized inside the cell. The half-life of mRNA is on the order of a few minutes, and
there are good reasons for this low value. In order to control the synthesis of proteins at the genetic level it
is important that mRNA be quite unstable, since otherwise translation of the mRNA could continue even
when the enzyme is not needed. Since it is much cheaper from an energetic point of view to synthesize
mRNA than protein, it is better for the cell to have a high turnover rate of mRNA rather than synthesize
unnecessary protein. The turnover rate of enzymes is not known exactly, and probably it is heavily
dependent on the cellular function of the enzyme. If the degradation of mRNA and protein are first-order
processes, the rate of turnover depends on the content of these two components inside the cell. Since the
content of enzymes and mRNA increases with the specific growth rate, the ATP requirement for turnover of
macromolecules is therefore increasing with the specific growth rate. Using a half-life for mRNA of 1 min
and a protein half-life of 10 hours, the ATP requirement for macromolecular turnover can be calculated to
be in the order of 6 mmoles ATP (g DW h )1. Consequently, only a minor part of the ATP requirement for
maintenance processes can be accounted for by turnover of macromolecules.
Stouthamer (1979) states that up to 50% of the total energy production during anaerobic growth of
E. coli
is
used for maintaining membrane potentials, i.e., maintenance of the proton and electrochemical gradient
across the cytoplasmic membrane. This corresponds to 49-59 mmoles of ATP per gram dry weight, and
only a minor fraction of the ATP is therefore "lost" in the futile cycles. During aerobic growth, membrane
energetization is ensured by the respiration (i.e., the oxidation of NADH), and this at least partly explains
why the operational P/O ratio is below the theoretical value, as discussed in Section 4.3._________________
5.2.2 Energetics of Anaerobic Processes
In anaerobic processes there is no consumption of oxygen, and microorganisms grown under
anaerobic conditions therefore have to rely on so-called substrate-level phosphorylation to obtain
ATP for growth (or they use external electron acceptors other than oxygen).
Obligate anaerobes
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