126
Chapter 5
processes, but below we list some of the most important processes that are customarily regarded as
maintenance processes:
Maintenance of gradients and electrical potential. In order to ensure proper function of
the cell it is necessary to maintain concentration gradients, e.g., a proton gradient across the
cellular membrane. Furthermore, it is necessary to maintain an electrical potential across the
cellular membrane. These processes require energy (and consequently substrate), but they
do not lead to formation of any new biomass, and they are therefore typical examples of
maintenance processes. We shall see (Note 5.2) that the major part of the maintenance
requirement originates in these processes.
Futile cycles. Inside the cells there are pairs of reactions that results in the net hydrolysis of
ATP. An example is the conversion of ffuctose-6-phosphate (F6P) to fructose 1,6-
bisphosphate (a reaction that requires ATP) followed by its hydrolysis back to F6P by a
phosphatase (a reaction that does not result in ATP formation). This two step futile cycle
represents a very simple situation and in practice the regulatory system of the cell ensures
that this futile cycle does not operate, e.g., the phosphatase is repressed in the presence of
glucose. However, there are more complex futile cycles that involve a large number of
reactions. The result is always a net hydrolysis of ATP. The exact function of such futile
cycles is not known, but they may serve to generate heat by the hydrolysis of ATP and
hence establish a higher temperature than that of the environment. Since futile cycles result
in utilization of energy without net formation of biomass, they may also be considered as
maintenance processes.
Turnover of macromolecules. Many macromolecules (e.g., mRNA) are degraded and
synthesized continuously inside the cell. This does not result in net formation of biomass
either,
but
substantial
amounts
of Gibbs
free
energy
are
used,
and
turnover of
macromolecules is therefore another typical example of a maintenance process.
Utilization o f energy, and consequently substrate, in each of the three processes listed above is
likely to be a function of the specific growth rate. When the specific growth rate is high there is a
high turnover of macromolecules, and with increasing activity level in the cell it is for example
necessary to pump more protons out of the cell. Furthermore, with a higher flux through the
cellular pathways there is a higher loss of energy in the futile cycles. This is biologically
reasonable since when the cells grow under limited conditions (a low specific growth rate) they
will try to use the substrate as efficiently as possible and the maintenance processes are therefore
curtailed. The energy expenditure in maintenance processes is therefore likely to be an increasing
function of the specific growth rate. Thus, part of the Gibbs free energy spent in these
maintenance processes may be included in the overall yield coefficient
Y
""e, and only the part of
free energy that is spent at zero-growth rate are included in the maintenance coefficient.
In 1960 Bauchop and Elsden introduced the concept of ATP requirements for biomass synthesis via
the yield coefficient
Y%Aie
(unit: mmoles ATP (g DW) '), and proposed a balance equation that is
analogous to Eq. (5.5):
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