From Cellular Function to Industrial Products
31
using a complex medium that contains some but not all of the building blocks. In physiological
characterization of cells it is always preferable to use well-defined growth media, or even minimal
media containing only glucose, ammonia, salts and a few vitamins.
Since protein synthesis requires a large input of free energy, it is important for the cell to adjust the
protein synthesis precisely to its needs. Proteins are synthesized by the so-called
protein
synthesizing system
(PSS) that primarily consists of ribosomes. In adjusting the size and activity of
the PSS, the cell can control the protein synthesis and thereby the energy expenditure. When the
cells experience a change from an energy-sufficient to an energy-deficient environment, they will
fairly rapidly (time constant of 1 -2 h) adjust the size of the PSS to the new conditions.
With the large costs of ATP for biomass synthesis it is important to have a continuous supply of
this compound. The total pool of adenylate phosphates is, however, small in the cell, and the
turnover time of ATP in living cells is therefore on the order of 1-10 s. Thus, if the continuous
generation of high-energy phosphate bonds in the form of ATP is halted for just a few seconds free
energy to drive biosynthesis will rapidly be used up, and consequently energy utilizing reactions
must be tightly coupled to energy producing reactions inside the cell. The regulatory mechanisms in
different pathways are very complex and involve many different components, and since ATP
formation and utilization involve a cyclic flow through ADP and/or AMP3, it is not surprising that
all three adenylates play a regulatory role in the cellular reactions. Some enzymes are regulated by
the concentration of one of the three components, whereas others are regulated by the concentration
ratio of two of the components, e.g., [ATP]/[ADP], Atkinson (1977) collected the control action of
the adenylates in a single variable, the
energy charge Ec.
\
a
tp
]+'
a
\
adp
]
c~ [
a
TP] + [
adp
] + [
amp
]
(2
9 )
Since ADP contains only one energized phosphate bond while ATP has two, a factor of
'A
is used
for ADP in the numerator. In theory the energy charge could vary between 0 and 1, but in living
cells at balanced growth conditions
Ec
varies only between 0.65 and 0.9. The pool of adenylates is
usually found to increase with the specific growth rate of the cells, whereas the energy charge is
almost invariant with the specific growth rate (Atkinson, 1977). Table 2.6 collects steady state
measurements of AMP, ADP and ATP at different specific growth rates of
Lactococcus lactis.
With both glucose and maltose as the limiting substrate the total adenylate pool is found to increase
with the specific growth rate, but the energy charge is almost constant at
0.68
for all dilution rates
and independent of the applied sugar.
AMP (adenosine monophosphate) is formed by cleavage of the anhydride-hound phosphate from ADP. The amount
of Gibbs free energy produced in the decomposition is slightly larger, i.e., 32.0 kJ/mole, than that of ATP hydrolysis.
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