From Cellular Function to Industrial Products
19
efficient utilization of glucose for biomass synthesis, and the drain of ATP due to the increased proton
influx with the presence of benzoic acid explains this. In another study, Schulze (1995) analyzed the
influence of benzoic acid on the ATP costs for biomass synthesis in anaerobic cultures of
S. cerevisiae.
He found that the ATP costs for biomass synthesis increased linearly with the benzoic acid
concentration, also a consequence of the increased proton influx when this acid is present in the medium.
Henriksen
et ai.
(1998) derived a set of equations that allows quantification of the ATP costs resulting
from uncoupling of the proton gradient by organic acids.
The aim of the study was to quantify the
uncoupling effect of phenoxyacetic acid, a precursor for penicillin V production, on the proton gradient
in
Penicillium chrysogenum
. Both forms of this acid may diffuse passively across the plasma membrane,
but the undissociated acid has a much larger solubility,
i.e.,
a larger partition coefficient, and is therefore
transported much faster.
To describe the mass flux of the two forms across the plasma membrane,
Henriksen
et al.
(1998) applied Eq. (2.3).
The specific cell area is about 2.5 m" (g DW
)'1
for
P.
chrysogenum,
and the permeability coefficients for the undissociated and dissociated forms of
phenoxyacetic acid have been estimated to be 3.2 x 10
"6
and 2.6 x 10
'10
m s'1, respectively (Nielsen,
1997).
Because the undissociated and dissociated forms of the acid are in equilibrium on each side of the
cytoplasmic membrane (HA ^ H* + A') with equilibrium constant
Ka,
Eq. (2.4) which correlates the two
forms with the total acid concentration can be written as
_ „
i
-pH
______
total
c dissl y j
1
+ \< y H ~pK“
(
1
)
where the pA'a for phenoxyacetic acid is 3.1. At pseudo-steady state conditions, the net influx of
undissociated acid will equal the net outflux of the dissociated form of the acid:
or
P
u n d is s
*■*
c e il
V
u n d iss
!,f>
)
i i s s ^ c e l t
:
(2)
where subscript
a
and
b
indicate the abiotic and biotic (cytosolic) side of the membrane, respectively. By
substituting for the undissociated and dissociated acid concentrations on the abiotic and cytosolic sides
of the membrane in terms of the total concentrations on the abiotic and cytosolic sides from eq. (
1
) and
rearranging, we obtain the following equation for the ratio of the total concentrations on the two sides of
the membrane:
-bjo t
1 + 10
pH*-pK°
1 + 1 0 ^ " ^
----------
- +
p
-----------------
1
4
-
10
^ " - ^ ”*
d'SS
1
+
I Q P K '- P 11-
lO'"*-'*-
+ p^„ ,
(3)
Because the permeability coefficient for the undissociated form of the acid is orders of magnitude greater
than that of the dissociated form, this equation can be reduced to
^
_ \ + l0 pH>-pK“
^ a.lot
1
+
10
pH‘~pK‘
(
4
)
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