Biochemical Reaction Networks
183
Calculate from the answer to b the amount of HAc that should be produced when the other
rates are as stated above. Note that the result fits remarkably well with the experimentally
determined
r ^ .
Next calculate the amounts of H2 and C 02
liberated between 10 and 15 h of
fermentation time. Check that rHLac ~ 0 to within the accuracy of the data.
e.
Between 15 and 20 hr of fermentation time, biomass growth appears to stop [a decrease in
x
is
even indicated in Reardon
et al.
(1987), Fig. 4]. Reardon
et al.
obtained the following changes
in concentrations:
Glucose
-93 mM
HBu
50 mM
HAc
35 mM
Acetoin
4.5 mM
Acetone
8 mM
Butanol
12 mM
Ethanol
2 mM
Do these data fit the metabolic model of b? If not, give a reasonable physiological explanation
for the lack of fit of, e.g., the carbon balance. Why is no extra biomass formed?
Reardon
et al.
(1987) notice a sharp increase in culture fluorescence after 15 h. They tentatively
explain this by an increase in NADH concentration in the cells. If this is true, does it point to
other flaws in the model of b after 15 h?
f.
Papoutsakis (1984) cites some very old (1930) data from van der Lek for a typical "solvent type
fermentation" of C.
acetobutylicum.
The biomass production is not stated, but for 100 moles
(18 kg) of glucose fermented the following amounts (in moles) of metabolic products are
found:
HBu
4.3
HAc
14
H2
139
co2
221
Acetoin
6.3
Acetone
22.4
Butanol
56
Ethanol
9.3
Use the calculated expression in b for
rHAc
to eliminate
rx
from the expressions for rH2 and
rc
in
b and check whether the model in b gives a satisfactory prediction of rH2 and rc.
g.
In the pathway diagram two reactions used by Reardon
et al.
(1987) in their Fig. 3 have been
left out. These reactions are:
-BuCoA - 0.5 HAc + 0.5 AcCoA + HBu = 0
(v14)
-AcetoAcCoA - 0.5 HAc + 0.5 AcCoA + 0.75 ac - 0.25 CO, - 0.25 ATP = 0
(vl5)
Reaction 14 is a parallel reaction to reaction 12, but without release of ATP. Reaction 15 is a