Thermodynamics of Bioreactions
117
b.
Nissen
et al.
(2001) measured the intracellular concentration of each of the four reactants in (1).
Actually only the cytosolic concentrations should have been measured since the inserted
transhydrogenase operates only in the cytosol, but the measurements in the table below are
thought to give an adequate representation of the relative values of the four nucleotides in the
cytosol.
Intracellular concentrations of NAD(H) and NADP(H) in p mol (g DW)'1
for the two control
strains TNI and TN26, and for the transhydrogenase-containing strain TN21, sampled during
exponential growth in anaerobic batch cultivations. Each measurement was carried out twice.
Strain
NAD+
NADP+
NADH
NADPH
NADH/NAD*
NADPH/NADP*
TNI
2.87 ± 0.09
0.23 ±0.01
0.44 ±0.01
1.21 ±0.07
0.15 ±0.01
5.26 ±0.55
TN26
2.85 ±0.11
0.24 ±0.01
0.43 ±0.01
1.19 ±0.07
0.15 ±0.01
4.96 ±0.52
TN21
2.17 ±0.07
0.27 ± 0.02
0.54 ± 0.02
0.80 ±0.10
0.17 ±0.01
2.96 ± 0.60
Calculate AG for reaction (1) using the very consistent data of the table for TN 1 and TN 26
(which should be almost identical).
c.
Based on the results of question 2: what is the expected value of the ratio between the numbers in
the last two columns of the table? Does the result for TN 21 correspond to the expected value of
the ratio? (A tentative explanation for the answer may be that the ratio NADH/NAD+ is highly
controlled by other mechanisms, and that it is not permitted to increase to its equilibrium value)
In any event the result of the genetic manipulation of the yeast strain is a total fiasco. The
secretion of 2-oxoglutarate observed in problem 3.6 for TN 21 could be a result of the lower
NADPH concentration in this strain, since NADPH is the preferred cofactor for conversion of 2-
oxaloacetate to glutamic acid.
Problem 4.4 Nitrification by
Nitrosomonas sp.
Fixation of C 02 to biomass can be driven by a simultaneous oxidation of NH3 to nitrite. The overall
reaction is
-C02 - (a + 0.2) NH3 - (1.5 a - 1.05) 0 2 + CHu O0.5N0, + a HNO, + (a-0.6) H20 = 0 (1)
Reaction (1) can be divided into two separate chemical reactions which run in parallel::
-C 0 2 - 0.2 NH3 - 0.6 H20 + CHu 0 0.5Na2 + 1.05 0 2 = 0
(2)
- NH3 - 1.5 0 2 + HN02 + H20 = 0
(3)
Determine AG for each of the two reactions (1) and (2). Determine the minimum value of parameter a in
(1)
for the whole process to be thermodynamically feasible. Chemolitotrophic bacteria such as
Nitrosomonas
are thermodynamically quite inefficient. Assume a thermodynamic efficiency of 30 % for
fixation of C 02 and calculate the maximum, yield of biomass on NH3.
This problem is inspired by work on Chemolitotrophs by dr. Lars K. Nielsen at University of
Queensland, Australia.
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