116
Chapter 4
(AHf , AGr) for OH" is - 54.97 and -39.59 kcal m ol
1
respectively.
Calculate AHC
and AGC
for N H /
Problem 4.2 Is a lysine yield of 6/7 on glucose possible from a thermodynamic point of view?
In Example 3.9 a maximum yield of 6/7 C-mole lysine (C-mole glucose
)'1
was predicted by imposing the
constraint that all yield coefficients in Eq (3.9.5) should be non-negative. Stephanopolous and Vallino
(1991) and (1993), and Marx
et al.
(1996) report that the maximum theoretical yield is only 3/4. The
reason could be that AG is positive for the reaction:
-CH20 -
(2/7) NH
3
+ (6/7) CH
7
/
3
O
1
/
3
N
10
+ (1/7) C 0
2
+ (3/7) H:0 =
0
(1)
a.
Use eq (4.16) to calculate an approximate value for (-AGc°)ly,ine.
b.
Use this value to determine AG° for reaction (1).
c.
Experimental values for AGC
and for
AHC
are difficult to find for lysine, but Morrero and Gani
(2001) have calculated AGf° and
AH?
for L-lysine by a group contribution method. Their
calculated values are:
- AGf° = 214 kJ mole
'1
and
- AHf° = 450 kj mole ’.
(2)
Use these data to calculate AGCD
and A
H °
for L-lysine. Data for C 0
2
and H20 are taken from the
examples of chapter 4. Note that the difference between AGC
calculated in question 1 and
question 3 is quite small.
d.
Will reaction (1) proceed voluntarily in the direction indicated in (1) ?
Problem 4.3 The effect of a transhydrogenase reaction in
Saccharomyces cerevisiae
.
In Problem 3.6 three different strains of
S. cerevisiae
were compared. The purpose of constructing strain
;
TN21 was to provide the anaerobically growing yeast culture with a sink for NADH. If NADH was
converted to NADPH it was thought that less glycerol would be formed - in TNI and TN26 glycerol
production is the only means of balancing the excess NADH formed in connection with the formation of
precursor metabolites like pyruvate and acetyl-CoA. Furthermore the produced NADPH might substitute
NADPH produced in the PP pathway and consequently diminish the glucose loss in this pathway.
a.
Consider the reaction
- NADH - NADP" + NADPH + NAD" = 0
(1)
which is catalysed by the transhydrogenase in TN2I. Is it reasonable to assume that AG° is
almost zero for reaction (
1
)?
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