280
Chapter 7
CAMP
CAP
RN/
POIS
ß-galactosidase
Lactose permease
► Thio galactoside transacytase
I
I
I
r
f
-
- o
lacZ
lacY
lacA
Repressor
Inducer
Figure 7.16 The lac-operon of
E. coli.
The operon includes a gene
(lacI)
for the repressor protein (r),
promotor (p), operator (o), and the three genes
lacZ, lacY,
and
lac A,
which code for different enzymes:
lacZ
codes for P-galactosidase,
lacY
for lactose permease, and
lacA
for thio galactoside transacylase. In its free
form the repressor protein may bind to the operator; when it complexes with the inducer (i), conformational
changes of the repressor protein prevent binding to the operator. cAMP complexes with CAP and the
complex may bind to the promoter, whereby the RNA polymerase may start the transcription from the
promotor.
The model in Eq. (7.44) gives a simplified description of the true system since there may be
different binding affinities for the repressor protein depending on how much lactose is bound to the
protein (see Problem 7.3). With the concentration of the species (indicated with squared brackets)
being in moles per gram dry weight, the equilibrium constants
Ka i=
1 ,2 ,3 ,4 are given by:
K
J
(7.45)
(7.46)
[X0X rnS,„]
1
[x,x,\s,J
(7.47)
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