196
Chapter 6
When S, is tightly bound to the enzyme form
E,
i.e. when the dissociation constant for
ESX
in
(6.9)
is small then the apparent affinity of
E
for
S
is small. The result is that
r
decreases unless
s
is large enough to make the denominator constant small compared to
s. Competitive inhibition -
the deposition of active enzyme in a “dead- end “compound
E SX-
can be alleviated by increasing
the substrate concentration, but this may not always be possible, e.g. in an enzymatic assay.
The competing substrate for E could be any foreign chemical or it could be the product
P
of the
reaction. Here a negative influence o f
P
on the rate is found also when the overall reaction is
completely irreversible. W hen the assay is run at different levels of
P
one may detect a possible
product inhibition.
If both
E
and
ES
can react with the foreign substance Si to form inactive complexes
E SX
and
ESSX
respectively, and if also
ESX
can react with
S
to form
E SSX
then the following reaction network is
found
S
E
ES
-► E + P
it
it
S,
(6.11)
ES,
S
ESS,
S
Here “the dead end complex” ESi can be “activated” through conversion to
ESSX
which again can
form the active enzyme complex
ES.
This type of indiscriminate inhibition where 5, binds
randomly to both primary forms of the enzyme does not lower the over all affinity of the enzyme
for
S,
but leads to a general decrease of the rate of the enzymatic reaction through a smaller
apparent rate constant.
Assume that
ESX
and
ESSX
have the same dissociation constant
K ^ x
to respectively
E
and
ES
and
also that the substrate
S
binds equally well to
E
and to
E SX
.
Then in analogy with (6.7)
e =
( e s s , )
(as)-?.
K
eq
1
( e s , )
(es)stK eq
K eqXs
(
6
.
12
)
( e s ) = -
K..
(
\
f
S
, 1
i
+ —1
-
+
1 + —1
-
(6.13)
/
K eq+s
l + ^ i
V
(6.14)
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