Environmental Engineering Reference
In-Depth Information
Cyanide (CN
), isoelectronic with CO, is a well-studied inhibitor of CODH
[
32
-
34
]. Several CODH crystal structures from different species with CN
or CO
bound to the Ni atom in the C-cluster have been determined: that of CN
-bound
CODH II
Ch
is included in Figure
3
[
14
]. A consensus is not achieved: compared to
the almost linear NC-Ni arrangement in CN
-bound CODH II
Ch
, the CN
-bound
CODH/ACS
Mt
and CO(formyl)-bound ACDS/CODH
Mb
reveal, respectively, a
bent
NC-Ni structure with bond angle ~114
and a
bent
OC-Ni structure with bond angle
~107
[
20
,
35
]. The OH
ligand remains on the pendant Fe in both structures.
Compared with the structure of CO
2
-bound CODH II
Ch
, the N-atom in CN
-bound
CODH/ACS
Mt
and the O-atom in CO-bound ACDS/CODH
Mb
each overlay with
the corresponding O-atom from CO
2
in CO
2
-bound CODH II
Ch
whereas the C-atom
in both CN
-bound ACS/CODH
Mt
and CO-bound ACDS/CODH
Mb
is displaced
from its position in CO
2
-bound CODH II
Ch
[
8
]. The shift in C-atom position could
facilitate the nucleophilic attack by OH
. Interestingly, the crystal structure of
CODH II
Ch
incubated with
n
-butyl isocyanide at -320 mV reveals that the C-atom
from the plausible product “
n
-butyl-isocyanate” binds to the Ni with a distorted
tetrahedral coordination geometry [
36
]. A second
n
-butyl-isocyanide is found in the
putative gas channel in CODH II
Ch
.
Introduction of CN
to a sample of CODH/ACS
Mt
poised in the C
red1
state
results in a new EPR signal with
g
av
¼
1.87, 1.78, 1.55) [
23
,
33
,
34
,
37
].
In contrast, no change in the EPR spectrum is observed when CN
is introduced to
the C
red2
state [
37
]. Further studies of
13
CN
-bound CODH/ACS
Mt
by electron
nuclear double resonance (ENDOR) spectroscopy revealed a doublet peak in the
C
red1
state - early evidence that CN
(
13
C nuclear spin I
1.72 (
g
¼
) binds directly to the
C-cluster [
38
,
39
]. Experiments to evaluate if cyanate (NCO
) is an inhibitor
indicated that it binds also to C
red1
even though (since it is an analogue of CO
2
not CO) it should display behavior opposite to that of CN
: this result was puzzling,
but as we explain below, it was clarified by PFE experiments.
Some crystal structures of CODH II
Ch
revealed a
¼
½
μ
-sulfido ligand bridging the Ni
atom and the pendant Fe atom (Figure
3d
), and this observation led to suggestions
that an additional (5th) sulfide is necessary for catalysis [
15
,
40
]. In place of
inorganic sulfide (S
2
), the thiolate sulfur from Cys531 was found to bridge the
Ni atom and the pendant Fe atom in the crystal structure of CODH
Rr
[
21
]. However,
crystal structures from recombinant CODH II expressed in
E. coli
and of CODH
from
Moorella thermoacetica
and
Methanosarcina barkeri
(
Mb
) revealed no pres-
ence of a 5th
-sulfido ligand [
20
,
35
]. The CO oxidation activities of CODH
measured by solution assays in the presence of sodium sulfide showed varying
results between different species and redox conditions [
33
,
41
]. The role of sulfide
is considered later when we discuss the PFE results.
Scheme
1
highlights a challenge for investigating enzymes such as CODHs that
catalyze redox reactions via rapid passage through a series of intermediates, the
presence (lifetime) of each of which depends on how fast or favorable are the electron
transfers that determine their existence. Direct coordination of a reactant or inhibitor,
or release of a product, should be selective for a particular oxidation state: the question
is, how to control and fine-tune the relative amounts of each state during catalysis and
simultaneously observe the effect on rate when reactant or inhibitor are introduced.
Not all inhibitors bind to active states: some exert their influence by facilitating redox
μ
Search WWH ::
Custom Search