Environmental Engineering Reference
In-Depth Information
2.2.4
Inhibited States of Cluster C
Light was shed upon a potential binding mode for CO by structural studies of
cyanide-inhibited cluster C [
97
,
98
]. Surprisingly, cyanide-inhibited structures for
CODH/ACS
Mt
and CODH II
Ch
revealed different binding modes (Figure
9
).
Figure 9 Inhibited states of CODH. (a) Ball-and-stick model of cyanide-inhibited CODH II
Ch
[
98
]. (b) Schematic drawing of cyanide-inhibited CODH II
Ch
.(c) Cyanide-inhibited cluster C of
ACS/CODH
Mt
[
97
]. (d) n-butyl-isocyanate bound to cluster C of CODH II
Ch
[
95
]. Major and
minor conformations for Fe
1
are depicted in black and grey colors, respectively.
In the structure of CODH II
Ch
-CN, cyanide binds to the open apical coordination
site of Ni in an approximately linear fashion (Ni-C-N angle of 170
) (Figure
9a
and 9b) [
98
]. The OH
x
ligand on Fe
1
is lost and Fe
1
shifts from the A position (Fe
1A
)
to an alternative position B (Fe
1B
). In untreated CODH II
Ch
crystals the Fe
1
atom
occupies the alternative positions with a ratio of 60/30 % for Fe
1A
/Fe
1B
, respec-
tively [
94
], while after CN treatment the cluster has inverted occupancies (10/70 %)
[
98
]. Cyanide binding is stabilized by hydrogen bonds with Lys
563
and His
93
.
A different coordination was found for the structure of CN-inhibited CODH/
ACS
Mt
: cyanide binds to Ni completing a distorted tetrahedral coordination with a
distinctly bent conformation (Ni-C-N angle of 114
, Figure
9c
)[
97
]. As this bent
angle has not been expected from inorganic model complexes [
99
], the authors
suggested that a conserved isoleucine residue in close proximity to the Ni atom
prevents a linear apical binding mode [
78
,
97
]. A patch of electron density in the
structure of the CODH component of the acetyl-CoA decarbonylase/synthase
(ACDS) complex from archaea
Methanosarcina barkeri
above the Ni ion was
Search WWH ::
Custom Search