Environmental Engineering Reference
In-Depth Information
3.2 Coordination and Oxidation States of the Nickel Center
The bound cofactor in MCR also interconverts among various states, which are
designated by their oxidation or ligation states; for example, MCR
red1m
indicates
that MCR has been reduced to the Ni(I) state and incubated with CH
3
-SCoM.
The “1” distinguishes this from another characterized Ni(I) state, called MCR
red2
.
Recent reviews include tables that provide the g values from the EPR spectra and
the coordination environments of the known MCR states [
65
,
66
].
The active Ni(I) form of the enzyme, termed MCR
red1
, has
g
values of 2.068,
2.082, 2.274 [
67
] and a visible spectrum with absorption maxima at 383 nm and
759 nm [
47
]. The Ni(I) has a 3d
9
configuration with the unpaired electron residing
mostly in the d
x
2
-
y
2
orbital with a hyperfine coupling value of 25-30 MHz from the
pyrrole nitrogens [
66
]. MCR
red1
can be generated within the cell by replacing
the 80%/20% (H
2
/CO
2
) gas phase with 100% H
2
[
68
,
69
] or 100% CO [
70
] prior
to harvesting the cells or by treating the MCR
ox1
state with sodium sulfide (Na
2
S)
[
71
]; alternatively, it can be generated
in vitro
by adding CH
3
-SCoM to the
MCR
red2
state [
72
].
There is no crystal structure of this form of the protein, due to its extreme
lability; however, it has been characterized by X-ray absorption spectroscopy,
which provides precise nickel-ligand bond lengths and geometries. As shown in
Figure
3
, which compares the Ni(I), Ni(II), and methyl-Ni(III) states, in the
MCR
red1
state, the Ni(I) is pentacoordinate, being ligated by the four nitrogen
atoms from the tetrapyrrole and the oxygen atom from the side chain of Gln
ʱ
'
147)
[
73
,
74
]. Several MCR
red1
sub-types have been described: MCR
red1c
,MCR
red1m
,
which are generated by incubating the Ni(I) form of the enzyme with CoMSH or
methyl-SCoM, respectively, and MCR
red1a
, generated in the absence of a CoM
derivative. The EPR spectra of these states differ slightly in that the MCR
red1m
and
MCR
red1c
forms exhibit sharper peaks and, thus, better resolved splitting of the
S-shaped resonance at
g
2.07 [
73
].
Another Ni(I) state, MCR
red2
, has been identified and, based on high-frequency
EPR studies and density functional theory (DFT) computations, the MCR
red2a
and
MCR
red2r
states were resolved [
56
,
75
,
76
]. In the MCR
red2r
state, the EPR spectrum
is markedly altered (
g
1
increases from the ~2.06 value seen in other MCR
red
states
to 2.175) and the nitrogen hyperfine splitting value (~14 MHz) of one of the pyrrole
nitrogens (from the A ring) is reduced relative to that of the other three nitrogens
(~24 MHz). These unusual features indicate a significant distortion of the tetrapyr-
role macrocycle that displaces the nitrogen in the A ring out of the plane containing
the other three nitrogens in the tetrapyrrole ring and removal of the glutamine
oxygen ligand. By
33
S isotope labeling and pulsed EPR studies, it was shown that
the thiolate sulfur of CoMS
ligates the Ni in the MCR
red2r
state [
56
,
77
].
An exchangeable proton with a strikingly large (42-43 MHz) hyperfine coupling
(
1
H nuclear spin I
¼
1/2) was identified in MCR
red2a
, indicating that this form of
the protein contains a 1.6 to 1.7
¼
Ni-H bond (perhaps a nickel hydride). In the
MCR
red2r
state, the proton coupling is reduced to 29 MHz, consistent with the
ionizable proton from CoMSH interacting with Ni and a tetrapyrrole nitrogen [
76
].
Å
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