Environmental Engineering Reference
In-Depth Information
To reveal the parallels among these seemingly different solutions to the same
problem, we will focus on the catalytic requirements for performing a CO oxidation
(Figures
4
and
10
):
(i) CO oxidation begins with binding and activating CO. By using Cu(I) and a
Ni(II) ion in a sulfur-rich coordination, both enzymes employ for this task a
metal with sufficient
π
-electron donor character and have an open coordination,
site for CO binding. Both nickel and copper are assumed to retain their redox
states during the catalytic cycle, restricting their role to bind and polarize CO.
(ii) The next task is to bind and activate water, which is achieved by Mo(VI)
(Cu,Mo-CODH) and Fe(II) (Ni,Fe-CODH). Water is “activated” by
deprotonation to give either a hydroxyl ligand (Ni,Fe-CODH) or an oxo ligand
(Cu,Mo-CODH). Deprotonation of the water ligand is necessary to increase its
nucleophilicity. The activated water must be sufficiently close and properly
oriented to react with the C atom of the bound CO, making the relative
arrangement of the open coordination sites for CO and water crucial.
(iii) Stabilization of bound CO
2
is likely achieved in both types of CODHs by
coordination to both metals. Ni,Fe-CODHs catalyze CO
2
reduction with
turnover numbers of up to 50 s
1
, with observed rate constants depending
on the electron donor and the presence of electron mediators [
146
]. However,
as only Ni,Fe-CODHs are able to reduce CO
2
, activation of CO
2
may not be
important or achievable for Cu,Mo-CODHs. Residues in the second coordi-
nation sphere stabilize CO
2
, as in the structure of CO
2
-bound Ni,Fe-CODH,
where the oxygen atoms of CO
2
are in hydrogen-bond distance to two protein
side chains [
94
].
(iv) In contrast to the water-gas shift reaction, where the two electrons and two
protons generated by CO oxidation are directly released as H
2
[
147
], CODHs
keep the protons and electrons separated. In Cu,Mo-CODHs Mo(VI) acts as
the electron acceptor taking both electrons originating from CO oxidation.
It is not clear where the electrons are stored in the C
red2
state of Ni,Fe-CODHs
and different electron acceptors within cluster C appear possible. The proposed
oxidation state of the C
red1
state is {[Ni
2+
Fe
2+
]:[Fe
3
S
4
]
}, which can be reconciled
with the S
1/2 spin state and M¨ssbauer data [
74
]. Two-electron uptake could
theoretically involve the formal reduction of Ni
2+
to Ni
0
, the formation of a
Ni-hydride or a Ni-Fe bridging hydride ion or, as recently suggested [
91
], formation
of a Ni-Fe bond. Both, the observation of an S
¼
1/2 EPR signal for the C
red2
state,
as well as the minor changes observed upon reduction of C
red1
to C
red2
by
M¨ssbauer spectroscopy, argue against an uptake of the electrons by the [Fe
3
S
4
]
moiety of cluster C. Certainly, further spectroscopic and computational investiga-
tions will be required to fully understand the differences in electronic structure
between C
red1
and C
red2
.
Both types of CODHs are capable of CO oxidation, but the rates with which they
achieve this task differ by approximately two orders of magnitude. While Cu,
Mo-CODH has a limiting turnover number of close to 100 s
1
[
50
,
55
], the most
active Ni,Fe-CODHs achieve turnover numbers of 30,000-40,000 s
1
[
34
]. It is not
¼
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