Environmental Engineering Reference
In-Depth Information
Figure 10 Mechanism of CO oxidation at cluster C. (1) A hydroxyl group is bound to Fe
1
in the
active state (C
red1
). (2) CO enters the active site from above and binds to Ni completing a square-
planar coordination. A hydrogen bond with histidine H
93
stabilizes this transient state, which orients
the nucleophilic hydroxyl group in close proximity to react with CO. (3) The oxidation product CO
2
is
bound to cluster C in a bridging conformation. (4) Uptake of a water molecule supports subsequent
release of CO
2
. Cluster C contains two additional electrons compared to the C
red1
state (C
red2
).
Successive release of two electrons restores cluster C in the catalytically competent C
red1
state.
2.3 Bifunctional Ni,Fe-Containing Carbon Monoxide
Dehydrogenases
2.3.1 Classification and Distribution
The structure and function of bifunctional carbon monoxide dehydrogenases have
been reviewed thoroughly during the last decade [
17
,
19
,
26
,
76
,
77
,
103
-
108
].
Here, the structural and mechanistic aspects of the bifunctional enzyme complex
are described, focusing on the ACS component. CODH/ACS catalyzes the final step
of the Wood-Ljungdahl pathway by coupling both enzyme activities. The CODH
component is responsible for the reversible reduction of CO
2
, while ACS catalyzes
the reversible condensation of CO, CoASH and a methyl group to acetyl-CoA [
26
]
(equation
2
).
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