Environmental Engineering Reference
In-Depth Information
Scheme 1 Two possible mechanisms of Ni-Fe CODH proposed by (a) Dobbek [
8
] and Lindahl
[
23
], in which the Ni subsite in C
red2
is formally Ni(0) or (b) as proposed by Fontecilla-
Camps et al. [
25
], in which the Ni subsite in C
red2
is bonded to a H atom, making what is formally
a Ni(II)-H species. The base 'B' refers to the amino acid that accepts/donates the proton, possibly
His or Lys. The red colors represent substrate-mimic inhibitors in which cyanide (CN
) binds to
the C
red1
state and cyanate (NCO
) binds to the C
red2
state.
(
g
av
~1.94) and the reduction potential for the [4Fe-4S]
2+/1+
couple is -440 mV in
the enzyme from
Moorella thermoacetica
and -418 mV in the enzyme from
Rhodospirillum rubrum
[
29
,
30
]. Significantly, the D-cluster remained undetected
until the first crystal structure of CODH was solved. One possibility is that this
cluster had escaped detection because the reduced [4Fe-4S]
1+
state has S
, but
studies of CODH
Rr
by magnetic circular dichroism (MCD) and resonance Raman
spectroscopy suggest that the D-cluster remains in the oxidized [4Fe-4S]
2+
state
even at -520 mV [
31
]. This information will be important when we consider the
results from PFE experiments later.
>
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