Environmental Engineering Reference
In-Depth Information
2.2.2 The P-cluster
The P-cluster is a [Fe
8
S
7
] cluster that is bridged between each
ʱʲ
-dimer of the
-subunit (Cys
ʱ
62
, Cys
ʱ
88
,
MoFe protein by six cysteinyl ligands: three from the
ʱ
Cys
ʱ
154
) and three from the
-subunit (Cys
ʲ
70
, Cys
ʲ
95
, Cys
ʲ
153
) (Figure
3
)[
7
,
10
].
In the presence of excess dithionite, the P-cluster adopts a so-called P
N
state, which
can be structurally viewed as two [Fe
4
S
4
] sub-cubanes sharing a
ʲ
ʼ
6
-sulfide
(Figure
3b
). EPR and M
¨
ssbauer spectroscopy further demonstrates that the
dithionite-reduced P
N
-cluster is essentially diamagnetic, which leads to the general
belief that all Fe atoms in this cluster are present in the all-ferrous state [
4
,
50
,
51
].
While there is no evidence that the P
N
-cluster can undergo further reduction,
it has been shown that this cluster can be oxidized stepwise into the so-called P
1+
-
and P
2+
-oxidation states. The P
1+
-state can be generated by an enzymatic
one-electron oxidation process, and it displays an interesting, mixed spin state of
S
5/2 [
52
]. The P
2+
-state, also known as the P
OX
-state, can be generated
by a two-electron oxidation process in the presence of IDS. This cluster is far
better characterized than the P
1+
-cluster. It displays a
S
¼
1/2,
S
¼
3 or 4 integer spin state,
which can be best recognized by a parallel-mode EPR signal at
g
¼
11.8 [
50
,
52
].
The crystal structure of the oxidized MoFe protein also reveals that the P
2+
-cluster
is structurally different than the P
N
-cluster in that it adopts a more open confirma-
tion than the latter (Figure
3b
). In particular, the sub-cubane coordinated by
the three cysteines from the
¼
ʲ
-subunit undergoes ligands exchange, with the
coordination of two of the
ʼ
6
-sulfide-ligated Fe atoms replaced by the coordination
of these atoms to two protein ligands: the O
atom from the side chain of Ser
ʲ
188
and the N atom from the backbone amide of Cys
ʱ
88
(Figure
3b
)[
10
,
46
]. Interest-
ingly, the P
N
- and P
OX
-states are readily reversible despite the significant structural
reorganization in this process, although it is not clear if such a redox/conforma-
tional change of the P-cluster is relevant to nitrogenase catalysis.
γ
2.2.3 The FeMoco
ʱ
The FeMoco is located within the
-subunit of the MoFe protein, and it is coordi-
nated in the protein by only two residues: His
ʱ
442
at the Mo end and Cys
ʱ
275
at the
opposite Fe end of the cluster (Figure
3b
)[
7
,
10
,
26
]. This cluster consists of a
[MoFe
7
S
9
] core, which contains a carbide (C
4
) atom in the center and a
homocitrate entity at the Mo end [
53
-
55
]. The metal-sulfur core of the FeMoco
can be viewed as [Fe
4
S
3
] and [MoFe
3
S
3
] sub-cubanes bridged by three
ʼ
2
-sulfides
and one
ʼ
4
-coordi-
nated by three inorganic sulfides and one additional ligand that is either provided by
the cysteinyl thio group (in the case of the terminal Fe atom) or the carbide (in the
cases of all other Fe atoms). The Mo atom, on the other hand, has a
ʼ
6
-carbide in between. This arrangement renders all Fe atoms
ʼ
6
-coordination,
with homocitrate acting as a bidentate ligand through two oxygen atoms (one
carboxyl group and one hydroxyl group) in addition to the ligation provided by
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