Environmental Engineering Reference
In-Depth Information
these two spin states is thought to be influenced by a number of factors, such as
temperature, viscosity, and ionic strength of the medium [
4
,
38
,
39
]. The [Fe
4
S
4
]
1+
cluster can be oxidized by one electron to the diamagnetic +2 state, either
enzymatically by its redox partner (i.e., MoFe protein), or chemically by oxidizing
dyes [e.g., indigodisulfonate (IDS)]. The [Fe
4
S
4
]
1+/2+
couple is believed to be
physiologically relevant, with a midpoint potential of approximately
290 mV
versus
NHE for the Fe protein of
A. vinelandii
[
40
]. Interestingly, the midpoint
potential of the [Fe
4
S
4
]
1+/2+
couple drops notably (by ~120 mV) to approxi-
mately
430 mV
versus
NHE [
40
]. While it is generally assumed that such a
significant decrease in redox potential is important for the electron transfer between
the Fe protein and the MoFe protein, the exact relevance of this change to nitroge-
nase catalysis remains elusive.
The [Fe
4
S
4
]
1+
cluster can be further reduced to the all-ferrous, [Fe
4
S
4
]
0
state by a
strong reductant like Ti(III) citrate, which has a redox potential of
800 mV
versus
NHE or lower [
41
,
42
]. This observation implies that the Fe protein may be able to
transfer two electrons per catalytic cycle via a [Fe
4
S
4
]
0/2+
redox couple, although
there is currently no evidence supporting the theory that the all-ferrous state is a
physiologically or catalytically relevant redox state of the [Fe
4
S
4
] cluster of the Fe
protein.
2.2 The MoFe Protein and Its Associated Metal Clusters
The MoFe protein is the catalytic component of Mo-nitrogenase. The structural
and spectroscopic properties of this protein and its associated metal clusters, which
are central to the understanding of N
2
activation and reduction, are discussed in
detail below.
2.2.1 The Polypeptide
The MoFe protein of
A. vinelandii
(Figure
3
)isan
ʱ
2
ʲ
2
-heterotetramer of ~220 kDa,
and its
ʱ
- and
ʲ
-subunits are encoded by the
nifD
and
nifK
genes, respectively.
The two
ʱʲ
-dimers are related by a two-fold rotational axis (Figure
3a
). Each
ʱʲ
-dimer of the MoFe protein contains a pair of P-cluster and FeMoco and,
therefore, can be viewed as a functional unit on its own. Each of the
ʱ
- and
ʲ
-subunits are divided into three structural domains:
ʱ
I,
ʱ
II,
ʱ
III, and
ʲ
I,
ʲ
II,
ʲ
III,
respectively, and all of these domains consist of
-sheet-type
polypeptide folds [
43
-
49
]. The P-cluster is located at the interfaces of the
ʱ
-helical- and parallel-
ʲ
ʱ
I- and
ʲ
I-domains, whereas the FeMoco is buried within a cavity between the
ʱ
I-,
ʱ
II-, and
ʱ
III-subunits, 14
Å
away from the P-cluster [
43
-
49
].
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