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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