Environmental Engineering Reference
In-Depth Information
(iv) addition of MgADP [
37
,
62
,
63
]. Studies of these structures reveal that, upon
complex formation, the Fe protein adopts different docking conformations, while the
MoFe protein remains entirely unchanged in conformation. Remarkably, the [Fe
4
S
4
]
cluster-P-cluster distance is shortened by ~20%whenMgAMPPCP orMgADP · AlF
4
is bound. This observation supports the proposal that the binding of MgATP induces a
conformational rearrangement of the Fe protein, which may in turn facilitate the
transfer of electrons from the Fe protein to the MoFe protein (Table
1
).
Table 1 Parameters
describing the docking
interactions between the Fe
protein and the MoFe protein
under different conditions.
a
(
)
b
)
c
Complex conditions
˕
r
(
Å
Chemically cross-linked
30
23.2
MgAMPPCP-stabilized
21
17.8
MgADP-stabilized
26-33
22.6-23.7
MgADP · AlF
4
-stabilized
12-13
17.5-17.6
a
This table was adapted from [
37
].
b
˕
is the angle between the helices from residue
γ
98 to
γ
112 of
0
when the
helices are in co-planar arrangement, where the Fe protein
docking interface is maximally flattened.
c
r
is the [Fe
4
S
4
]-P-cluster distance and it is defined between the
centroids of these clusters in each complex.
each of the Fe protein subunits. It is defined that
˕¼
Additionally, comparison of the MgAMPPCP- or MgADP · AlF
4
-bound
structures, which mimic the stage II and III, respectively, of the Fe protein cycle,
shows that the binding surface of the Fe protein is substantially flattened, thereby
allowing the Fe protein to dock more snugly onto the MoFe protein (Table
1
). These
structural insights highlight the role of the Fe protein in energetically coupling
different nucleotide-binding states to nitrogenase catalysis, which may have some
general relevance to other nucleotide-dependent molecular motor systems [
63
].
3.1.2 The MoFe Protein Cycle
The MoFe protein cycle consists of eight distinct steps, each representing one of the
eight electron/proton transfer steps that are required to reduce one molecule of N
2
to
two molecules of NH
3
while evolving one molecule of H
2
as the side product
(Scheme
2
). In this cycle, the resting state of one functional
-dimer of the MoFe
protein is designated M
0
, which contains a FeMoco in the dithionite-reduced state.
Upon receipt of electrons from the Fe protein, the M
0
state is converted stepwise to
a series of turnover states. These turnover states are designated M
n
, where n
represents the number of proton/electron pairs added on M
0
.
In the absence of any substrate, the two-electron-reduced state, M
2
, is cycled
back to M
0
through the release of H
2
(Scheme
2
a). As such, in the absence of N
2
, the
MoFe protein only cycles between M
0
and M
2
, and the enzyme can be viewed as an
ATP-dependent hydrogenase. Kinetic data [
4
,
28
] suggest that the binding of N
2
to
the MoFe protein occurs at a state that is a 3- or 4-electrons more reduced state than
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