Environmental Engineering Reference
In-Depth Information
three sulfides and one nitrogen atom of His
ʱ
442
(Figure
3
). The homocitrate entity
forms an extensive hydrogen-bonding network with the sulfides of the metal-sulfur
core [
4
], and it is thought to be responsible for the overall negative charge of the
FeMoco, which in turn is important for the insertion of FeMoco along a positively
charged path into its binding site within the MoFe protein [
56
,
57
].
The protein-bound FeMoco exhibits a well-characterized
S
¼
3/2 EPR signal at
g
¼
4.7, 3.7 and 2.0 in the presence of excess dithionite [
4
]. Electrochemical
analysis shows that FeMoco can be either reduced or oxidized by one electron to
a diamagnetic state [
4
,
52
]. The EPR signal of the resting-state FeMoco disappears
when the MoFe protein is enzymatically reduced by the Fe protein, which is
interpreted as FeMoco being further reduced to facilitate substrate turnover.
The FeMoco can be extracted as an intact entity into organic solvents, such as
N
-methylformamide (NMF) [
3
,
4
,
58
-
60
]. Upon extraction, the FeMoco exhibits an
S
5.94, 4.66, and 3.50, which is similar to, yet much broader
in shape, than that of its protein-bound counterpart [
58
], suggesting that the
polypeptide surroundings of FeMoco have a significant impact on its electronic
properties. The solvent-extracted FeMoco can be used to reconstitute and activate
the FeMoco-deficient, apo MoFe protein. More excitingly, it can be used directly to
reduce certain substrates, such as protons, cyanide, and CO, in the absence of ATP
and both protein components of the Mo-nitrogenase [
61
]. While it is yet to be
demonstrated that the extracted FeMoco can reduce N
2
, it is conceivable that
conditions can be established that allow this cofactor to serve as a catalyst for N
2
reduction in the isolated state.
¼
3/2 EPR signal at
g
¼
3 The Catalytic Mechanism of Mo-Nitrogenase
The reaction of N
2
reduction by Mo-nitrogenase is generally depicted as follows:
8H
þ
þ
8e
þ
N
2
þ
16 ATP
!
2NH
3
þ
H
2
þ
16 ADP
þ
16 P
i
ð
1
Þ
In the case of the V-nitrogenase, however, the ratio of H
2
and NH
3
formation was
shown to differ from what has been proposed for its Mo-counterpart (see Section
4
below). Structural and biochemical analyses have provided significant insights
into the catalytic mechanism of Mo-nitrogenase. The crystal structure of the
MgADP · AlF
4
-stabilized Mo-nitrogenase complex has effectively captured a
“transition state” of this nitrogenase and provided compelling evidence for the
formation of an electron transfer pathway during catalysis [
62
] (Figure
4
).
The large body of biochemical experiments conducted by Thorneley and Lowe,
on the other hand, has allowed the successful construction of a classical catalytic
scheme of Mo-nitrogenase (see Section
3.1
below). This model provided a great
framework for recent spectroscopic studies, which led to the characterization of a
number of potential intermediates of N
2
reduction (see Section
3.2
below).
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