Environmental Engineering Reference
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mechanism of biological nitrogen fixation centers on the Mo-nitrogenase.
The V-nitrogenase, on the other hand, was much less studied than its
Mo-counterpart. This nitrogenase displays structural and catalytic features that
are distinct from those of the Mo-nitrogenase [ 5 , 22 ]. Most notably, it reduces
carbon monoxide to short-chain hydrocarbons, such as ethane, ethylene, propane,
and propylene, at much higher efficiencies than its Mo-counterpart [ 22 ]. Given the
isoelectronic properties of CO and N 2 , it is conceivable that knowledge of the
mechanism of CO reduction may contribute, if indirectly, to our understanding of
the mechanism of N 2 reduction.
The objective of this article is to review the most recent advances toward the
understanding of the reaction mechanism of nitrogenase, with sections introducing
the structural properties of Mo-nitrogenase (Section 2 ), presenting the classical
model of nitrogenase mechanism and the recent development of catalysis by
Mo-nitrogenase (Section 3 ), comparing the structural and catalytic features of
Mo- and V-nitrogenases (Section 4 ) and briefly summarizing the current status of
this research topic (Section 5 ).
2 The Structural and Biochemical Properties
of Mo-Nitrogenase
Mo-nitrogenase is the best studied member of this enzyme family and it has
a wide prevalence in Nature. This nitrogenase is found in diazotrophic
(or nitrogen-fixing) microorganisms, such as Azotobacter chroococcum,
Azotobacter vinelandii , Klebsiella pneumoniae ,and Clostridium pasteurianum
[ 9 , 10 , 25 ]. Biochemical analysis has long established Mo-nitrogenase as a
two-component enzyme system, with one component referred to as the iron
(Fe) protein and the other referred to as the molybdenum-iron (MoFe) protein
(Figure 1 ). Both component proteins contain FeS clusters: the Fe protein carries a
ferrodoxin-type [Fe 4 S 4 ] cluster; whereas the MoFe protein carries two types of
complex FeS clusters: the P-cluster ([Fe 8 S 7 ]) and the iron-molybdenum cofactor
(or FeMoco; [MoFe 7 S 9 C-homocitrate]) (Figure 1 ).
The three FeS clusters of nitrogenase are essential for catalysis, as they can be
alignedintoanelectrontransferpathway upon ATP-dependent docking of the
Fe protein on the MoFe protein, which then allows electrons to be transferred
from the [Fe 4 S 4 ] cluster of the former protein, via the P-cluster, to the FeMoco
of the latter protein, where substrate reduction eventually occurs (Figure 1 )
[ 1 - 16 ]. To better understand this ATP-dependent, inter-protein electron transfer
process, it is crucial for us to take a look at the structural and chemical properties
of the two component proteins of Mo-nitrogenase and their associated
metal clusters.
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