Environmental Engineering Reference
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in the MgADP-bound state, and it also utilizes the [Fe
4
S
4
]
1+/2+
couple for the
one-electron transfer event during catalysis.
Figure 7 EXAFS-derived models of [Fe
4
S
4
] clusters in NifH (a) and VnfH (b). The [Fe
4
S
4
]
clusters are shown in ball-and-stick (
left
) and line (
middle
and
right
) presentations. Both clusters
have been rotated to show the overlapping of the [Fe
2
S
2
] rhomboids (
right
). In the case of NifH,
the two rhomboids are unequal and bent to a greater degree out of plane relative to each other (a);
whereas in VnfH, two identical [Fe
2
S
2
] rhomboids are stacked and offset by 90
(b). Atoms are
colored as follows: Fe, orange; S, yellow. PYMOL was used to generate this figure, with
coordinates of the crystal structure of the [Fe
4
S
4
] cluster in NifH (PDB entry 1NIP) modified on
the basis of the EXAFS fits given in [
95
].
Surprisingly, despite the high homology between the two Fe proteins, Fe K-edge
XAS/EXAFS analysis shows distinct differences between the geometric arrange-
ments of their associated [Fe
4
S
4
] clusters [
91
] (Figure
7
). The [Fe
4
S
4
] cluster in
VnfH can be best modeled by two identical [Fe
2
S
2
] rhomboids, which are stacked
on and offset by 90
from each other (Figure
7b
); whereas the [Fe
4
S
4
] cluster in
NifH can be modeled alternatively by two unequal [Fe
2
S
2
] rhomboids, which are
bent to a greater degree out of plane relative to each other (Figure
7a
). The exact
effect of these subtle differences on the catalytic capacities of the V- and
Mo-nitrogenases is still unclear, and further biochemical or structural studies
need to be performed on these proteins to gain a better understanding in this regard.
4.1.2 The VFe Protein
The
-subunits of the VFe protein are encoded by
vnfD
,
vnfK
,and
vnfG
genes, respectively. Contrary to the
ʱ
-,
ʲ
-, and
ʴ
ʱ
2
ʲ
2
-tetrameric MoFe protein, the VFe protein
from
A. vinelandii
(Figure
8a, b
) has the composition of an
ʱ
2
ʲ
2
ʴ
4
-octamer, with its
ʱ
32 % of sequence identity with their respec-
tive counterparts in the MoFe protein and its
-and
ʲ
-subunits sharing
33 % and
-subunit being a unique feature of the
VFe protein [
5
,
22
]. In addition, there is a high degree of sequence identity between
the MoFe protein and the VFe protein, which centers around the residues that are
either ligated to or in the surroundings of the P-cluster and the cofactor [
5
,
22
].
ʴ
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