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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