Environmental Engineering Reference
In-Depth Information
X-ray crystal structures of both the nucleotide-free and the MgADP-bound forms of
the Fe protein have been solved (Figure
2a
)[
29
], the structure of the MgATP-bound
form of this protein is still unavailable. Nevertheless, biochemical and mutagenetic
studies have revealed the potential involvement of a so-called P-loop at the Walker
A motif in the binding of MgATP to the Fe protein [
6
,
10
,
27
,
30
-
32
]. The P-loop
features Ser
16
, which coordinates a Mg
2+
ion and a conserved Lys
10
residue through
its hydroxyl group, thereby forming a salt bridge with the
ʲ
-phosphate of the
nucleotide and consequently locking the nucleotide in its binding site (Figure
2a, b
).
Upon ATP binding, the Fe protein undergoes a conformational rearrangement
that renders its [Fe
4
S
4
] cluster more exposed to solvent and, consequently, there is
an increase in the reactivity of the [Fe
4
S
4
] cluster toward metal chelators [
33
].
Consistent with this observation, electron paramagnetic resonance (EPR) analysis
also suggests a potential change of the environment of the [Fe
4
S
4
] cluster in the Fe
protein upon incubation with MgATP [
33
-
36
]. Interestingly, the crystal structures
of the Fe protein show that the nucleotide-binding sites are 15-19
away from the
[Fe
4
S
4
] cluster [
4
,
6
,
9
,
29
]. The lack of direct contact between the nucleotide- and
cluster-binding sites provides further support for the proposed conformational
change of the Fe protein upon MgATP binding that alters the location of the
[Fe
4
S
4
] cluster [
4
]. While the structural information of such a conformational
rearrangement has not been acquired so far, there is indirect evidence from muta-
genic studies that certain site-directed variants of the Fe protein cannot undergo
conformational changes due to the perturbation of the signal transduction between
the nucleotide- and cluster-binding sites [
31
,
32
]. Further investigations of these
variants suggest that the Walker A motif, the so-called Switch I (residues 38-44)
and Switch II (residues 125-129) regions, and a conserved region around the
residue Ala
157
are all crucial for the MgATP-dependent protein conformational
changes of the Fe protein (Figure
2b
)[
4
,
6
,
10
,
27
]. Based on these results, as well
as the crystal structure of the MgADP-bound Fe protein, it has been proposed that
(i) Switch I acts as a direct link that communicates between the nucleotide-binding
site and the Fe protein/MoFe protein docking interface; and (ii) Switch II mediates
signal transduction between the nucleotide-binding site and the [Fe
4
S
4
] cluster
[
29
,
37
]. The purpose of this MgATP binding-induced change in protein confor-
mation is still under debate; however, this conformational change is clearly essen-
tial for catalysis, as any mutation that prevents it from occurring renders the Fe
protein inactive in substrate reduction.
Å
2.1.2 The [Fe
4
S
4
] Cluster
The [Fe
4
S
4
] cluster is symmetrically ligated by two cysteine residues, Cys
97
and
Cys
132
, from each subunit of the Fe protein (Figure
2c
). It has a +1 oxidization state
when the protein is isolated in excess dithionite. In this state, the cluster displays a
mixed electronic spin state of
S
¼
1/2 and
S
¼
3/2 [
38
,
39
]. The distribution of
Search WWH ::
Custom Search