Figure 39.This domain contains a
slightly distorted tetrahedral [Fe(Cys)4] site that has a high
homology to D. gigas desulforedoxin (Dx).364b This site, which
is often referred to as "center I", was initially proposed to
mediate intradomain electron transfer between a 2Fe-SOR
redox partner and its SOR active center, but no evidence
supporting this claim has been produced so far. In fact, the Fe-
Fe distance is about 25 A, too large to allow catalytically
significant intramolecular electron transfer. Instead, Emerson et
al. have shown that the disruption of center I has no effect on
the in vitro or in vivo activity of the D. vulgaris 2Fe-SOR.367b
Additionally, the T. pallidum 1Fe-SOR, which can be viewed as
a "naturally mutated" 2Fe-SOR that lost center I, retains its
efficient in vitro and in vivo SOR activity.386b The ~15 residues
loop connecting the two domains in the 2Fe-SORs or between
the short 310 helix and the catalytic domain contains the mostly
conserved (E)(K)HxP-motif.The
change in conformation upon oxidation/reduction is expected
to also occur in 2Fe-SORs, and indeed Fourier transform
infrared (FTIR) studies suggest that the glutamate binds the
Fe3+ center in the SORs from D. baarsii and T. pallidum and
that this process is coupled to the change in the Fe redoxstate.385b Unfortunately, no crystallographic data have so far
supported the binding of the glutamate ligand to the center in
2Fe-SORs, probably due to the difficulty of maintaining these
enzymes in the Fe3+ state, in part due to X-ray-induced
photoreduction of the protein during structural data
collection.71b,389a,390
7.3.3.(A) Superimposition of 1Fe-SOR (blue) and 2Fe-SOR (red) monomers (PDB codes: 1DO6 and
1DFX); (B) structural conservation of amino acid residues in the monomer of 1Fe-SOR, mapped over P. furiosus structure and made using
ConSurf;398,420 and (C) ribbon diagram of the same monomer in B (PDB code: 1DO6).While the SOR reduces superoxide to hydrogen peroxide, P450
binds O2 and catalyzes its two-electron reduction and double
protonation to cleave the O-O bond, yielding 1 equiv of water
and an enzyme intermediate known as Compound I (see
section 7.5).The
structure of the reduced center was determined after incubation
of P. furiosus SOR crystals with sodium dithionite, showing the
typical [Fe(His)4Cys] configuration (PDB code: 1DDK71b),
albeit with low Fe occupancy at two of its subunits (~20%).Redox-Linked Structural Changes in the SOR
Active Center.Structural conservation in SORs.7.3.2.