  Superoxide reductase (SOR) is an Fe protein that catalyzes the reduction of
superoxide to give H(2)O(2). Recently, the mutation of the Glu47 residue into
alanine (E47A) in the active site of SOR from Desulfoarculus baarsii has
allowed the stabilization of an iron-peroxo species when quickly reacted with
H(2)O(2) [Math{\'e} et al. (2002) J. Am. Chem. Soc. 124, 4966-4967]. To further
investigate this non-heme peroxo-iron species, we have carried out a
M{\"o}ssbauer study of the (57)Fe-enriched E47A SOR from D. baarsii reacted
quickly with H(2)O(2). Considering the M{\"o}ssbauer data, we conclude, in
conjunction with the other spectroscopic data available and with the results of
density functional calculations on related models, that this species
corresponds to a high-spin side-on peroxo-Fe(3+) complex. This is one of the
first examples of such a species in a biological system for which M{\"o}ssbauer
parameters are now available: delta(/Fe) = 0.54 (1) mm/s, DeltaE(Q) = -0.80 (5)
mm/s, and the asymmetry parameter eta = 0.60 (5) mm/s. The M{\"o}ssbauer and
spin Hamiltonian parameters have been evaluated on a model from the side-on
peroxo complex (model 2) issued from the oxidized iron center in SOR from
Pyrococcus furiosus, for which structural data are available in the literature
[Yeh et al. (2000) Biochemistry 39, 2499-2508]. For comparison, similar
calculations have been carried out on a model derived from 2 (model 3), where
the [CH(3)-S](1)(-) group has been replaced by the neutral [NH(3)](0) group
[Neese and Solomon (1998) J. Am. Chem. Soc. 120, 12829-12848]. Both models 2
and 3 contain a formally high-spin Fe(3+) ion (i.e., with empty minority spin
orbitals). We found, however, a significant fraction (approximately 0.6 for 2,
approximately 0.8 for 3) of spin (equivalently charge) spread over two occupied
(minority spin) orbitals. The quadrupole splitting value for 2 is found to be
negative and matches quite well the experimental value. The computed quadrupole
tensors are rhombic in the case of 2 and axial in the case of 3. This
difference originates directly from the presence of the thiolate ligand in 2. A
correlation between experimental isomer shifts for Fe(3+) mononuclear complexes
with computed electron densities at the iron nucleus has been built and used to
evaluate the isomer shift values for 2 and 3 (0.56 and 0.63 mm/s,
respectively). A significant increase of isomer shift value is found upon going
from a methylthiolate to a nitrogen ligand for the Fe(3+) ion, consistent with
covalency effects due to the presence of the axial thiolate ligand. Considering
that the isomer shift value for 3 is likely to be in the 0.61-0.65 mm/s range
[Horner et al. (2002) Eur. J. Inorg. Chem., 3278-3283], the isomer shift value
for a high-spin eta(2)-O(2) Fe(3+) complex with an axial thiolate group can be
estimated to be in the 0.54-0.58 mm/s range. The occurrence of a side-on peroxo
intermediate in SOR is discussed in relation to the recent data published for a
side-on peroxo-Fe(3+) species in another biological system [Karlsson et al.
(2003) Science 299, 1039-1042].
