EPR and ligand field studies of iron superoxide dismutases and iron-substituted manganese superoxide dismutases: Relationships between electronic structure of the active site and activity
Jp. Renault et al., EPR and ligand field studies of iron superoxide dismutases and iron-substituted manganese superoxide dismutases: Relationships between electronic structure of the active site and activity, INORG CHEM, 39(12), 2000, pp. 2666-2675
The problem of metal selectivity of iron/manganese superoxide dismutases (S
ODs) is addressed through the electronic structures of active sites using e
lectron paramagnetic resonance and ligand field calculations. Studies of wi
ld-type iron(III) SOD (FeSOD) from Escherichia coil and from Methanobacteri
um thermoautotrophicum and iron-substituted manganese(III) SOD (Fe(sub)MnSO
D) from E. coli and from Serratia marcescens are reported. EPR spectroscopy
of wild-type enzymes shows transitions within all three Kramers doublets i
dentified by their g values. From the temperature dependence of the observe
d transitions, the zero-field splitting is found to be negative, D = -2 +/-
0.2 cm(-1). The electronic structure is typical of a distorted trigonal bi
pyramid, all the EPR features being reproduced by ligand field analysis. Th
is unique and necessary electronic structure characterizes wild-type enzyme
s whatever their classification from the amino acid sequence into iron or m
anganese types, as E. coli FeSOD or M. thermoautotrophicum FeSOD. In iron-s
ubstituted manganese SODs, reduced catalytic activity is found. We describe
how inhomogeneity of all reported substituted MnSODs might explain the act
ivity decrease. EPR spectra of substituted enzymes show several overlapping
components. From simulation of these spectra, one component is identified
which shares the same electronic structure of the wild-type FeSODs, with th
e proportion depending on pH. Ligand field calculations were performed to i
nvestigate distortions of the active site geometry which induce variation o
f the excitation energy of the lowest quartet state. The corresponding coup
ling between the ground state and the elicited state is found to be maximum
in the geometry of the native SODs. We conjecture that such couplings shou
ld be considered in the electron-transfer process and in the contribution o
f the typical electronic structure of FeSOD to the activity.