M{'o}ssbauer characterization of an unusual high-spin side-on peroxo-Fe3+ species in the active site of superoxide reductase from Desulfoarculus Baarsii. Density functional calculations on related models

摘要

Superoxide reductase (SOR) is an Fe protein that catalyzes the reduction ofsuperoxide to give H(2)O(2). Recently, the mutation of the Glu47 residue intoalanine (E47A) in the active site of SOR from Desulfoarculus baarsii hasallowed the stabilization of an iron-peroxo species when quickly reacted withH(2)O(2) [Math{\'e} et al. (2002) J. Am. Chem. Soc. 124, 4966-4967]. To furtherinvestigate this non-heme peroxo-iron species, we have carried out aM{\"o}ssbauer study of the (57)Fe-enriched E47A SOR from D. baarsii reactedquickly with H(2)O(2). Considering the M{\"o}ssbauer data, we conclude, inconjunction with the other spectroscopic data available and with the results ofdensity functional calculations on related models, that this speciescorresponds to a high-spin side-on peroxo-Fe(3+) complex. This is one of thefirst examples of such a species in a biological system for which M{\"o}ssbauerparameters 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 andspin Hamiltonian parameters have been evaluated on a model from the side-onperoxo complex (model 2) issued from the oxidized iron center in SOR fromPyrococcus furiosus, for which structural data are available in the literature[Yeh et al. (2000) Biochemistry 39, 2499-2508]. For comparison, similarcalculations have been carried out on a model derived from 2 (model 3), wherethe [CH(3)-S](1)(-) group has been replaced by the neutral [NH(3)](0) group[Neese and Solomon (1998) J. Am.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 2and 3 contain a formally high-spin Fe(3+) ion (i.e., with empty minority spinorbitals). 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 benegative and matches quite well the experimental value. The computed quadrupoletensors are rhombic in the case of 2 and axial in the case of 3. Thisdifference originates directly from the presence of the thiolate ligand in 2. Acorrelation between experimental isomer shifts for Fe(3+) mononuclear complexeswith computed electron densities at the iron nucleus has been built and used toevaluate 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 goingfrom a methylthiolate to a nitrogen ligand for the Fe(3+) ion, consistent withcovalency effects due to the presence of the axial thiolate ligand. Consideringthat 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 valuefor a high-spin eta(2)-O(2) Fe(3+) complex with an axial thiolate group can beestimated to be in the 0.54-0.58 mm/s range. The occurrence of a side-on peroxointermediate in SOR is discussed in relation to the recent data published for aside-on peroxo-Fe(3+) species in another biological system [Karlsson et al.(2003) Science 299, 1039-1042].