M. Costas, M. P. Mehn, M. P. Jensen, and L. Que, Dioxygen Activation at Mononuclear Nonheme Iron Active Sites:?? Enzymes, Models, and Intermediates, Chemical Reviews, vol.104, issue.2, p.939, 2004.
DOI : 10.1021/cr020628n

I. G. Denisov, T. M. Makris, S. G. Sligar, and I. Schlichting, Structure and Chemistry of Cytochrome P450, Chemical Reviews, vol.105, issue.6, p.2253, 2005.
DOI : 10.1021/cr0307143

F. E. Jenney, J. , M. F. Verhagen, X. Cui, and M. W. Adams, Anaerobic Microbes: Oxygen Detoxification Without Superoxide Dismutase, Science, vol.286, issue.5438, p.306, 1999.
DOI : 10.1126/science.286.5438.306

I. Schlichting, The Catalytic Pathway of Cytochrome P450cam at Atomic Resolution, Science, vol.287, issue.5458, p.1615, 2000.
DOI : 10.1126/science.287.5458.1615

K. Kuhnel, E. Derat, J. Terner, S. Shaik, and I. Schlichting, Structure and quantum chemical characterization of chloroperoxidase compound 0, a common reaction intermediate of diverse heme enzymes, Proceedings of the National Academy of Sciences, vol.104, issue.1, p.99, 2007.
DOI : 10.1073/pnas.0606285103

A. Karlsson, Crystal Structure of Naphthalene Dioxygenase: Side-on Binding of Dioxygen to Iron, Science, vol.299, issue.5609, p.1039, 2003.
DOI : 10.1126/science.1078020

D. Bourgeois and A. Royant, Advances in kinetic protein crystallography, Current Opinion in Structural Biology, vol.15, issue.5, p.538, 2005.
DOI : 10.1016/j.sbi.2005.08.002

V. Adam, A. Royant, V. Niviere, F. P. Molina-heredia, and D. Bourgeois, Structure of Superoxide Reductase Bound to Ferrocyanide and Active Site Expansion upon X-Ray-Induced Photo-Reduction, Structure, vol.12, issue.9, p.1729, 2004.
DOI : 10.1016/j.str.2004.07.013
URL : https://hal.archives-ouvertes.fr/hal-01075786

M. D. Clay, Superoxide Reductase:?? Implications for Active-Site Structures and the Catalytic Mechanism, Journal of the American Chemical Society, vol.124, issue.5, p.788, 2002.
DOI : 10.1021/ja016889g

C. Mathe, V. Niviere, C. Houee-levin, and T. A. Mattioli, Fe3+?????2???peroxo species in superoxide reductase from Treponema pallidum. Comparison with Desulfoarculus baarsii, Biophysical Chemistry, vol.119, issue.1, p.38, 2006.
DOI : 10.1016/j.bpc.2005.06.013
URL : https://hal.archives-ouvertes.fr/hal-00088121

G. H. Loew and D. L. Harris, Role of the Heme Active Site and Protein Environment in Structure, Spectra, and Function of the Cytochrome P450s, Chemical Reviews, vol.100, issue.2, p.407, 2000.
DOI : 10.1021/cr980389x

C. Mathe, V. Niviere, and T. A. Mattioli, Is Associated with pH Dependent Spectral Changes, Journal of the American Chemical Society, vol.127, issue.47, p.16436, 2005.
DOI : 10.1021/ja053808y
URL : https://hal.archives-ouvertes.fr/hal-01075779

G. Roelfes, End-On and Side-On Peroxo Derivatives of Non-Heme Iron Complexes with Pentadentate Ligands:?? Models for Putative Intermediates in Biological Iron/Dioxygen Chemistry, Inorganic Chemistry, vol.42, issue.8, p.2639, 2003.
DOI : 10.1021/ic034065p

J. Girerd, F. Banse, and A. J. Simaan, Characterization and Properties of Non-Heme Iron Peroxo Complexes, Struct. Bonding (Berlin), vol.97, p.145, 2000.
DOI : 10.1007/3-540-46592-8_6

T. C. Brunold and E. I. Solomon, Reversible Dioxygen Binding to Hemerythrin. 1. Electronic Structures of Deoxy- and Oxyhemerythrin, Journal of the American Chemical Society, vol.121, issue.36, p.8277, 1999.
DOI : 10.1021/ja990334s

O. and S. Wt, Fe(II) have been deposited in the RCSB Protein Data Bank with pdb codes 2ji2, 2ji3 and 2ji1, respectively. The ESRF is greatly acknowledged for continuous support of 14

M. Field, L. David, H. Jouve, and J. , Mahi for providing us with H 2 18 O 2 . G.K. acknowledges support by an EMBO long-term fellowship, and D.B. acknowledges support by an, ACI Jeune Chercheurs " from the French Ministry of Research. V. N. acknowledges support by the " Toxicologie Nucléaire " program from the CEA