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Superoxide reductases (SORs) are superoxide (O2-)-detoxifying enzymes thatcatalyse the reduction of O2- into hydrogen peroxide. Three different classesof SOR have been reported on the basis of the presence or not of an additionalN-terminal domain. They all share a similar active site, with an unusualnon-heme Fe atom coordinated by four equatorial histidines and one axialcysteine residues. Crucial catalytic reaction intermediates of SOR arepurported to be Fe(3+)-(hydro)peroxo species. Using resonance Ramanspectroscopy, we compared the vibrational properties of the Fe3+ active site oftwo different classes of SOR, from Desulfoarculus baarsii and Treponemapallidum, along with their ferrocyanide and their peroxo complexes. In bothspecies, rapid treatment with H2O2 results in the stabilization of a side-onhigh spin Fe(3+)-(eta(2)-OO) peroxo species. Comparison of these two peroxospecies reveals significant differences in vibrational frequencies and bondstrengths of the Fe-O2 (weaker) and O-O (stronger) bonds for the T. pallidumenzyme. Thus, the two peroxo adducts in these two SORs have differentstabilities which are also seen to be correlated with differences in the Fe-Scoordination strengths as gauged by the Fe-S vibrational frequencies. This wasinterpreted from structural variations in the two active sites, resulting indifferences in the electron donating properties of the trans cysteine ligand.Our results suggest that the structural differences observed in the active siteof different classes of SORs should be a determining factor for the rate ofrelease of the iron-peroxo intermediate during enzymatic turnover.

Fe(3+)-eta(2)-peroxo species in superoxide reductase from Treponema pallidum. Comparison with Desulfoarculus baarsii