Abstract : The superoxide reductase (SOR) is an enzyme of detoxification present only at certain types of micro-organisms. It eliminates the radical superoxide by reducing it to give hydrogen peroxide without molecular oxygen formation. The active site of the SOR consists of a mononuclear chelated ferro-iron center by 4 histidines in equatorial positions and a cysteine in axial position. The presence and the positioning of this cysteine within the active site are completely atypical and could be at the origin of this particular reactivity with superoxide. Various structures of SOR showed the existence of hydrogen bonds between the sulphur of this cysteine and the peptide NH of connections of certain amino acids of which isoleucine 118 on which we centered our work. Four mutants of isoleucine 118 were built and purified. The characterizations by spectroscopies UV-visible, FTIR, Raman Resonance, by the determination of the pKa and the potential redox of the iron of the active site showed that the changes on this position induce a reinforcement of the connection S-Fe. This could be associated a weakening of the hydrogen bond between the peptide NH of the mutants of isoleucine 118 and the sulphur of cysteine. The studies of fast kinetics by pulse radiolysis made it possible to show that the changes facilitate the protonation of the reactional intermediaries of type Fe3+- peroxo and Fe3+- hydroxo. These results are in agreement with the increase in density electronic around iron of the active site, which induces a more important alkalinity of these reactional intermediaries. Thus, we could show that the reinforcement of the connection S-Fe has as a consequence the acceleration of the reaction of the SOR with superoxide. The axial ligand cysteine has thus as a role to control the electronic density around iron of the active site and thus to influence the reduction of superoxyde.