In this paper, we investigate the first step of the copper-zinc superoxide dismutase enzymatic cycle, involving the binding of a superoxide anion, the transfer of one electron toward the copper, and the simultaneous detachment of His63. By means of combining the perturbed matrix method (PMM) [Chem. Phys. Lett. 2001, 365, 450-456] with basic statistical mechanical relations, presented in the accompanying paper, we describe the coupling between these chemical events and the atomic motions of the complex environment of the reaction center. Results clearly show that the protein-solvent environment fluctuations are essential to understand the reaction mechanism which is based on the concerted rupture of the copper-histidine coordination bond and the copper-superoxide bond in the active site.