Control of the Evolution of Iron Peroxide Intermediate in Superoxide Reductase from Desulfoarculus baarsii. Involvement of Lysine 48 in Protonation

Superoxide reductase is a nonheme iron metalloenzyme that detoxifies superoxide anion radicals O₂^•– in some microorganisms. Its catalytic mechanism was previously proposed to involve a single ferric iron (hydro)­peroxo intermediate, which is protonated to form the reaction product H₂O₂. Here, we show by pulse radiolysis that the mutation of the well-conserved lysine 48 into isoleucine in the SOR from Desulfoarculus baarsii dramatically affects its reaction with O₂^•–. Although the first reaction intermediate and its decay are not affected by the mutation, H₂O₂ is no longer the reaction product. In addition, in contrast to the wild-type SOR, the lysine mutant catalyzes a two-electron oxidation of an olefin into epoxide in the presence of H₂O₂, suggesting the formation of iron-oxo intermediate species in this mutant. In agreement with the recent X-ray structures of the peroxide intermediates trapped in a SOR crystal, these data support the involvement of lysine 48 in the specific protonation of the proximal oxygen of the peroxide intermediate to generate H₂O₂, thus avoiding formation of iron-oxo species, as is observed in cytochrome P450. In addition, we proposed that the first reaction intermediate observed by pulse radiolysis is a ferrous-iron superoxo species, in agreement with TD-DFT calculations of the absorption spectrum of this intermediate. A new reaction scheme for the catalytical mechanism of SOR with O₂^•– is presented in which ferrous iron-superoxo and ferric hydroperoxide species are reaction intermediates, and the lysine 48 plays a key role in the control of the evolution of iron peroxide intermediate to form H₂O<sub>2