Superoxide Reduction Mechanism of Archaeoglobus fulgidus One-Iron Superoxide Reductase

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Abstract
Superoxide reductases (SORs), iron-centered enzymes responsible for reducing superoxide (O2-) to hydrogen peroxide, are found in many anaerobic and microaerophilic prokaryotes. The rapid reaction with an exogenous electron donor renders the reductase activity catalytic. Here, we demonstrate using pulse radiolysis that the initial reaction between O2- and Archaeoglobus fulgidus neelaredoxin, a one-iron SOR, leads to a short-lived transient that immediately disappears to yield a solvent-bound ferric species in acid−base equilibrium. Through comparison of wild-type neelaredoxin with mutants lacking the ferric ion coordinating glutamate, we demonstrate that the remaining step is related to the final coordination of this ligand to the oxidized metal center and kinetically characterize it for the first time, by pulse radiolysis and stopped-flow kinetics. The way exogenous phosphate perturbs the kinetics of superoxide reduction by neelaredoxin and mutant proteins was also investigated.

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