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Superoxide reductase (SOR) is a small metalloenzyme that catalyzes reductionof O(2)(*)(-) to H(2)O(2) and thus provides an antioxidant mechanism againstsuperoxide radicals. Its active site contains an unusual mononuclear ferrouscenter, which is very efficient during electron transfer to O(2)(*)(-)[Lombard, M., Fontecave, M., Touati, D., and Nivi{\`e}re, V. (2000) J. Biol.Chem. 275, 115-121]. The reaction of the enzyme from Desulfoarculus baarsiiwith superoxide was studied by pulse radiolysis methods. The first step is anextremely fast bimolecular reaction of superoxide reductase with superoxide,with a rate constant of (1.1 +/- 0.3) x 10(9) M(-1) s(-1). A first intermediateis formed which is converted to a second one at a much slower rate constant of500 +/- 50 s(-1). Decay of the second intermediate occurs with a rate constantof 25 +/- 5 s(-1). These intermediates are suggested to be iron-superoxide andiron-peroxide species. Furthermore, the role of glutamate 47 and lysine 48,which are the closest charged residues to the vacant sixth iron coordinationsite, has been investigated by site-directed mutagenesis. Mutation of glutamate47 into alanine has no effect on the rates of the reaction. On the contrary,mutation of lysine 48 into an isoleucine led to a 20-30-fold decrease of therate constant of the bimolecular reaction, suggesting that lysine 48 plays animportant role during guiding and binding of superoxide to the iron center II.In addition, we report that expression of the lysine 48 sor mutant gene hardlyrestored to a superoxide dismutase-deficient Escherichia coli mutant theability to grow under aerobic conditions.

Superoxide reductase from Desulfoarculus baarsii: reaction mechanism and role of glutamate 47 and lysine 48 in catalysis