Saturation behavior of the manganese-containing superoxide dismutase from Paracoccus denitrificans.

A pulse radiolysis study of the Mn-superoxide dismutase from Paracoccus denitrificans has shown that, at concentration of 0(2)-. below 0.8 x 10(-4)M, the catalyzed dismutation of 0(2)-. is a first order reaction with regard to 0(2)-.. At concentration of 0(2)-. above 0.8 x 10(-4)M, the Mn-superoxide dismutase is shown to catalyze superoxide dismutation with a mechanism which exhibits saturation kinetics. This behavior was previously found in the bovine Cu/Zn-superoxide dismutase and in the Fe-superoxide dismutase from Photobacterium leiognathi. Two parameters of catalysis were determined from pH 5 to pH 11: the rate constant k was pH independent at basic pH. The variation of Km with pH indicated that the enzyme possessed an ionizable group with pK 9.8 which participates to the substrate binding.