Modulation of the catalytic rate of Cu,Zn superoxide dismutase in single and double mutants of conserved positively and negatively charged residues.

The catalytic rate of four single and three double mutants of Xenopus laevis Cu,Zn superoxide dismutase B, neutralized at Lys120, Asp130, Glu131, and Lys134, has been determined by pulse radiolysis as a function of ionic strength. Neutralization of Glu131 increases the catalytic rate by 80% at low ionic strength, but the effect is reduced to 50% at physiological ionic strength. The rate is unperturbed upon neutralization of Asp130, while neutralization of either of the two lysines drastically decreases the enzyme activity. The Lys120Leu-Lys134Thr and Lys134Thr-Asp130Gln double mutations have an additive and a compensative effect, respectively, on the activity values, while neutralization of the Glu131-Lys134 pair, which also has a compensative effect, gives rise to a faster enzyme at any ionic strength value. The effects observed in the single Asp130Gln and Lys120Leu mutants differ from those reported on human or bovine enzymes [Getzoff et al. (1992) Nature (London) 358, 347-351; Sines et al. (1990) Biochemistry 29, 9403-9412], indicating that some residues occupying the same position in the linear sequence of different Cu,Zn superoxide dismutases have a different functional weight. Our results also suggest that the strategy of multiple charge mutation may be a promising approach in order to increase the catalytic rate of Cu,Zn SODs independently of ionic strength.