Role of the electrostatic loop charged residues in Cu,Zn superoxide dismutase

We have expressed and characterized a mutant of Xenopus laevis Cu,Zn superoxide dismutase in which four highly conserved charged residues belonging to the electrostatic loop have been replaced by neutral side chains: Lys120 --> Leu, Asp130 --> Gln, Glu131 --> Gln, and Lys134 --> Thr. At low ionic strength, the mutant enzyme is one of the fastest superoxide dismutases ever assayed (k = 6.7 x 10(9) M(-1) s(-1), at pH 7 and mu = 0.02 M). Brownian dynamics simulations give rise to identical enzyme-substrate association rates for both wild-type and mutant enzymes, ruling out the possibility that enhancement of the activity is due to pure electrostatic factors. Comparative analysis of the experimental catalytic rate of the quadruple and single mutants reveals the nonadditivity of the mutation effects, indicating that the hyperefficiency of the mutant is due to a decrease of the energy barrier and/or to an alternative pathway for the diffusion of superoxide within the active site channel. At physiological ionic strength the catalytic rate of the mutant at neutral pH is similar to that of the wild-type enzyme as it is to the catalytic rate pH dependence. Moreover, mutation effects are additive. These results show that, at physiological salt conditions, electrostatic loop charged residues do not influence the diffusion pathway of the substrate and, if concomitantly neutralized, are not essential for high catalytic efficiency of the enzyme, pointing out the role of the metal cluster and of the invariant Arg141 in determining the local electrostatic forces facilitating the diffusion of the substrate towards the active site.