INVESTIGATION OF A NEW CU,ZN SUPEROXIDE-DISMUTASE MUTANT - THE THR-]ARG 137 DERIVATIVE

The preparation and biophysical characterization of a mutant of supero xide dismutase in which the native Thr 137 has been substituted with a positive Arg residue are reported. Thr 137 forms, together with Arg 1 43, a bottleneck at the entrance to the active-site Cu ion. The geomet ry of the Cu ligands shows only minor change, after the above substitu tion. However, the enzymatic activity of the Arg 137 mutant is smaller than that of the wild type at physiological ionic strength and approa ches that of wild type in the limit of zero ionic strength. The bindin g constant of the anion N3-, which had previously been shown to be a g ood probe of the O2- substrate, is increased about 20-fold in the muta nt with respect to the value found in the wild type. These results are discussed on the bases of the whole charge of the cavity and the poss ible change in the conformation of the active-site channel.