Identification of the residues responsible for the alkaline inhibition of the activity of Cu,Zn superoxide dismutase: a study of native and chemically modified enzymes.

The pH dependence of the activity of Cu,Zn superoxide dismutases from bovine erythrocytes and shark liver was studied by pulse radiolysis in both the native enzymes and those chemically modified at lysine side chains. The study was aimed at identifying the residues responsible for the activity decrease at pH > 9, observed in all native Cu,Zn superoxide dismutases, and is based on the Lys-->Arg substitution present in the shark protein at position 134, which has been established to be critical for the catalytic efficiency of the enzyme. Both native enzymes display a pH dependence that can be deconvoluted by three deprotonation equilibria, at pH 9-9.5 (pK1), at pH 10.2 (pK2), and at pH 11.5 (pK3). pK1 is lacking in both the modified enzymes and thus can be assigned to activity-linked lysine residues. pK2 is clearly dominated by Arg134 in the modified shark enzyme and can be assigned to surface arginine residues. pK3 is shared by all four enzyme forms and is likely to be due to the invariant Arg141.