Primary structure of a cationic Cu,Zn Superoxide dismutase

The complete amino acid sequence of Cu,Zn superoxide dismutase from sheep erythrocytes has been determined. The sequence is very similar to that of the bovine enzyme, having the same number of residues (151) and only two substitutions in the 'hypervariable' region (residues 17-30). The 5 overall substitutions confer a positive charge on the sheep enzyme at neutral pH (pI approximately equal to 8). This charge is localized outside the active site region. The catalytic efficiency of the sheep enzyme is 15% less than that of the cow enzyme, confirming the hypothesis that the enzyme activity is related to the concentration of positive surface charge near the active site channel.