Evolutionary relationships in superoxide dismutase

Two distinct types of superoxide dismutase have been isolated from aerobic organisms. A blue-green Cu-Zn protein with a mol. wt of 32 000 and comprising two identical subunits has been obtained from a wide range of eukaryotes (e.g. [3,4]) while a pinkish-purple Mncontaining enzyme, also composed of identical subunits, has been isolated from eukaryotic mitochondria [S] and from prokaryotes [6,7]. In addition, an Fecontaining superoxide dismutase, similar to its Mncontaining counterpart, has been obtained from,!?. coli [8]. The prokaryotic Mn and Fe-containing enzymes are dimers with mol. wts of 40 000 and the mitochondrial Mn-enzyme, with a mol. wt of 80 000, is a tetramer. The subunits of the Mn and Fe-containing dismutases are of the same chain length, comprising approx. 185 to 190 residues [6-81. In addition they possess similar amino acid compositions and N-terminal sequences [9]. The Cu-Zn enzyme from bovine erythrocytes on the other band possesses a smaller subunit (of 150 residues [ 1 O]), with a different amino acid composition [ 111 and N-terminal sequence [9]. These results led Steinman and Hill [9] to suggest that the two classes of dismutase are probably composed of entirely different proteins of independent evolutionary origin. Insofar as the sequence comparisons of the respective protein chains were confined to the N-terminal segments of 25 to 30 residues, the possibility