Structural and spectroscopic comparison of manganese-containing superoxide dismutases

Predicted secondary structures and optical properties of four manganese-containing superoxide dismutases isolated from Saccharomyces cerevisiae, Bacillus stearothermophilus, Escherichia coli and human liver are compared. The structural predictions are further compared with the known crystal structure of the manganese-containing superoxide dismutase from Thermus thermophilus HB8. The secondary structures of the four dismutases are predicted by the methods of Chou and Fasman (Adv. Enzymol. 47 (1978) 45-148), Garnier et al. (J. Mol. Biol. 120 (1978) 97-120) and Lim (J. Mol. Biol. 88 (1974) 873-894). The three models show satisfactory agreement and predict that the enzymes have a mixed alpha-helix and beta-sheet structure, and that they have homologous structures. The former conclusion is also reached from an analysis of the hydrophobic character of the amino-acid sequences of the four proteins according to Kyte and Doolittle (J. Mol. Biol. 157 (1982) 105-132). The calculation of the secondary structure based on the 185-260 nm circular dichroism spectrum of manganese-containing superoxide dismutase from S. cerevisiae reveals that the enzyme consists of 61% alpha-helix, 13% beta-sheet, 11% turn and 8% random coil conformations, which is in good accordance with the prediction based on the amino-acid sequences. Comparison of the 400-700 nm circular dichroism spectra of manganese-containing superoxide dismutase from S. cerevisiae, E. coli and T. thermophilus demonstrates that manganese atoms have homologous coordination in the three enzymes. This investigation based on primary structures and spectral properties indicates that the four dismutases have the same overall structure. Since the structural predictions are in good agreement with the structure found for the manganese-containing superoxide dismutase from T. thermophilus HB8, it can be concluded that this structure is representative for the four enzymes and probably for manganese-containing superoxide dismutases in general.