Amino acid sequence of copper-zinc superoxide dismutase from horse liver

The complete amino acid sequence of copper-zinc superoxide dismutase from horse liver is reported. The molecule consists of 153 amino acids and has a Mr = 16,000. The primary structure was determined by automated and manual sequence analysis on fragments produced by cleavage of the S-carboxymethylated protein with cyanogen bromide and on peptides obtained by digestion with trypsin, thermolysin, Staphylococcus aureus protease, or subtilisin. The protein is devoid of tryptophan and tyrosine and displays an acetylated NH2 terminus. Comparison of its primary structure with the known sequences of copper-zinc superoxide dismutases from bovine and human erythrocytes and from yeast reveals a high degree of sequence homology among the four enzymes. This is especially borne out in the regions containing the amino acid residues involved in the metal binding and the half-cystine residues forming the intramolecular disulfide bridge. The striking conservation of the preponderant glycine residues known to be important for the pronounced protein folding in bovine erythrocyte superoxide dismutase suggests similar three-dimensional structures for human erythrocyte, horse liver, and yeast copper-zinc superoxide dismutases.