Primary structure of Cu-Zn superoxide dismutase of Brassica oleracea proves homology with corresponding enzymes of animals, fungi and prokaryotes.

The complete amino-acid sequence of Cu-Zn superoxide dismutase from white cabbage (Brassica oleracea) is reported. The polypeptide chain consists of 151 amino acids and has a molecular mass of 15,604 Da. The primary structure of the reduced and S-carboxymethylated protein was determined by automated solid phase sequence analysis of tryptic fragments and peptides obtained by digestion with Staphylococcus aureus proteinase V8. The protein shows a free amino terminus as was found for all non-mammalian Cu-Zn enzymes so far sequenced. Comparison of the amino-acid sequence from the plant Cu-Zn enzyme with those from nine eukaryotic enzymes reveals a high degree of homology (50-64%) among these enzymes. As already described for all the eukaryotic Cu-Zn superoxide dismutases also the plant enzyme shows a low homology (about 28%) with the bacteriocuprein of Photobacterium leiognathi. However, the amino-acid residues involved in metal binding, the half-cystine residues forming the intermolecular disulfide bridge, one of the arginine and some glycine and proline residues are conserved in all eleven Cu-Zn superoxide dismutases. Although the precise role of the 23 completely conserved residues is not yet completely understood, they appear to almost define the minimum structural requirements for optimizing the superoxide dismutation at the catalytic site, since functional differences between the eleven enzymes are not detectable.