Evolutionary Aspects of Superoxide Dismutase: The Copper/Zinc Enzyme

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Abstract
Copper/zinc superoxide dismutase is typically an enzyme of eukaryotes. The presence of the enzyme in the ponyfish symbiont Photobocterium leiognothi and some free living bacteria does not have an immediate explanation. Amino acid sequence alignment of 19 Cu/Zn superoxide disrnutases shows 21 invariant residues in key positions related to maintenance of the β-barrel fold, the active site structure including the electrostatic channel loop, and dimer contacts. Nineteen other residues are invariant in 18 of the 19 sequences. Thirteen of these nearly invariant residues show substitutions in Photobocterium Cu/Zn superoxide dismutase. Copper/zinc superoxide disrnutase from the trematode Schisiosoma mansoni shows an N-terminal sub-domain with a hydrophobic leader peptide. as in human extracellular superoxide dismutase which is a Cu/Zn enzyme. The latter also has a C-terminal sub-domain with preponderance of hydrophilic and positively charged residues. The amino acid sequence of this superoxide dismutase between the N-terminal and C-terminal regions shares many features of cytosolic Cu/Zn superoxide dismutase. including 20 of the 21 invariant residues found in 19 Cu/Zn enzymes, suggesting a similar type of β-barrel fold and active site structure for the extracellular enzyme.