title: The rat extracellular superoxide dismutase dimer is converted to a tetramer by the exchange of a single amino acid.
abstract: Extracellular superoxide dismutase (EC-SOD) is a secreted Cu and Zn-containing glycoprotein. While EC-SOD from most mammals is tetrameric and has a high affinity for heparin and heparan sulfate, rat EC-SOD has a low affinity for heparin, does not bind to heparan sulfate in vivo, and is apparently dimeric. To examine the molecular basis of the deviant physical properties of rat EC-SOD, the cDNAs of the rat and mouse EC-SODs were isolated and the deduced amino acid sequences were compared with that of human EC-SOD. Comparison of the sequences offered no obvious explanation of the differences. Analysis of a series of chimeric and point mutated EC-SODs showed that the N-terminal region contributes to the oligomeric state of the EC-SODs, and that a single amino acid, a valine (human amino acid position 24), is essential for the tetramerization. This residue is replaced by an aspartate in the rat. Rat EC-SOD carrying an Asp --> Val mutation is tetrameric and has a high heparin affinity, while mouse EC-SOD with a Val --> Asp mutation is dimeric and has lost its high heparin affinity. Thus, the rat EC-SOD dimer is converted to a tetramer by the exchange of a single amino acid. Furthermore, the cooperative action of four heparin-binding domains is necessary for high heparin affinity. These results also suggest that tetrameric EC-SODs are not symmetrical tetrahedrons, but composed of two interacting dimers, further supporting an evolutionary relationship with the dimeric cytosolic Cu and Zn-containing SODs.
authors.affiliation: Department of Clinical Chemistry, Umeå University Hospital, Sweden.
authors.forename: L M
authors.initials: LM
authors.lastname: Carlsson
authors.forename: S L
authors.initials: SL
authors.lastname: Marklund
authors.forename: T
authors.initials: T
authors.lastname: Edlund
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001224:Aspartic Acid; D016466:CHO Cells; D002846:Chromatography, Affinity; D002850:Chromatography, Gel; D003001:Cloning, Molecular; D006224:Cricetinae; D003600:Cytosol / Q000201:enzymology; D005110:Extracellular Space / Q000201:enzymology; D006801:Humans; D008168:Lung / Q000201:enzymology; D046911:Macromolecular Substances; D051379:Mice; D008969:Molecular Sequence Data; D017354:Point Mutation; D051381:Rats; D017207:Rats, Sprague-Dawley; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000302:isolation & purification; D017386:Sequence Homology, Amino Acid; D013482:Superoxide Dismutase / Q000096:biosynthesis* / Q000737:chemistry* / Q000302:isolation & purification; D014162:Transfection; D014633:Valine
chemical_list: D046911:Macromolecular Substances; D011994:Recombinant Proteins; D001224:Aspartic Acid; D013482:Superoxide Dismutase; D014633:Valine