title: Selective metal binding by Vanabin2 from the vanadium-rich ascidian, Ascidia sydneiensis samea.
abstract: Vanadium-binding proteins, or Vanabins, have recently been isolated from the vanadium-rich ascidian, Ascidia sydneiensis samea. Recent reports indicate that Vanabin2 binds twenty V(IV) ions at pH 7.5, and that it has a novel bow-shaped conformation. However, the role of Vanabin2 in vanadium accumulation by the ascidian has not yet been determined. In the present study, the effects of acidic pH on selective metal binding to Vanabin2 and on the secondary structure of Vanabin2 were examined. Vanabin2 selectively bound to V(IV), Fe(III), and Cu(II) ions under acidic conditions. In contrast, Co(II), Ni(II), and Zn(II) ions were bound at pH 6.5 but not at pH 4.5. Changes in pH had no detectable effect on the secondary structure of Vanabin2 under acidic conditions, as determined by circular dichroism spectroscopy, and little variation in the dissociation constant for V(IV) ions was observed in the pH range 4.5-7.5, suggesting that the binding state of the ligands is not affected by acidification. Taken together, these results suggest that the reason for metal ion dissociation upon acidification is attributable not to a change in secondary structure but, rather, that it is caused by protonation of the amino acid ligands that complex with V(IV) ions.
authors.affiliation: Molecular Physiology Laboratory, Department of Biological Science, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan.
authors.forename: Norifumi
authors.initials: N
authors.lastname: Kawakami
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Ueki
authors.forename: Koichi
authors.initials: K
authors.lastname: Matsuo
authors.forename: Kunihiko
authors.initials: K
authors.lastname: Gekko
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Michibata
mesh_terms: D000818:Animals; D027682:Cation Transport Proteins / Q000737:chemistry*; D002845:Chromatography; D003001:Cloning, Molecular; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008024:Ligands; D008670:Metals / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D013056:Spectrophotometry, Ultraviolet; D014561:Urochordata / Q000378:metabolism*; D014639:Vanadium / Q000737:chemistry*
chemical_list: D027682:Cation Transport Proteins; D007477:Ions; D008024:Ligands; D008670:Metals; D014639:Vanadium