Kawakami Norifumi, Ueki Tatsuya, Matsuo Koichi, Gekko Kunihiko, Michibata Hitoshi
Molecular Physiology Laboratory, Department of Biological Science, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan.
Biochim Biophys Acta. 2006 Jul;1760(7):1096-101. doi: 10.1016/j.bbagen.2006.03.013. Epub 2006 Apr 4.
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.
钒结合蛋白，即钒蛋白，最近已从富含钒的海鞘——萨氏海鞘（Ascidia sydneiensis samea）中分离出来。最近的报告表明，钒蛋白2在pH 7.5时能结合20个四价钒离子，并且具有一种新颖的弓形构象。然而，钒蛋白2在海鞘积累钒的过程中所起的作用尚未确定。在本研究中，研究了酸性pH对钒蛋白2选择性结合金属以及对其二级结构的影响。在酸性条件下，钒蛋白2能选择性地结合四价钒、三价铁和二价铜离子。相比之下，二价钴、二价镍和二价锌离子在pH 6.5时能结合，但在pH 4.5时不能结合。通过圆二色光谱法测定，在酸性条件下，pH的变化对钒蛋白2的二级结构没有可检测到的影响，并且在4.5 - 7.5的pH范围内，四价钒离子的解离常数几乎没有变化，这表明配体的结合状态不受酸化影响。综上所述，这些结果表明酸化时金属离子解离的原因并非二级结构的改变，而是与四价钒离子络合的氨基酸配体发生了质子化。
