1H-NMR studies of calmodulin: The character of the calcium binding sites.
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The effects of various divalent cations on the Ca<SUP>2+</SUP>-binding sites of calmodulin were observed by 400 MHz <SUP>1</SUP>H-NMR. The first and second Ca ions bound to sites III and IV (stage I), while the third and fourth bound to sites I and II (stage II). Zn<SUP>2+</SUP>, Hg<SUP>2+</SUP> and Mn<SUP>2+</SUP> bound to the first and third Ca<SUP>2+</SUP>-binding sites, but not to the second and fourth. Zn<SUP>2+</SUP>, Hg<SUP>2+</SUP> or Mn<SUP>2+</SUP> coula bind to the first Ca<SUP>2+</SUP>-binding site by themselves and could bind to the third site only after the conformational change which occurs when two Ca<SUP>2+</SUP> ions bind to first and second sites. Although Mg<SUP>2+</SUP> did not bind to the first, second or fourth Ca<SUP>2+</SUP>-binding sites, it did bind to the third site. These results suggest that the order of Ca<SUP>2+</SUP>-binding and the order of affinity of the binding sites are not parallel; the first and third Ca<SUP>2+</SUP>-binding sites have high Ca<SUP>2+</SUP>-affinity, with the third being highest, whereas the second and fourth sites are of lower affinity. Also, we suggest in this study that the first and second sites are exposed on the surface of the protein, while the third and fourth ones are buried in the interior; the latter are exposed by the conformational change accompanying the binding of calcium to the first and second sites. Furthermore, the form of the interface by which calmodulin binds to target enzyme was altered slowly and continuously by the calcium-induced conformational change. The target enzyme was chosen and bound selectively to calmodulin among various enzymes by each interface form.
- 公益社団法人 日本薬理学会の論文
公益社団法人 日本薬理学会 | 論文
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