Walsh M, Stevens F C, Oikawa K, Kay C M
Biochemistry. 1978 Sep 19;17(19):3928-30.
The structural features and Ca2+-binding properties of native and N-chlorosuccinimide-oxidized modulator protein were compared by circular dichroism. In the presence of Ca2+,the far-UV spectra of native and oxidized modulator protein are virtually indistinguishable, indicating that oxidation of surface methionine residues does not alter the overall conformation of the molecule. In the absence of Ca2+, however, the circular dichroism spectra of native and oxidized modulator are different with calculated helical contents of 40% and 26%, respectively. As judged by circular dichroism titration studies, the native modulator contains both high-(Kd = 1.9 X 10(-7) M) and low-affinity (Kd = 4 X 10(-4) M) Ca2+-binding sites, whereas the modified modulator appears to possess only low-affinity sites (Kd = 3.8 X 10(-4) M). The reduced secondary structure in Ca2+-free oxidized modulator protein may account for the absence of high affinity Ca2+ binding sites.
通过圆二色性比较了天然和N-氯代琥珀酰亚胺氧化的调节蛋白的结构特征及Ca2+结合特性。在Ca2+存在的情况下，天然调节蛋白和氧化调节蛋白的远紫外光谱几乎无法区分，这表明表面甲硫氨酸残基的氧化不会改变分子的整体构象。然而，在没有Ca2+的情况下，天然调节蛋白和氧化调节蛋白的圆二色光谱不同，计算得到的螺旋含量分别为40%和26%。通过圆二色性滴定研究判断，天然调节蛋白含有高亲和力（Kd = 1.9×10(-7) M）和低亲和力（Kd = 4×10(-4) M）的Ca2+结合位点，而修饰后的调节蛋白似乎仅具有低亲和力位点（Kd = 3.8×10(-4) M）。无Ca2+的氧化调节蛋白中二级结构的减少可能是高亲和力Ca2+结合位点缺失的原因。
