Rao S T, Satyshur K A, Greaser M L, Sundaralingam M
Department of Biochemistry, College of Agricultural & Life Sciences, University of Wisconsin-Madison, 53706, USA.
Acta Crystallogr D Biol Crystallogr. 1996 Sep 1;52(Pt 5):916-22. doi: 10.1107/S0907444996006166.
The crystal structures of three metal complexes of troponin C (TnC) have been determined and refined where the two occupied structural Ca(2+) sites in the C domain have been substituted by Mn(2+), Cd(2+) and Tb(3+). The X-ray intensity data were collected to 2.1, 1.8 and 1.8 A resolution, respectively, on the three metal complexes, which are isomorphous with Ca-TnC. The three complexes have r.m.s. deviations of 0.27, 0.25 and 0.35 A, respectively, for all protein atoms, from Ca-TnC. Irrespective of the charge on the metal (+2 or +3), the occupied sites 3 and 4 exhibit a distorted pentagonal bipyramidal coordination, like Ca-TnC, with seven ligands, six from the 12-residue binding loop and the seventh from a water molecule. Mn(2+) at site 4 seems to display a longer distance to one of the carboxyl bidentate ligands representing an intermediate coordination simulating the six-coordinate Mg(2+). The carboxyl O atoms of the bidentate Glu12 are displaced on the side of the equatorial plane passing through the remaining three ligands with one O atom closer to the plane (Delta of 0.11 to 0.76 A) than the other (Delta of 0.93 to 1.38 A). The two axial ligands are an aspartic carboxyl O atom and a water molecule. The metal is displaced (0.18 to 0.56 A) towards the water facing the water channel.
已测定并精修了肌钙蛋白C（TnC）的三种金属配合物的晶体结构，其中C结构域中两个被占据的结构Ca(2+)位点已被Mn(2+)、Cd(2+)和Tb(3+)取代。分别在三种与Ca-TnC同晶型的金属配合物上收集了X射线强度数据，分辨率分别为2.1、1.8和1.8 Å。对于所有蛋白质原子，这三种配合物与Ca-TnC的均方根偏差分别为0.27、0.25和0.35 Å。无论金属的电荷是+2还是+3，被占据的位点3和4都呈现出扭曲的五角双锥配位，类似于Ca-TnC，有七个配体，六个来自12个残基的结合环，第七个来自水分子。位点4的Mn(2+)似乎与一个羧基双齿配体的距离更长，代表了模拟六配位Mg(2+)的中间配位。双齿Glu12的羧基O原子在穿过其余三个配体的赤道平面一侧发生位移，其中一个O原子比另一个更靠近平面（差值为0.11至0.76 Å）（另一个差值为0.93至1.38 Å）。两个轴向配体是一个天冬氨酸羧基O原子和一个水分子。金属朝着面向水通道的水位移（0.18至0.56 Å）。
