Yoshino H, Wakita M, Izumi Y
Department of Chemistry, Sapporo Medical College, Japan.
J Biol Chem. 1993 Jun 5;268(16):12123-8.
Solution x-ray scattering using synchrotron radiation as an x-ray source has been used to study the solution structure of calmodulin complexed with substance P, a undecapeptide neurotransmitter. The x-ray data indicate that the complex has a compact globular structure, the formation of which is dependent upon the binding of Ca2+ to calmodulin. In the Ca(2+)-saturated condition, the radius of gyration of complexed calmodulin was 4.2 A smaller than that of uncomplexed calmodulin. The Ca(2+)-dependent change in radius of gyration of calmodulin with substance P is complete by the third and fourth Ca2+ binding. The behavior of the Guinier plot at small-to-moderate angles for uncomplexed calmodulin corresponds to a dumbbell shape. The Guinier plot for complexed calmodulin, however, corresponds to a non-dumbbell shape. The susceptibility of calmodulin to proteolytic attack with trypsin was used to examine the nature of the calmodulin complexed with substance P. In the presence of equimolar substance P, the first and second Ca2+ binding to calmodulin was enough to form a trypsin-resistant complex. These biochemical and x-ray data suggest that the binding of substance P to calmodulin is completed when the C-terminal half of calmodulin is occupied by Ca2+, while a significant structural change of calmodulin in the complex is still induced by successive Ca2+ occupancy on the N-terminal half of this molecule.
使用同步辐射作为X射线源的溶液X射线散射已被用于研究与P物质（一种十一肽神经递质）复合的钙调蛋白的溶液结构。X射线数据表明该复合物具有紧密的球状结构，其形成取决于Ca2+与钙调蛋白的结合。在Ca(2+)饱和条件下，复合钙调蛋白的回转半径比未复合钙调蛋白的回转半径小4.2 Å。钙调蛋白与P物质结合时，回转半径的Ca(2+)依赖性变化在第三个和第四个Ca2+结合时完成。未复合钙调蛋白在小到中等角度下的吉尼尔图行为对应于哑铃形状。然而，复合钙调蛋白的吉尼尔图对应于非哑铃形状。利用钙调蛋白对胰蛋白酶蛋白水解攻击的敏感性来研究与P物质复合的钙调蛋白的性质。在等摩尔P物质存在下，钙调蛋白与第一个和第二个Ca2+结合足以形成抗胰蛋白酶复合物。这些生化和X射线数据表明，当钙调蛋白的C端一半被Ca2+占据时，P物质与钙调蛋白的结合完成，而该分子N端一半上连续的Ca2+占据仍会诱导复合物中钙调蛋白发生显著的结构变化。
