ENERGETICS OF TARGET PEPTIDE RECOGNITION BY CALMODULIN: CALORIMETRIC STUDY

钙调素是一种小分子蛋白质，参与调节广泛的 各种各样的细胞内过程。 Ca2+的协同结合 与钙调素的两个Ca 2+结合结构域诱导构象变化， 这些变化允许钙调蛋白激活特定的靶酶。 钙调蛋白与对应于钙调蛋白的肽的结合， 兔平滑肌肌球蛋白轻链钙调素结合位点 用等温滴定法研究smMLCKp 微量热法 结合能对 温度、pH、Ca2+浓度和NaCl浓度 测定 发现钙调素与smMLCKp的结合 焓（H）、热容（Cp） 和熵（S）接近室温，表明它是一个 在熵上是不利的。 从 这些结果可以得出结论，疏水效应- 有利于去除非极性蛋白质基团的熵效应 不是钙调素-smMLCKp的主要驱动力 识别. 虽然大量的非极性侧链是 结合后埋下的这些稳定复合物主要通过形成 紧密堆积的货车范德华相互作用。 结合 研究了在酸性pH值下， 静电相互作用结合。 据发现， 酸性pH值导致吉布斯自由能大幅下降， 结合，但没有变化的焓，表明静电 相互作用仅对结合能学有熵贡献。 的可及表面积和原子堆积密度， 钙调素-smMLCKp晶体 结构进行了分析，并讨论了有关的结果 实验数据

Calmodulin is a small protein involved in the regulation of a wide variety of intracellular processes. The cooperative binding of Ca2+ to calmodulin's two Ca2+ binding domains induces conformational changes which allow calmodulin to activate specific target enzymes. The association of calmodulin with a peptide corresponding to the calmodulin binding site of rabbit smooth muscle myosin light chain kinase (smMLCKp) was studied using isothermal titration microcalorimetry. The dependence of the binding energetics on temperature, pH, Ca2+ concentration, and NaCl concentration were determined. It is found that the binding of calmodulin to smMLCKp proceeds with negative changes in enthalpy ( H), heat capacity ( Cp), and entropy ( S) near room temperature, indicating that it is an enthalpically driven process that is entropically unfavorable. From these results it is concluded that the hydrophobic effect - an entropic effect which favors the removal of nonpolar protein groups from water - is not a major driving force in calmodulin-smMLCKp recognition. Although a large number of nonpolar side chains are buried upon binding these stabilize the complex primarily by forming tightly packed van der Waals interactions with one another. Binding at acidic pH was studied in order to assess the contribution of electrostatic interactions to binding. It is found that moving to acidic pH results in a large decrease in the Gibbs free energy of binding but no change in the enthalpy, indicating that electrostatic interactions contribute only entropically to the binding energetics. The accessible surface area and atomic packing density of the calmodulin-smMLCKp crystal structure are analyzed, and the results discussed in relation to the experimental data.