去辅基天青蛋白两种天然构象共存的热力学证据 ———差热温度扫描和圆二色的研究

天然态蛋白质能否在溶液中存在多种构象是一个有争议的问题 . 在前报道中已经鉴定出绿脓杆菌去辅基天青蛋白突变体 M121L 可以多种构象共存 . 用差热扫描量热和圆二色性的方法研究了野生型去辅基天青蛋白的热变性 . 结果表明在 pH 从 4.0 到 9.0 的范围内存在着两个摩尔热容最大值 . 较低温度下的去折叠反应在所研究 pH 范围内均部分可逆，而较高温度下的去折叠反应均不可逆 . 蛋白质去折叠的热容变化双峰用可相互转化的两种构象共存模型进行拟合 . 较低温度下能够去折叠的构象在 pH 4.0 时占 64% ，在 pH 9.0 时占 55%. 监测热变性过程中圆二色谱在 219 nm 处的信号变化也可以观测到两个独立的去折叠变化 . 信号变化的比值与在相同条件下差热扫描法测得的两种构象摩尔比一致 . 上述结果进一步支持了前文提出的去辅基天青蛋白在溶液中至少存在着两种构象的设想 .

The Thermodynamic Evidence of Two Native Conformations Coexisting in Solution For Apoazuirn: a DSC and CD Study

Whether protein could adopt multiple conformations coexisting in solution is disputable. In a previous report, the conformation heterogeneity of apoazurin mutant M121L had been identified. The thermal unfolding of wild type apoazurin from Pseudomonas aeruginosa is re-investigated with differential scanning calorimetry (DSC) and circular dichroism (CD) methods. The results show that unfolding in the pH range from 4 to 9 is associated with two heat capacity maxima. The low temperature transitions are reversible at all pH conditions used, while the high temperature transitions are irreversible. The two unfolding transitions were analyzed by the two-interchangeable-conformation model with the fraction for the first transition (N1) from 64% at pH 4.0 to 55% at pH 9.0. Temperature induced unfolding monitored at 219 nm shows also two separate transitions. The ratio of the signal changes is consistent with the fractions obtained from the corresponding DSC measurements. These results provide further support for the hypothesis that at least two conformations of apoazurin coexists in solution.