The high resolution crystal structure of a native thermostable serpin reveals the complex mechanism underpinning the stressed to relaxed transition.

Serpins fold into a native metastable state and utilize a complex conformational change to inhibit target proteases. An undesirable result of this conformational flexibility is that most inhibitory serpins are heat sensitive, forming inactive polymers at elevated temperatures. However, the prokaryote serpin, thermopin, from Thermobifida fusca is able to function in a heated environment. We have determined the 1.8 A x-ray crystal structure of thermopin in the native, inhibitory conformation. A structural comparison with the previously determined 1.5 A structure of cleaved thermopin provides detailed insight into the complex mechanism of conformational change in serpins. Flexibility in the shutter region and electrostatic interactions at the top of the A beta-sheet (the breach) involving the C-terminal tail, a unique structural feature of thermopin, are postulated to be important for controlling inhibitory activity and triggering conformational change, respectively, in the native state. Here we have discussed the structural basis of how this serpin reconciles the thermodynamic instability necessary for function with the stability required to withstand elevated temperatures.

丝氨酸蛋白酶抑制剂（Serpins）可折叠为天然亚稳态，并通过复杂的构象变化抑制靶蛋白酶。这种构象灵活性带来的一项不利后果是，多数具有抑制活性的Serpins对热敏感，在高温环境下会形成无活性的聚合物。然而，来自解热纤菌（Thermobifida fusca）的原核丝氨酸蛋白酶抑制剂热平素（thermopin）可在高温环境下行使功能。本研究解析了处于天然抑制构象的热平素的1.8埃X射线晶体结构。本研究通过与此前解析的裂解型热平素的1.5埃晶体结构进行结构比对，为Serpins构象变化的复杂机制提供了详尽的认知。热平素特有的C端尾所介导的Aβ折叠片顶端（缺口区）的静电相互作用，以及快门区的构象灵活性，被认为分别在天然状态下调控抑制活性和触发构象变化中发挥关键作用。本研究还阐明了该丝氨酸蛋白酶抑制剂如何在行使功能所必需的热力学不稳定性与耐受高温所需的结构稳定性之间实现平衡的结构基础。