The high resolution crystal structure of a native thermostable serpin reveals the complex mechanism underpinning the stressed to relaxed transition. (1SNG)

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）会折叠形成天然亚稳态，并通过复杂的构象变化来抑制靶蛋白酶。这种构象灵活性带来的一个不利后果是，大多数具有抑制活性的丝氨酸蛋白酶抑制剂均为热敏感型，在高温环境下会形成无活性的聚合物。然而，来自嗜热毁丝霉（Thermobifida fusca）的原核生物丝氨酸蛋白酶抑制剂热调蛋白（thermopin），却能够在高温环境下行使功能。本研究已解析出处于天然抑制构象的热调蛋白1.8埃分辨率的X射线晶体结构。通过与此前解析的1.5埃分辨率切割型热调蛋白晶体结构进行结构比对，我们得以深入阐明丝氨酸蛋白酶抑制剂的复杂构象变化机制。研究推测，快门区域的构象灵活性，以及热调蛋白特有的结构特征——即Aβ折叠片顶部（缺口区域）涉及C端尾的静电相互作用——在天然状态下分别对抑制活性的调控与构象变化的触发具有关键作用。本文就此探讨了该丝氨酸蛋白酶抑制剂如何在行使功能所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。