Department of Haematology, University of Cambridge, Cambridge Institute for Medical Research, Cambridge, UK.
Biol Chem. 2010 Aug;391(8):973-82. doi: 10.1515/BC.2010.085.
Members of the serpin (serine protease inhibitor) superfamily fold into a metastable conformation that is crucial for proper function. As a consequence, serpins are susceptible to mutations that cause misfolding and the intracellular accumulation of pathogenic polymers. The mechanism of serpin polymerisation remains to be resolved, however, over the past two decades the 'loop-sheet' hypothesis has gained wide acceptance. In this mechanism the reactive centre loop of one serpin monomer inserts into the beta-sheet A of another (in trans), in a manner similar to what is seen for reactive centre loop-cleaved and latent conformations (in cis). The hypothesis has been refined in response to certain experimental data, but it has proved difficult to assess the various propositions without creating molecular models. Here we evaluate the loop-sheet mechanism by creating models of pentamers of the archetypal serpin alpha(1)-antitrypsin. We conclude that an inescapable consequence of the loop-sheet mechanism is polymer compaction and rigidity, properties that are inconsistent with the 'beads-on-a-string' morphology of polymers obtained from human tissue. The recent crystal structure of a domain-swapped serpin dimer suggests an alternative mechanism that is consistent with known polymer properties, including the requirement of partial unfolding to induce polymer formation in vitro, and polymerisation from a folding intermediate in vivo.
丝氨酸蛋白酶抑制剂（serpin）超家族的成员折叠成一种亚稳态构象，这种构象对于正确的功能至关重要。因此，丝氨酸蛋白酶抑制剂容易受到突变的影响，这些突变会导致错误折叠和致病性聚合物的细胞内积累。然而，丝氨酸蛋白酶抑制剂聚合的机制仍有待解决，尽管在过去的二十年中，“环片假说”已经得到了广泛的认可。在该机制中，一个丝氨酸蛋白酶抑制剂单体的反应中心环插入到另一个（反式）的β-片层 A 中，这种方式类似于反应中心环被切割和潜伏构象（顺式）中所见的方式。该假说已经根据某些实验数据进行了改进，但在不创建分子模型的情况下，评估各种假设一直很困难。在这里，我们通过创建典型丝氨酸蛋白酶抑制剂α1-抗胰蛋白酶五聚体的模型来评估环片机制。我们得出的结论是，环片机制不可避免的结果是聚合物的紧缩和刚性，这些性质与从人体组织中获得的聚合物的“串珠”形态不一致。最近域交换丝氨酸蛋白酶抑制剂二聚体的晶体结构表明了一种替代机制，该机制与已知的聚合物性质一致，包括体外诱导聚合物形成需要部分展开，以及体内从折叠中间产物聚合。
