Membrane-bound styrene oxide isomerase (SOI) catalyses the Meinwald rearrangement—a Lewis-acid-catalysed isomerization of an epoxide to a carbonyl compound—and has been used in single and cascade reactions. However, the structural information that explains its reaction mechanism has remained elusive. Here we determine cryo-electron microscopy (cryo-EM) structures of SOI bound to a single-domain antibody with and without the competitive inhibitor benzylamine, and elucidate the catalytic mechanism using electron paramagnetic resonance spectroscopy, functional assays, biophysical methods and docking experiments. We find ferric haem b bound at the subunit interface of the trimeric enzyme through H58, where Fe(III) acts as the Lewis acid by binding to the epoxide oxygen. Y103 and N64 and a hydrophobic pocket binding the oxygen of the epoxide and the aryl group, respectively, position substrates in a manner that explains the high regio-selectivity and stereo-specificity of SOI. Our findings can support extending the range of epoxide substrates and be used to potentially repurpose SOI for the catalysis of new-to-nature Fe-based chemical reactions.

膜结合苯乙烯氧化物异构酶 (SOI) 催化 Meinwald 重排（路易斯酸催化环氧化物异构化为羰基化合物），并已用于单一和级联反应。然而，解释其反应机制的结构信息仍然难以捉摸。在这里，我们确定了在有或没有竞争性抑制剂苯甲胺的情况下与单域抗体结合的 SOI 的冷冻电子显微镜 (cryo-EM) 结构，并利用电子顺磁共振波谱、功能测定、生物物理方法和对接实验阐明了催化机制。我们发现铁血红素 b 通过 H58 结合在三聚酶的亚基界面上，其中 Fe(III) 通过与环氧化物氧结合而充当路易斯酸。 Y103 和 N64 以及分别与环氧化物和芳基的氧结合的疏水口袋以解释 SOI 的高区域选择性和立体特异性的方式定位底物。我们的研究结果可以支持扩展环氧化物底物的范围，并可用于潜在地重新利用 SOI 来催化新的铁基化学反应。