Class II terpene cyclases, such as oxidosqualene and squalene-hopene cyclases, catalyse some of the most complex polycyclization reactions. They minimally exhibit a β,γ-didomain architecture that has been evolutionarily repurposed in a wide range of terpene-processing enzymes and likely resulted from a fusion of unidentified monodomain proteins. Although single domain class I terpene cyclases have already been identified, the corresponding class II counterparts have not been previously reported. Here we present high-resolution X-ray structures of a monodomain class II cyclase, merosterolic acid synthase (MstE). With a minimalistic β-domain architecture, this cyanobacterial enzyme is able to construct four rings in cytotoxic meroterpenoids with a sterol-like topology. The structures with bound substrate, product, and inhibitor provide detailed snapshots of a cyclization mechanism largely governed by residues located in a noncanonical enzyme region. Our results complement the few known class II cyclase crystal structures, while also indicating that archaic monodomain cyclases might have already catalyzed complex reaction cascades.

II 类萜环化酶，例如氧化角鲨烯和角鲨烯-藿烯环化酶，可催化一些最复杂的多环化反应。它们最低限度地表现出 β，γ-双域结构，该结构已在广泛的萜烯加工酶中进化地重新利用，并且可能是由未识别的单域蛋白融合产生的。尽管已经鉴定出单域 I 类萜环化酶，但之前没有报道过相应的 II 类对应物。在这里，我们提出了单域 II 类环化酶、类固醇酸合酶 (MstE) 的高分辨率 X 射线结构。凭借简约的 β 结构域结构，这种蓝藻酶能够在具有类固醇拓扑结构的细胞毒性类二萜中构建四个环。具有结合底物、产物的结构，和抑制剂提供了主要由位于非规范酶区域中的残基控制的环化机制的详细快照。我们的结果补充了少数已知的 II 类环化酶晶体结构，同时也表明古老的单域环化酶可能已经催化了复杂的反应级联。