Department of Cell and Molecular Biology, Uppsala University, Uppsala, Sweden.
Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, UiT - The Arctic University of Norway, Tromsø, Norway.
Elife. 2021 Feb 10;10:e61818. doi: 10.7554/eLife.61818.
The first S-adenosyl methionine (SAM) degrading enzyme (SAMase) was discovered in bacteriophage T3, as a counter-defense against the bacterial restriction-modification system, and annotated as a SAM hydrolase forming 5'-methyl-thioadenosine (MTA) and L-homoserine. From environmental phages, we recently discovered three SAMases with barely detectable sequence similarity to T3 SAMase and without homology to proteins of known structure. Here, we present the very first phage SAMase structures, in complex with a substrate analogue and the product MTA. The structure shows a trimer of alpha-beta sandwiches similar to the GlnB-like superfamily, with active sites formed at the trimer interfaces. Quantum-mechanical calculations, thin-layer chromatography, and nuclear magnetic resonance spectroscopy demonstrate that this family of enzymes are not hydrolases but lyases forming MTA and L-homoserine lactone in a unimolecular reaction mechanism. Sequence analysis and in vitro and in vivo mutagenesis support that T3 SAMase belongs to the same structural family and utilizes the same reaction mechanism.
第一个 S-腺苷甲硫氨酸（SAM）降解酶（SAMase）是在噬菌体 T3 中发现的，作为对抗细菌限制修饰系统的反防御机制，并被注释为 SAM 水解酶，形成 5'-甲基硫代腺苷（MTA）和 L-高丝氨酸。最近，我们从环境噬菌体中发现了三种 SAMase，它们与 T3 SAMase 的序列相似度几乎难以检测，与已知结构的蛋白质没有同源性。在这里，我们首次展示了噬菌体 SAMase 与底物类似物和产物 MTA 的复合物结构。该结构显示了一种类似于 GlnB 超家族的三聚体α-β三明治结构，活性位点形成在三聚体界面上。量子力学计算、薄层层析和核磁共振波谱表明，该酶家族不是水解酶，而是通过单一分子反应机制形成 MTA 和 L-高丝氨酸内酯的裂解酶。序列分析以及体外和体内诱变支持 T3 SAMase 属于相同的结构家族，并利用相同的反应机制。
