Dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) are widespread in marine environment, and are important participants in the global sulfur cycle. Microbiol oxidation of DMS to DMSO represents a major sink of DMS in marine surface waters. The SAR11 clade and the marine Roseobacter clade (MRC) are the most abundant heterotrophic bacteria in the ocean surface seawater. It has been reported that trimethylamine monooxygenase (Tmm, EC 1.14.13.148) from both MRC and SAR11 bacteria likely oxidizes DMS to generate DMSO. However, the structural basis of DMS oxidation has not been explained. Here, we characterized a Tmm homolog from the SAR11 bacterium Pelagibacter sp. HTCC7211 (Tmm₇₂₁₁). Tmm₇₂₁₁ exhibits DMS oxidation activity in vitro. We further solved the crystal structures of Tmm₇₂₁₁ and Tmm₇₂₁₁ soaked with DMS, and proposed the catalytic mechanism of Tmm₇₂₁₁, which comprises a reductive half-reaction and an oxidative half-reaction. FAD and NADPH molecules are essential for the catalysis of Tmm₇₂₁₁. In the reductive half-reaction, FAD is reduced by NADPH. In the oxidative half-reaction, the reduced FAD reacts with O₂ to form the C4a-(hydro)peroxyflavin. The binding of DMS may repel the nicotinamide ring of NADP⁺, and make NADP⁺ generate a conformational change, shutting off the substrate entrance and exposing the active C4a-(hydro)peroxyflavin to DMS to complete the oxidation of DMS. The proposed catalytic mechanism of Tmm₇₂₁₁ may be widely adopted by MRC and SAR11 bacteria. This study provides important insight into the conversion of DMS into DMSO in marine bacteria, leading to a better understanding of the global sulfur cycle.

二甲硫醚（DMS）和二甲亚砜（DMSO）广泛存在于海洋环境中，是全球硫循环的重要参与者。 DMS 微生物氧化为 DMSO 是海洋表层水中 DMS 的主要汇。 SAR11进化枝和海洋玫瑰杆菌属进化枝（MRC）是海洋表面海水中最丰富的异养细菌。据报道，来自 MRC 和 SAR11 细菌的三甲胺单加氧酶（Tmm，EC 1.14.13.148）可能会氧化 DMS 生成 DMSO。然而，DMS氧化的结构基础尚未得到解释。在这里，我们鉴定了 SAR11 细菌的 Tmm 同源物远洋杆菌属sp。 HTCC7211（Tmm ₇₂₁₁ ）。 Tmm ₇₂₁₁表现出 DMS 氧化活性体外。我们进一步解析了Tmm ₇₂₁₁和​​DMS浸泡后的Tmm ₇₂₁₁的晶体结构，并提出了Tmm ₇₂₁₁的催化机理，包括还原半反应和氧化半反应。 FAD 和 NADPH 分子对于 Tmm ₇₂₁₁的催化至关重要。在还原半反应中，FAD 被 NADPH 还原。在氧化半反应中，还原的FAD与O ₂反应形成C4a-(氢)过氧黄素。 DMS的结合可能会排斥NADP ⁺的烟酰胺环，使NADP ⁺产生构象变化，关闭底物入口，将活性C4a-(氢)过氧黄素暴露于DMS，完成DMS的氧化。 Tmm ₇₂₁₁所提出的催化机制可能被 MRC 和 SAR11 细菌广泛采用。 这项研究提供了关于海洋细菌中 DMS 转化为 DMSO 的重要见解，从而更好地了解全球硫循环。