Sulfur is an essential element for life. Bacteria can obtain sulfur from inorganic sulfate; but in the sulfur starvation–induced response, Pseudomonads employ two-component flavin-dependent monooxygenases (TC-FMOs) from the msu and sfn operons to assimilate sulfur from environmental compounds including alkanesulfonates and dialkylsulfones. Here, we report binding studies of oxidized FMN to enzymes involved within the P. fluorescens enzymatic pathway responsible for converting dimethylsulfone (DMSO 2 ) to sulfite. In this catabolic pathway, SfnG serves as the initial TC-FMO for sulfur assimilation, which is investigated in detail by solving the 2.6-Å resolution crystal structure of unliganded SfnG and the 1.75-Å resolution crystal structure of the SfnG ternary complex containing FMN and DMSO 2 . We find that SfnG adopts a (β/α) 8 barrel fold with a distinct quaternary configuration from other tetrameric class C TC-FMOs. To probe the unexpected tetramer arrangement, structural heterogeneity is assessed by chromatography and light scattering to confirm ligand binding correlates with a tetramer. Binding of FMN and DMSO 2 accompanies ordering of the active site, with DMSO 2 bound on the si -face of the flavin. A previously unobserved protein backbone conformation is found within the oxygen-binding site on the re -face of the flavin. Functional assays and the positioning of ligands with respect to the oxygen-binding site are consistent with use of an N5-(hydro)peroxyflavin pathway. Biochemical endpoint assays and docking studies reveal SfnG breaks the C–S bond of a range of dialkylsulfones.

中文翻译：

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二甲基砜单加氧酶裂解 C-S 键的结构、生物物理和生化见解


硫是生命必不可少的元素。细菌可以从无机硫酸盐中获得硫;但在硫饥饿诱导的反应中，假单胞菌利用来自 msu 和 sfn 操纵子的双组分黄素依赖性单加氧酶 （TC-FMO） 从环境化合物（包括烷基磺酸盐和二烷基砜）中吸收硫。在这里，我们报道了氧化 FMN 与负责将二甲基砜 （DMSO 2） 转化为亚硫酸盐的荧光假单胞菌酶途径中涉及的酶的结合研究。在该分解代谢途径中，SfnG 用作硫同化的初始 TC-FMO，通过解析未配体 SfnG 的 2.6 Å 分辨率晶体结构和含有 FMN 和 DMSO 2 的 SfnG 三元复合物的 1.75 Å 分辨率晶体结构来详细研究硫同化。我们发现 SfnG 采用 （β/α） 8 桶形折叠，与其他四聚体 C 类 TC-FMO 具有不同的四元构型。为了探测意外的四聚体排列，通过色谱和光散射评估结构异质性，以确认配体结合与四聚体的相关性。FMN 和 DMSO 2 的结合伴随着活性位点的排序，DMSO 2 结合在黄素的 si 面上。以前未观察到的蛋白质骨架构象是在黄素背面的氧结合位点内发现的。功能测定和配体相对于氧结合位点的定位与使用 N5-（氢）过氧黄素途径一致。生化终点测定和对接研究表明，SfnG 会破坏一系列二烷基砜的 C-S 键。