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 (DMSO2) 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 DMSO2. 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 DMSO2 accompanies ordering of the active site, with DMSO2 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.
Keywords: FMN; N5-(hydro)peroxyflavin; TIM barrel; dialkylsulfone; flavin.