The reduced flavin-dependent monooxygenase SfnG converts dimethylsulfone to methanesulfinate

Abstract

The biochemical pathway through which sulfur may be assimilated from dimethylsulfide (DMS) is proposed to proceed via oxidation of DMS to dimethylsulfoxide (DMSO) and subsequent conversion of DMSO to dimethylsulfone (DMSO2). Analogous chemical oxidation processes involving biogenic DMS in the atmosphere result in the deposition of DMSO2 into the terrestrial environment. Elucidating the enzymatic pathways that involve DMSO2 contribute to our understanding of the global sulfur cycle. Dimethylsulfone monooxygenase SfnG and flavin mononucleotide (FMN) reductase MsuE from the genome of the aerobic soil bacterium Pseudomonas fluorescens Pf0-1 were produced in Escherichia coli, purified, and biochemically characterized. The enzyme MsuE functions as a reduced nicotinamide adenine dinucleotide (NADH)-dependent FMN reductase with apparent steady state kinetic parameters of Km = 69 μM and kcat/Km = 9 min−1 μM −1 using NADH as the variable substrate, and Km = 8 μM and kcat/Km = 105 min−1 μM −1 using FMN as the variable substrate. The enzyme SfnG functions as a flavoprotein monooxygenase and converts DMSO2 to methanesulfinate in the presence of FMN, NADH, and MsuE, as evidenced by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. The results suggest that methanesulfinate is a biochemical intermediate in sulfur assimilation.