Date of Award
Campus Access Thesis
Master of Science (MS)
Daniel P. Dowling
Dimethylsulfide (DMS) is a product of phytoplankton and marine algae metabolism that accounts for 50% of biogenic sulfur emissions. Volatile DMS and its oxidation products are components of sulfate aerosols and cloud condensation nuclei, thus they play a role in the governance of planetary albedo. These compounds enter the terrestrial environment via deposition, where they are thought to serve as a nutrient source for soil-dwelling bacteria. Recently, a metabolic pathway for sulfur acquisition from dimethylsulfone (DMSO2) has been discovered in Pseudomonads, highlighting a biological process involved in the breakdown of oxidized DMS. In Pseudomonas fluorescens, this pathway involves three reduced flavin-dependent monooxygenases: SfnG converts DMSO2 to methanesulfinate, and MsuC and MsuD subsequently oxidize methanesulfinate to sulfite, which can be used in downstream pathways to synthesize biomolecules such as cysteine or the gasotransmitter H2S. The reduced flavin-dependent monooxygenases use reduced flavin mononucleotide (FMNH-) supplied by the reductase MsuE to oxidize their respective substrates with concomitant reduction of molecular oxygen. Characterization of these pathways will lead to a better understanding of the biochemical processes involved in the global sulfur cycle.
This thesis reports the 2.4- and 2.1-Å resolution crystal structures of SfnG and MsuC, respectively. SfnG catalyzes a carbon-sulfur bond cleavage that converts DMSO2 to methanesulfinate and adopts a TIM-barrel fold. A putative active site, which shares features with homologous enzymes that catalyze cleavage of carbon-sulfur or carbon-nitrogen bonds, has been identified at the C-terminal side of the barrel. MsuC catalyzes oxidation of methanesulfinate, the C1 (+2) oxyanion, to methanesulfonate, the C1 (+4) oxyanion. The enzyme adopts an acyl-CoA dehydrogenase fold and a putative active site has been proposed. Work towards the development of a continuous spectroscopic assay for characterization of steady-state kinetic parameters of reduced flavin-dependent monooxygenases is presented using the MsuEC system. Initial MsuC kinetic parameters suggest that the enzyme has moderate affinity for methanesulfinate (KM = 11 + 3 µM) and exhibits slow turnover (kcat = 3 + 1 min-1).
Soule, Jess, "Structural and Kinetic Characterization of Two Reduced-Flavin Dependent Monooxygenases Involved in Bacterial Sulfur Assimilation and Their Link to Climate Regulation" (2019). Graduate Masters Theses. 586.