Date of Award

8-2019

Document Type

Campus Access Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Advisor

Daniel P. Dowling

Second Advisor

Jason Green

Third Advisor

Marianna Török

Abstract

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).

Comments

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