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Texas A&M University, Postdoctoral Associate, 2001
Our group is interested in understanding the structural and physical dynamics of enzymes that contribute to their functional properties. Our lab has focused our research efforts on a bacterial two-component system involved in sulfur acquisition, and a mammalian metabolic pathway important in taurine biosynthesis. These catalytically distinct reactions play a vital role in maintaining appropriate sulfur levels in bacterial and mammalian systems.
Alkanesulfonate Monooxygenase System
For many bacterial organisms, inorganic sulfate is an important metabolite in the biological synthesis of sulfur-containing macromolecules. Inorganic sulfur is poorly represented in aerobic soil therefore bacteria in soil environments must have alternative sources for obtaining this element. When Escherichia coli is deprived of inorganic sulfur or cysteine, a set of sulfate-starvation-induced (Ssi) proteins is produced at increased levels. Two of these proteins include an FMN reductase (SsuE) and an FMNH2-utilizing monooxygenase (SsuD). This alkanesulfonate monooxygenase system is involved in the acquisition of sulfur through the reduction of alkanesulfonates to sulfites and the corresponding aldehyde. The system belongs to a family of two-component enzyme systems that utilize FMN as a substrate rather than a bound prosthetic group. The number of bacterial flavin-dependent monooxygenases that utilize flavin as a substrate has increased significantly with new systems continually being identified. We are interested in the novel mechanism of desulfonation by this system, and identifying how conformational flexibility of these enzymes contributes to their function.
Taurine Biosynthetic Pathway
The sulfur-containing nonprotein amino acid, taurine, plays an important protective role in a wide variety of mammalian physiological functions. Many of the biological functions of taurine rely on the intracellular concentrations of taurine, which is determined by the cells capacity to synthesize this metabolite. Taurine can be synthesized by several different mechanisms, but is primarily produced by the cysteine dioxygenase pathway. In the initial reaction, cysteine is converted to cysteine sulfinic acid by the enzyme cysteine dioxygenase. The reaction catalyzed by cysteine dioxygenase (CDO) has been shown to be a primary step in the production of taurine. The cysteine sulfinic acid formed can branch to form pyruvate and sulfite by aspartate amino transferase or taurine by cysteine sulfinate decarboxylase (CSD). By catalyzing the first step in the pathway, CDO plays a critical role in the maintenance of cysteine and taurine levels in cells. The focus of our research is to obtain a detailed understanding of the catalytic mechanism of each enzyme in the taurine biosynthetic pathway to determine how this important metabolite is properly maintained in the cell.
Ellis, H. R. “Mechanism for Sulfur Acquisition by the Alkanesulfonate Monooxygenase System.” Biorg. Chem., 2011, 39, 178-184.
Carpenter, R. A.; Xiong, J.; Robbins, J.M.; Ellis, H.R. “Functional role of a conserved arginine residue located on a mobile loop of the alkanesulfonate monooxygenase.” Biochemistry, 2011, 50, 6469-6477.
Ellis, H. R. “Two-component flavin-dependent monooxygenase enzymes.” Arch. Biochem. Biophys., 2010, 497, 1-12.
Carpenter, R. A.; Zhan, X.; Ellis, H. R. “Catalytic role of a conserved cysteine residue in the desulfonation reaction by the alkanesulfonate monooxygenase enzyme.” Biochim Biophys. Acta., 2009, 1804, 97-105.
Zhan, X.; Carpenter, R. A.; Ellis, H.R. “Catalytic importance of the substrate binding order for the FMNH2-dependent alkanesulfonate monooxygenase enzyme.” Biochemistry, 2008, 47, 2221-2230.