Thermophilic bacterial sulfur metabolism: a structural and functional investigation of sulfite-oxidizing enzymes

As an intermediate of sulfur metabolism, sulfite is both toxic and a metabolic substrate. Hence, organisms throughout all domains of life have evolved their respective enzymes in response to sulfite presence. Regardless of the ultimate aim, sulfiteis converted to sulfate either directly by sulfite dehydrogenase/oxidase, or indirectly by APS reductase and ATP sulfurylase with the formation of the intermediate APS. In this work, the thermophilic sulfite dehydrogenase (SDH) fromThermus thermophilus along with its own electron carrier cytochrome c550, was produced recombinantly in Escherichia coli. Subsequently, both proteins were purified using a combination of affinity, ion exchange and gel filtration chromatographic techniques. The purified samples were then used for protein crystallization in order to determine their three-dimensional structures and understand the electron transfer dynamics. Using X-ray diffraction, the structures of SDH and two individual domains of cytochrome c550 have been determined to 1.8, 1.2 and 3.8 Å, respectively. Proteins have also been used for biophysical characterization using Electron Paramagnetic Resonance and Electrochemistry. Additionally, an alternative dehydrogenase of the SoxCD family was identified bioinformatically. The enzyme (SoxCTt) and the mono-heme cytochrome SoxDTt were cloned and expressed in E. coli, separately. Purified samples have been used to probe their crystallogenesis leading to single crystals of cytochrome c (SoxD). A preliminary dataset was collected from X-ray diffraction experiments. Together, these data will inform the future mutagenesis study to confirm the protein-protein interaction and electron transfer kinetics of these proteins.