Abstract
Sulfite dehydrogenases (SDHs) are enzymes that catalyze the oxidation of the toxic and mutagenic compound sulfite to sulfate, thereby protecting cells from adverse effects associated with sulfite exposure. While some bacterial SDHs that have been characterized to date are able to use cytochrome c as an electron acceptor, the majority of these enzymes prefer ferricyanide as an electron acceptor and have therefore been termed “atypical” SDHs. Identifying the natural electron acceptor of these enzymes, however, is crucial for understanding how the “atypical” SDHs are integrated into cell metabolism. The SorT sulfite dehydrogenase from Sinorhizobium meliloti is a representative of this enzyme type and we have investigated the interactions of SorT with two small redox proteins, a cytochrome c and a Cu containing pseudoazurin, that are encoded in the same operon and are co-transcribed with the sorT gene. Both potential acceptor proteins have been purified and characterized in terms of their biochemical and electrochemical properties, and interactions and enzymatic studies with both the purified SorT sulfite dehydrogenase and components of the respiratory chain have been carried out. We were able to show for the first time that an “atypical” sulfite dehydrogenase can couple efficiently to a cytochrome c isolated from the same organism despite being unable to efficiently reduce horse heart cytochrome c, however, at present the role of the pseudoazurin in SorT electron transfer is unclear, but it is possible that it acts as an intermediate electron shuttle between. The SorT system appears to couple directly to the respiratory chain, most likely to a cytochrome oxidase.
Highlights
Sulfite dehydrogenases (SDHs) are enzymes that catalyze the oxidation of sulfite, which can cause damage to important cellular components such as proteins, DNA, and lipids, to the inert and non-toxic compound sulfate (Kappler and Dahl, 2001; Kappler, 2011)
Characterization of recombinant SorT The S. meliloti SDH, SorT, can only be purified with very low yields as several proteins co-purify with SorT (Wilson and Kappler, 2009), and this precludes detailed characterization of this protein both in terms of enzyme kinetics and structure as well as spectroscopic characterization of the protein. rSorT was produced in E. coli TP1000 as a cytoplasmic protein and purified to homogeneity using a combination of affinity chromatography and size exclusion chromatography
The SorT sulfite dehydrogenase from S. meliloti is a representative of the “atypical” bacterial SDHs that show highest activities with the artificial electron acceptor ferricyanide, our data suggest that in vivo the reaction of SorT is coupled to a cognate cytochrome c that is found in the sorT operon
Summary
Sulfite dehydrogenases (SDHs) are enzymes that catalyze the oxidation of sulfite, which can cause damage to important cellular components such as proteins, DNA, and lipids, to the inert and non-toxic compound sulfate (Kappler and Dahl, 2001; Kappler, 2011). One of the distinctive features of molybdenum enzymes is that they can catalyze the transfer of an oxygen atom to a substrate using water as the oxygen donor (Hille, 1996). The reaction catalyzed by SOs and SDHs proceeds according to the general equation SO32− + H2O → SO 2− 4 2H+ 2e− (1)
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