Introduction: Iron-containing persulfide dioxygenases (PDO) catalyze the oxidation of glutathione persulfide (GSSH) to sulfite and GSH. They are abundant in Bacteria and mitochondria.Sulfur oxygenase reductases (SOR) catalyze an O2-dependent elemental sulfur (S0) dismutation to sulfite and H2S. They occur in chemolithotrophic sulfur oxidizers and are structurally unrelated to PDOs. Both enzymes contain mononuclear non-heme Fe active sites, each with two His and one carboxylate ligands. Here, we compare structural and spectroscopic properties of the AcPDO from Acidithiobacillus caldus and three different SORs from Bacteria and Archaea in order to better understand the reaction mechanisms.
Results: GSSH binds to the PDO active site in a cleft at the enzyme's surface with the iron at its bottom. Incubation with GSSH resulted in the formation of S-Fe charge transfer absorption spectra; which was not observed in the SORs. Modeling of GSSH binding to the AcPDO crystal structure suggested that a transient persulfide-peroxo intermediate is formed, which is resolved by proton shuffling with a second coordination sphere His. Cysteines do not participate in catalysis, a disulfide present at the AcPDO surface stabilizes its C-terminus.
In contrast, SORs have a spacious and deeply embedded active site cavity with a narrow entrance pore comprising 3 conserved cysteines. X-ray crystallography of SORs showed that each of the cysteines could be persulfurated and that SOR mutants can become stuck in that state. Free and/or cysteine-bound polysulfide chains seem to be binding to Fe with the terminal S atom. WE suggest that sulfane S dismutation occurs, when the Fe-bound peroxo oxygen attacks the penultimate sulfane S atom of the chain, thus splitting off the terminal sulfide, while the oxidation steps proceeds in a similar way as in PDO.
Conclusion: The results suggest that the glutathione moiety positions the persulfide for terminal sulfane sulfur oxidation at the open PDO active site, whereas the closed cavity of the SORs is required to keep the more mobile free persulfides trapped in the vicinity of the Fe atom in order to get S0 dismutation to work.