Understanding the mechanism of H₂S oxidation by flavin-dependent sulfide oxidases: a DFT/IEF-PCM study

In the last years, H₂S has been recognized as a signaling molecule in mammals, which can synthesize and catabolize (by oxidation) such species. The latter process is accelerated by a sulfide:quinone oxidoreductase (SQR, E.C. 1.8.5.4), a flavin-dependent sulfide oxidase (FDSO). FDSOs catalyze electron transfer from H₂S to an acceptor in catalytic cycles involving two phases: (I) reduction of FAD by H₂S (SH^-) and (II) electron transfer from FADH^- to the electron acceptor. The first step of FAD reduction consists on the reaction of SH^- with a catalytic disulfide at the active site of the enzyme, to yield a thiolate and a persulfide in the protein. This step is ca. 10⁶ times faster than the analogous reaction with low-molecular-weight disulfides (LMWDs) and the causes of such extraordinary acceleration remain unknown. Using the IEF-PCM(ε ≈ 10)/M06-2X-D3/6-31+G(d,p) level of theory, we have modeled the reaction of SH^- with a disulfide as located in a representative model of the active site extracted from a prokaryotic SQR, assessing the effects of partial covalent interactions (PCIs) between the leaving sulfur atom and flavin ring on the activation Gibbs free-energy barrier at 298 K (∆^‡G_298K). To also evaluate the importance of entropic penalties on the first step, we have modeled at the same level of theory the reaction of (bis)hydroxyethyl disulfide in aqueous solution, a LMWD for which experimental data is available. Our results show that PCIs between the leaving sulfur atom and the flavin group only have a minor effect (∆^‡G_298K reduced by 1.6 kcal mol^-1) while compensating entropic penalties could have a much larger effect (up to 8.3 kcal mol^-1). Finally, we also present here a first model of some of further steps in the phase I of the catalytic cycle as in mammalian FDSOs, providing some light about their detailed mechanism. Graphical abstract .