Theoretical study of the oxidation reaction and electron spin resonance parameters involving sulfite oxidase

We present a density functional theory (DFT) study on the conversion of sulfite to sulfate with a model complex representing the active site of the molybdenum-containing enzyme sulfite oxidase (SO). This study considers the attack of the sulfur lone pair from SO(3)(2-) on the equatorial oxo ligand of the model complex as the initial step in the oxidation process. The good agreement between our energy profile and data derived from experimental kinetic parameters provides some support for the reaction mechanism of the oxidative half-reaction of SO proposed in this study. The enzymatic reductive half-reaction involves the formation of an Electron Spin Resonance (ESR) active Mo(V) species. Experimentally, differences in ESR parameters (g-values and (1)H hyperfine coupling constant) of the low- and high-pH forms of the enzyme have been found. The current study also presents DFT-based calculations on ESR parameters for three model complexes representing the paramagnetic center Mo(V) of SO in its possible low- and high-pH forms. We provide an analysis of the magnetic orbital coupling responsible for the calculated g-values. Finally, we suggest how the conformation and hydrogen bonding interactions of the hydroxyl ligand can explain the different ESR parameters at low- and high-pH.