Theoretical Study of the Oxidation Reaction and Electron Spin Resonance Parameters Involving Sulfite Oxidase

Abstract

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.

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

“…The essential step is a nucleophilic attack of the substrate SO 3 2− [17]. This suggestion was refuted later [18,19]. The lowest-energy transition states connecting starting material and product involve a hydrogen-bond of HSO 3 − either to the cysteinato ligand or to an external base (Scheme 2c and d).…”

“…The lowest-energy transition states connecting starting material and product involve a hydrogen-bond of HSO 3 − either to the cysteinato ligand or to an external base (Scheme 2c and d). These hydrogen bonds dramatically lower the transition states for the nucleophilic attack of the sulfur lone pair at the oxido ligand [18,19]. Obviously, the nucleophilicity of the substrate is increased by hydrogen bonding.…”

“…(a-d) Alternative reaction mechanisms of SO oxidizing hydrogen sulfite[13,[16][17][18]; (e) proposed mechanism of XO acting on xanthine[1,10].…”

Comparison of the Active-Site Design of Molybdenum Oxo-Transfer Enzymes by Quantum Mechanical Calculations

“…19,27,36 To reduce the computational load, the MPT ligand was modeled by 1,2-dimethyldithiolene ([(MeCS)2] 2-; DMDT), which also has been used in many of the previous studies. 12,17,19,20,23,24,26,28,33 The protein-derived cysteine and serine ligands were modeled by MeS -and MeO -, respectively, whereas other groups were not truncated.…”

“…27 We have made some experiments with adding a model of an arginine residue to the SO model system. However, the results are sensitive to where this group is placed (there are actually five arginine residues and one lysine within 10 Å of the Mo ion in SO 41 ) and which restraints are used to fix it there (making the model specific for a certain protein and therefore not reflecting the intrinsic reactivity of the isolated active site).…”