Structure of the Michaelis complex and function of the catalytic center in the reductive Half-reaction of computational and synthetic models of sulfite oxidase

Abstract

By using frontier-molecular-orbital and electrostatic (nucleophilic) interactions as well as relaxed potential-energy surface scans, it is shown that the initial step in the oxygen-atom transfer (OAT) reaction of [Mo^VIO₂-(S₂C₂Me₂)SMe]^-1 (1) and [Mo^VIO₂-{(S₂C₂(CN)₂}₂]^2- (2) with HSO₃- takes place by oxoanionic binding of the substrate to the MoVI center with the formation of a stable Michaelis complex. The gas-phase and solvent-corrected enthalpy profile with fully optimized minima and transition states for the OAT reaction of 1 and 2 with HSO3¯ showed the release of reaction energy for both complexes. The optimized geometries of 1 and 2 in the respective enzyme-substrate complexes showed a common feature with the participation of hydrogen bonding of the substrate with the axial (spectator) oxo group in the subsequent formation of the six-membered MoO₂HOS transition state. The enzyme-substrate complex of 2 shows heptacoordination as proposed earlier, although the trans (to axial oxo)-Mo—S(dithiolene) bond is elongated to 2.948 Å.