A density functional study of the completion of the methane monooxygenase catalytic cycle. Methanol complex to MMOH resting state

In the final stage of the hydroxylation of methane to methanol by the methane monooxygenase (MMO) enzyme, a binuclear iron-methanol complex (II), expels the methanol and restores the resting state (RS) Fe^III-Fe^III form of the hydroxylate (MMOH) component to complete the catalytic cycle. The overall process, II → RS, can include protonation, addition of water, expulsion of methanol, and ligand rearrangement, all in an unknown order of occurrence. A model previously applied successfully to describe the hydroxylation mechanism that produces II is used here with the density functional theory to examine a series of intermediate structures and reaction steps to find the lowest-energy reaction path for II → RS. Two different groups of structures and reaction steps are considered; those involving proton transfer from the protein/environment to the complexes (group A) and those that are charge neutral with consideration of internal hydrogen atom migration among the oxygen ligands of the diferric complexes (group B). The lowest-energy paths for group A were found to be paths II - CH₃OH → III + H⁺ → VII + H₂O → IX, II - CH₃OH → III + H₂O → V + H⁺ → IX, and II + H⁺ → VI - CH₃OH → VII + H₂O → IX, with overall reaction energies of -19.9 (⁹A) and -35.0 (¹¹A) kcal/mol. The methanol dissociation step gives a small thermodynamic barrier to all these mechanisms. In group B, the preferred reaction path is II + H₂O → IV → X {or XI} - CH₃OH → XII, as well as II - CH₃OH → III + H₂O → V → XII, with reaction energies of -15.7 (⁹A) and -18.8 (¹¹A) kcal/mol. The IX and XIII structures have similar geometries and can be identified with the RS of MMOH, depending on its actual charge. The ¹¹A structures are consistently more stable than their ⁹A counterparts, as expected for Fe^III-Fe^III complexes, and the II → RS process will proceed entirely on the ¹¹A energy surface.