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 metha ne monooxygenase (MMO) enzyme, a binuclear iron-methanol complex (II), expe ls the methanol and restores the resting state (RS) Fe-II-Fe-II form of the hydroxylate (MMOH) component to complete the catalytic cycle. The overall process, II --> RS, can include protonation, addition of water, expulsion o f methanol, and ligand rearrangement, all in an unknown order of occurrence . A model previously applied successfully to describe the hydroxylation mec hanism 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 structur es and reaction steps are considered; those involving proton transfer from the protein/environment to the complexes (group A) and those that are charg e 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 - CH3 OH --> VII + H2O --> IX, with overall reaction energies of -19.9 ((9)A) and -35.0 ((11)A) kcal/mol. The methanol dissociation step gives; a small ther modynamic barrier to all these mechanisms. In group B, the preferred reacti on path is II + H2O --> IV --> X {or XI} - CH3OH --> XII, as well as II - C H3OH --> III + H2O --> V --> XII, with reaction energies of -15.7 ((9)A) an d -18.8 ((11)A) kcal/mol. The IX and XIII structures have similar geometrie s and can be identified with the RS of MMOH, depending on its actual charge . The (11)A structures are consistently more stable than their (9)A counter parts, as expected for Fe-III-Fe-III complexes, and the II --> RS process w ill proceed entirely on the (11)A energy surface.