Structural Model Studies for the Peroxo Intermediate P and the Reaction Pathway from P → Q of Methane Monooxygenase Using Broken-Symmetry Density Functional Calculations

Several structural models for the active site of the peroxo intermediate state “P” of the hydroxylase component of soluble methane monooxygenase (MMOH) have been studied, using two DFT functionals OPBE and PW91 with broken-symmetry methodology and the conductor-like screening (COSMO) solvation model. These active site models have different O2 binding modes to the diiron center, such as the μ-η2,η2, trans-μ−1,2 and cis-μ−1,2 conformations. The calculated properties, including optimized geometries, electronic energies, Fe net spin populations, and Mössbauer isomer shift and quadrupole splitting values, have been reported and compared with available experimental results. The high-spin antiferromagnetically (AF) coupled Fe3+ sites are correctly predicted by both OPBE and PW91 methods for all active site models. Our data analysis and comparisons favor a cis-μ−1,2 structure (model cis-μ−1,2a shown in Figure ) likely to represent the active site of MMOH-P. Feasible structural changes from MMOH-P to another intermediate state MMOH-Q are also proposed, where the carboxylate group of Glu243 side chain has to open up from the mono-oxygen bridging position, and the dissociations of the terminal H2O ligand from Fe1 and of the oxygen atom in the carboxylate group of Glu144 from Fe2 are also necessary for the O2 binding mode changes from cis to trans. The O−O bond is proposed to break in the trans-conformation and forms two μ-oxo bridges in MMOH-Q. The terminal H2O molecule and the Glu144 side chain then rebind with Fe1 and Fe2, respectively, in Q.