Investigation of the dinuclear Fe center of methane monooxygenase by advanced paramagnetic resonance techniques: On the geometry of DMSO binding

We report an extensive advanced paramagnetic resonance characterization of the mixed-valence dinuclear Fe center of methane monooxygenase hydroxylase (MMOH(mv)) from Methylococcus capsulatus (Mc) (Bath) and of binding to it by the exogenous ligand DMSO. We employ continuous wave and pulsed electron nuclear double resonance (ENDOR) spectroscopy, both at Q-band microwave frequencies, to examine N-14,N-15, H-1,H-2, C-13, and Fe-57 nuclei. Preliminary H-1 ENDOR results were communicated previously (DeRose, V. J.; Liu, K. E.; Hoffman, B. M.; Lippard, S.J. J. Am. Chem. Soc. 1993, 115, 6440-6441). ENDOR-derived N-14,N-15 hyperfine tensors are interpreted in terms of the spin distribution on histidyl ligands bound to the dinuclear center. Determination of the Fe-57 hyperfine tensors gives a complete picture of the spin-coupled Fe2+ and Fe3+ ions. The H-1,H-2 ENDOR results disclose the presence of a bridging hydroxide and an aqua ligand in both native and DMSO-treated enzyme. A novel procedure for describing the H-1 hyperfine tensor of the bridge gives the orientation of the g-tensor relative to the cluster framework in both enzyme forms, information that is normally obtained only from full single-crystal EPR studies. DMSO is found to cause small perturbations of both histidyl ligands, and little change in the Fe-57 hyperfine tensors. However, Q-band pulsed H-2 and C-13 ENDOR measurements of labeled DMSO show that this exogenous ligand binds in a distinct site with a well-ordered structure, and further indicate that it is O-bound to the Fe3+ ion of the mixed-valence cluster. The analysis, coupled with H-2 X-band electron spin-echo envelope modulation data, places limitations on the possible orientation of the bound DMSO. These geometric restrictions have been used to guide molecular modeling of DMSO bound to the MMOH(mv) diiron active site. The results reported here provide a basis with which to study other dinuclear Fe-carboxylate proteins.