Combined NMR and computational study for azide binding to human manganese superoxide dismutase.

Human manganese superoxide dismutase (MnSOD) labeled with 3-fluorotyrosine (Tyf) was complexed with the 15N-labeled inhibitor azide ([15N3−]). The sample was characterized by solid-state NMR (SSNMR) spectroscopy (19F-MAS and 15N-CPMAS). Employing 19F-15N-REDOR spectroscopy, we determined the distances between the fluorine label in Tyrosine-34 and the three 15N-nuclei of the azide and the relative orientation of the azide in the binding pocket of the MnSOD. A distance of R1=4.85Å between the 19F-label of Tyf34 and the nearest 15N of the azide and an azide–fluorotyrosine Tyf34 angle of 90° were determined. These geometry data are employed as input for molecular modeling of the location of the inhibitor in the active site of the enzyme. In the computations, several possible binding geometries of the azide near the Mn-complex were assumed. Only when the azide replaces the water ligand at the Mn-complex we obtained a geometry of the azide–Mn-complex, which is consistent with the present NMR data. This indicates that the water molecule ligating to the Mn-complex is removed and the azide is placed at this position. As a consequence the azide forms an H bond with Gln143 instead with Tyf34, in contrast to non-19F-labeled MnSOD, where the azide is hydrogen bonded to the hydroxy group of Tyr34.