Second-sphere contributions to substrate-analogue binding in iron(III) superoxide dismutase

A combination of spectroscopic and computational methods has been employed to explore the nature of the yellow and pink low-temperature azide adducts of iron(III) superoxide dismutase (N₃-FeSOD), which have been known for more than two decades. Variable-temperature variable-field magnetic circular dichroism (MCD) data suggest that both species possess similar ferric centers with a single azide ligand bound, contradicting previous proposals invoking two azide ligands in the pink form. Complementary data obtained on the azide complex of the Q69E FeSOD mutant reveal that relatively minor perturbations in the metal-center environment are sufficient to produce significant spectral changes; the Q69E N₃-FeSOD species is red in color at all temperatures. Resonance Raman (RR) spectra of the wild-type and Q69E mutant N₃-FeSOD complexes are consistent with similar Fe-N₃ units in all three species; however, variations in energies and relative intensities of the RR features associated with this unit reveal subtle differences in (N₃^-)-Fe³⁺ bonding. To understand these differences on a quantitative level, density functional theory and semiempirical INDO/S-CI calculations have been performed on N₃-FeSOD models. These computations support our model that a single azide ligand is present in all three N₃-FeSOD adducts and suggest that their different appearances reflect differences in the Fe-N-N bond angle. A 10° increase in the Fe-N-N bond angle is sufficient to account for the spectral differences between the yellow and pink wild-type N₃-FeSOD species. We show that this bond angle is strongly affected by the second coordination sphere, which therefore might also play an important role in orienting incoming substrate for reaction with the FeSOD active site.