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

Area of Science:

• Biochemistry
• Bioinorganic Chemistry
• Computational Chemistry

Background:

• Iron(III) superoxide dismutase (FeSOD) is crucial for cellular defense against reactive oxygen species.
• Low-temperature azide adducts of FeSOD exhibit distinct yellow and pink forms, with their structural basis remaining unclear for over two decades.

Purpose of the Study:

• To elucidate the structural and electronic properties of yellow and pink azide adducts of iron(III) superoxide dismutase (N(3)-FeSOD).
• To resolve discrepancies regarding the number of azide ligands and understand the factors contributing to spectral variations.

Main Methods:

• Variable-temperature, variable-field magnetic circular dichroism (MCD) spectroscopy.
• Resonance Raman (RR) spectroscopy.
• Density functional theory (DFT) and semiempirical INDO/S-CI computational modeling.

Main Results:

• Spectroscopic data indicate a single azide ligand bound to the ferric center in both yellow and pink N(3)-FeSOD species, challenging prior hypotheses.
• Resonance Raman spectra reveal subtle differences in the Fe-N(3) bonding, consistent across wild-type and Q69E mutant forms.
• Computational studies support a single azide model and attribute spectral variations to differences in the Fe-N-N bond angle, with a ~10° variation explaining the color difference.

Conclusions:

• The distinct colors of N(3)-FeSOD adducts arise from variations in the Fe-N-N bond angle, influenced by the second coordination sphere.
• The second coordination sphere plays a role in modulating the active site geometry and potentially substrate orientation for enzymatic activity.