Spectroscopic and Computational Characterization of the NO Adduct of Substrate-Bound Fe(II) Cysteine Dioxygenase: Insights into the Mechanism of O₂ Activation

Cysteine dioxygenase (CDO) is a mononuclear nonheme iron­(II)-dependent enzyme critical for maintaining appropriate cysteine (Cys) and taurine levels in eukaryotic systems. Because CDO possesses both an unusual 3-His facial ligation sphere to the iron center and a rare Cys–Tyr cross-link near the active site, the mechanism by which it converts Cys and molecular oxygen to cysteine sulfinic acid is of broad interest. However, as of yet, direct experimental support for any of the proposed mechanisms is still lacking. In this study, we have used NO as a substrate analogue for O₂ to prepare a species that mimics the geometric and electronic structures of an early reaction intermediate. The resultant unusual <i>S</i> = ¹/₂ {FeNO}⁷ species was characterized by magnetic circular dichroism, electron paramagnetic resonance, and electronic absorption spectroscopies as well as computational methods including density functional theory and semiempirical calculations. The NO adducts of Cys- and selenocysteine (Sec)-bound Fe­(II)­CDO exhibit virtually identical electronic properties; yet, CDO is unable to oxidize Sec. To explore the differences in reactivity between Cys- and Sec-bound CDO, the geometries and energies of viable O₂-bound intermediates were evaluated computationally, and it was found that a low-energy quintet-spin intermediate on the Cys reaction pathway adopts a different geometry for the Sec-bound adduct. The absence of a low-energy O₂ adduct for Sec-bound CDO is consistent with our experimental data and may explain why Sec is not oxidized by CDO