Binding of Nitric Oxide to a Synthetic Model of Iron-Containing Nitrile Hydratase (Fe-NHase) and Its Photorelease: Relevance to Photoregulation of Fe-NHase by NO

The activity of the non-heme iron enzyme nitrile hydratase (Fe-NHase) is modulated by nitric oxide (NO). The inactive (dark form) NO-bound enzyme is activated when exposed to light via the release of NO from the iron center. In order to determine whether oxygenation of active site Fe-bound Cys-S centers are involved in this process of NO regulation, a model complex (Et4N)[(Cl2PhPepS)Fe(NO)(DMAP)] (8) has been synthesized and structurally characterized. Complex 8 does not exhibit any NO photolability. However, following oxygenation of the Fe-bound thiolato-S centers to sulfinates (with the aid of oxaziridine), the resulting complex (Et4N)[(Cl2PhPep{SO2}2)Fe(NO)(DMAP)] (9) releases NO readily upon illumination with visible light. Spectroscopic properties of 8 and 9 confirm that these species do mimic the active site of Fe-NHase closely, and the results indicate that NO photolability is related to S-oxygenation. Results of density functional theory and time-dependent DFT studies on both 8 and 9 indicate that S-oxygenation weakens Fe−S bonding and that strong transitions near 470 nm transfer an electron from a carboxamido-N/sulfinato-SO2 MO to a dπ(Fe)-π*(NO)/dz2(Fe)-σ*(NO) antibonding orbital in 9. In case of 8, strong S−Fe−NO bonding interactions prevent the release of NO upon illumination. Together, the results of this work strongly suggest that oxygenated Cys-S centers play an important role in the process of NO regulation of Fe-NHases.