title: 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.
abstract: 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 (Et(4)N)[(Cl(2)PhPepS)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 (Et(4)N)[(Cl(2)PhPep{SO(2)}(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-SO(2) MO to a dpi(Fe)-pi*(NO)/d(z)2(Fe)-sigma*(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.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California 95064, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Rose
authors.forename: Nolan M
authors.initials: NM
authors.lastname: Betterley
authors.forename: Allen G
authors.initials: AG
authors.lastname: Oliver
authors.forename: Pradip K
authors.initials: PK
authors.lastname: Mascharak
mesh_terms: D001665:Binding Sites; D020134:Catalytic Domain; D018360:Crystallography, X-Ray / Q000379:methods; D005290:Ferric Compounds / Q000737:chemistry*; D006836:Hydro-Lyases / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008027:Light; D008958:Models, Molecular; D009569:Nitric Oxide / Q000737:chemistry*; D055668:Photochemical Processes; D010777:Photochemistry / Q000379:methods; D011487:Protein Conformation
chemical_list: D005290:Ferric Compounds; D009569:Nitric Oxide; D007501:Iron; D006836:Hydro-Lyases; C048831:nitrile hydratase