Nitrosyliron(III) hemoglobin: autoreduction and spectroscopy

Abstract

Nitrosyl complexes of the iron(III) forms of myoglobin, human hemoglobin, Glycera dibranchiata hemoglobins (Hbm and Hbh), and model iron(II) and iron(III) synthetic porphyrins including octaethylporphyrin (OEP) have been prepared. The iron(III) heme proteins are electron spin (paramagnetic) resonance (ESR) silent, while hexacoordinate solution structures are indicated for [Fe(OEP)(NO)2]ClO4 and for Hbm(II)NO, which has an ESR spectrum similar to that of Mb(II)NO and the hexacoordinate iron(II) model complex Fe(OEP)NO(BzIm). The splitting of the alpha- and beta-bands in the optical spectrum of Mb(III)NO and Hbh(III)NO contrasts markedly with the sharp, single bands observed in that of Hbm-(III)NO. The nondegeneracy of the dxz and dyz orbitals in Mb(III)NO and Hbh(III)NO is attributed to the influence of the distal histidine. Circular dichroism spectra were obtained for Hbm(III)NO, Hbm(II)NO, Hbh(III)NO, Hbh(II)NO, Mb(II)NO, and Mb(III)NO. The vicinal chiral center contribution that governs the heme protein CD leads to low Kuhn anisotropies, which have been used to assign certain electronic transitions. The Hb(III)NO spectrum is not stable but transforms into that of Hb(II)NO. This autoredox process follows kinetics that are first order in FeIIINO. The relative rates of autoreduction (25 degrees C, 1 atm NO) are Mb(III)NO less than Hbm(III)NO less than Hb alpha(III)NO less than HbA(III)NO. At high NO partial pressure or after "recycling" of HbA, the rates of reduction decrease. The first step in the reaction of NO with the ferric heme is the reversible formation of the formally iron(III) adduct. This reacts with another molecule of NO, generating the final heme(II)-NO via nitrosylation of NO itself or of an endogenous nucleophile. Kinetic and spectroscopic evidence shows involvement of trans-heme-(NO)2 in the reaction. The activation parameters delta H and delta S were determined. The overall reaction is photoenhanced.

Routes to S -nitroso-hemoglobin formation with heme redox and preferential reactivity in the β subunits

“…The observation of Wade and Castro (23) that met-Hb is extremely ineffective, as compared with, for example, horse metmyoglobin, in promoting the apparent nitrosation of exogenous nucleophiles may, in part, reflect interference from the intramolecular nitrosothiol-forming reaction. Our results also clarify certain observations of Addison and Stephanos (22), which were interpreted as indicating that the protein product obtained by complete reductive nitrosylation of methemoglobin is modified in the course of the reaction: Addison and Stephanos, however, suggested that this modification entails nitrosation of a histidine or lysine rather than the thiol. Finally, the variation of the levels of heme-Fe(II)NO hemoglobin complexes that have been detected in blood by EPR and other methods (33-40) may be influenced by the levels of heme-Fe(III)NO and other EPR undetectable species that can interchange with the heme Fe(II)NO species under different methods of sample treatment.…”

“…Although the reductive nitrosylation of oxidized heme-proteins has been studied for Ͼ60 years (18)(19)(20)(21)(22)(23)(24), the reaction of NO with methemoglobin remains only partially understood. In the case of simple monomeric heme-proteins, reductive nitrosylation has been most simply summarized by the equation:…”

Activation of Heme-regulated Eukaryotic Initiation Factor 2α Kinase by Nitric Oxide Is Induced by the Formation of a Five-coordinate NO-Heme Complex

“…1B). The addition of NO to the Fe(III)-HRI complex resulted in a shift in the Soret peak to 398 nm, indicating reductive nitrosylation of the complex, leading to the formation of a Fe(II)-HRI(NO) complex (data not shown), as reported for Hb (26). The Soret band of the Fe(II)-HRI(NO) complex at around 400 nm is characteristic of a five-coordinate NO-heme complex, analogous to sGC (27), CBS (28), CooA (29), and cytochrome cЈ from denitrifying bacteria (30,31), and is distinct from spectra of the six-coordinate NO-heme, which contain Soret bands at around 415-425 nm (Table I).…”

Interaction of Nitric Oxide with Human Heme Oxygenase-1

“…This suggests that coordination of the NO to the heme iron is similar in both hemoproteins, although the broader bandwidth observed with the hHO-1 complex suggests that the NO has greater mobility in that protein. Generally, as is the case for the binding of NO to ferric hemoproteins, these results strongly argue that the NO is bound perpendicular to the heme face rather than at an angle (53)(54)(55)(56), in contrast to its orientation when bound to ferrous hemoproteins. Furthermore, in both met-Mb and ferric hHO-1-heme, the iron in the absence of NO is coordinated to a water molecule.…”