Linkage isomerization in heme-NOx compounds: understanding NO, nitrite, and hyponitrite interactions with iron porphyrins

Abstract

Nitric oxide (NO) and its derivatives such as nitrite and hyponitrite are biologically important species of relevance to human health. Much of their physiological relevance stems from their interactions with the iron centers in heme proteins. The chemical reactivities displayed by the heme-NOx species (NOx = NO, nitrite, hyponitrite) are a function of the binding modes of the NOx ligands. Hence, an understanding of the types of binding modes extant in heme-NOx compounds is important if we are to unravel the inherent chemical properties of these NOx metabolites. In this Forum Article, the experimentally characterized linkage isomers of heme-NOx models and proteins are presented and reviewed. Nitrosyl linkage isomers of synthetic iron and ruthenium porphyrins have been generated by photolysis at low temperatures and characterized by spectroscopy and density functional theory calculations. Nitrite linkage isomers in synthetic metalloporphyrin derivatives have been generated from photolysis experiments and in low-temperature matrices. In the case of nitrite adducts of heme proteins, both N and O binding have been determined crystallographically, and the role of the distal H-bonding residue in myoglobin in directing the O-binding mode of nitrite has been explored using mutagenesis. To date, only one synthetic metalloporphyrin complex containing a hyponitrite ligand (displaying an O-binding mode) has been characterized by crystallography. This is contrasted with other hyponitrite binding modes experimentally determined for coordination compounds and computationally for NO reductase enzymes. Although linkage isomerism in heme-NOx derivatives is still in its infancy, opportunities now exist for a detailed exploration of the existence and stabilities of the metastable states in both heme models and heme proteins.

Figure 1

Representative approaches of the NO molecule towards the Fe center in heme proteins.

Figure 2

Metal-NO binding modes.

Figure 3

Difference spectra (spectrum after 15 min irradiation minus spectrum prior to irradiation) for the ¹⁴NO (bottom) and ¹⁵NO (top) labeled (OEP)Ru(NO)(O-i-C₅H₁₁) compound.

Figure 4

Light-induced generation of metastable nitrosyl linkage isomers during irradiation of (OEP)Ru(NO)(O-i-C₅H₁₁) as a KBr pellet.

Figure 5

Optimized geometry of the metastable side-on nitrosyl linkage isomer of (OEP)Ru(NO)Cl.

Figure 6

Light-induced generation of the metastable isonitrosyl linkage isomer of (TTP)Fe(NO) as a KBr pellet.

Figure 7

Calculated geometries of the ground-state (porphine)Fe(NO) (top) and its metastable isonitrosyl derivative (bottom) viewed from three different directions (reproduced in part from reference ). Valence shells of the H, C, N, and O atoms were described by a double-ζSTO basis set extended with a polarization function, whereas the 3s, 3p, and 3d shells on Fe were described by a triple-ζSTO basis set (ADF program package).

Figure 8

Electron localization function (ELF) of the ground-state (porphine)Fe(NO)(NO₂), with a plotted isosurface value of 0.8 (Reproduced with permission from reference . Copyright 2006 The American Chemical Society.)

Figure 9

Light-induced transformations upon irradiation of (TPP)Fe(NO)(NO₂) as a KBr pellet at low temperatures.

Figure 10

Calculated energies and representative structures for the linkage isomers of (porphine)Fe(NO)(NO₂).//= axial ligand planes are coplanar; ⊥= axial ligand planes are mutually perpendicular; MSa_L and MSc_L are the isomers displaying linear FeNO and FeON groups, respectively. (Reproduced from reference . Copyright 2004 The American Chemical Society.)

Figure 11

Nitrite binding modes to monometallic centers.

Figure 12

Proposed mechanisms for the linkage isomerization reactions after photolysis of the ground-state nitro (TPP)Co(NO₂) (A) and nitrito (TPP)Mn(ONO) (B) compounds.

Figure 13

Reaction of nitrogen dioxide with sublimed layers of (TPP)Fe to give initially (TPP)Fe(ONO) followed by exogenous ligand-induced isomerization to the nitro isomer.

Figure 14

The heme active sites of the N-bound nitrite adducts of cytochrome cd1 NiR from Paraccocus pantotrophus (top left; 1.8 Å resolution; PDB access code 1AAQ), the sulfite reductase hemoprotein from Escherichia coli (top right; 2.1 Å resolution; PDB access code 3GEO), cytochrome c NiR from Wolinella succinogenes (bottom left; 1.6 Å resolution) and its Y218F mutant (bottom right; 1.75 Å resolution; PDB access code 3BNH).

Figure 15

The F_o – F_c omit electron density maps (contoured at 3σ) and final models of the heme environments of the O-bound nitrite adducts of (A) wildtype horse heart ferric Mb (1.20 Å resolution; PDB access code 2FRF), (B) Mn^III-substituted Mb (1.60 Å resolution; PDB access code 2O5O), and (C) Co^III-substituted Mb (1.60 Å resolution; PDB access code 2O5S).

Figure 16

The F_o – F_c omit electron density maps (contoured at 3σ) and final models of the heme environments of the O-bound nitrite adduct of ferric human Hb (1.80 Å resolution; PDB access code 3D7O).74

Figure 17

Sketches of the trans- and cis-FeONO conformations.

Figure 18

Sketches of the actives sites of wild-type Mb (left), the H64V mutant (middle), and the H64V/V67R double-mutant (right).

Figure 19

The F_o – F_c omit electron density maps (contoured at 3σ) and final models of the heme environments of (A) the N-bound nitrite adduct of the ferric Mb H64V mutant (1.95 Å resolution; PDB access code 3HEP), and (B) the Obound nitrite adduct the ferric Mb H64V/V67R double mutant (2.0 Å resolution; PDB access code 3HEO).

Figure 20

Probable protonation pathways involving the bound nitrite ligands to generate FeNO (left; I) and free NO (right, II).

Figure 21

Redox congeners of the NO dimer.

Figure 22

The cis and trans forms of the hyponitrite dianion.

Figure 23

Structurally characterized metal hyponitrite binding modes in inorganic coordination compounds.

Figure 24

Intermediates in the three putative mechanisms for NO reduction by NORs and HCOs.

Figure 25

A proposed intermediate (E) and a structurally characterized (F) bimetallic heme-hyponitrite moiety.

Figure 26

Molecular structure of [(OEP)Fe]₂(μ-ONNO). Top: Hydrogen atoms and the CH₂Cl₂ solvates have been omitted for clarity. Bottom: With CH₂Cl₂ solvates, but without non-solvate H atoms. Selected bond lengths (Å) and angles (°): Fe–O = 1.889(2), O–N = 1.375(2), N–N = 1.250(3), Fe–N(por) = 2.049(2)-2.064(2), ∠FeON = 118.56(12), ∠NNO = 108.5(2). (Reproduced from reference . Copyright 2009 The American Chemical Society.)

Figure 27

Frontier spin orbitals for high-spin [(porphine)Fe]₂(μ-ONNO). HOSO and LUSO denote the highest occupied and the lowest unoccupied spin orbitals, respectively (reproduced from reference 124).