Spectroscopic and functional characterization of nitrophorin 7 from the blood-feeding insect Rhodnius prolixus reveals an important role of its isoform-specific N-terminus for proper protein function

Nitrophorins (NPs) are a class of NO-transporting and histamine-sequestering heme b proteins that occur in the saliva of the bloodsucking insect Rhodnius prolixus. A detailed study of the newly described member, NP7, is presented herein. NO association constants for NP7 [KIII(eq)(NO)] reveal a drastic change when the pH is varied from 5.5 (reflecting the insect's saliva) to slightly above plasma pH (7.5) (>10(9) M-1 --> 4.0 x 10(6) M-1); thus, the protein promotes the storage of NO in the insect's saliva and its release inside the victim's tissues. In contrast to the other nitrophorins, NP1-4, histamine sequestering cannot be accomplished in vivo due to the low binding constant [KIII(eq)(histamine)] of 10(5) M-1 compared to the histamine concentration of 1-10 x 10(-9) M in the blood. A major part of this study deals with the N-terminus, 1Leu-Pro-Gly-Glu-Cys5 of NP7, which is not found in NP1-4. Since NP7 has not been isolated from the insects but was recognized in a cDNA library instead, the N-terminal site of signal peptidase cleavage upon protein secretion was predicted by the program SIGNALP [Andersen, J. F., Gudderra, N. P., Francischetti, I. M. B., Valenzuela, J. G., and Ribeiro, J. M. C. (2004) Biochemistry 43, 6987-6994]. In marked contrast to wild-type NP7, NP7(Delta1-3) exhibits a very high NO affinity at pH 7.5 [KIII(eq)(NO) approximately 10(9) M-1], suggesting that the release of NO in the plasma cannot efficiently be accomplished by the truncated form. Comparison of the reduction potentials of both constructs by spectroelectrochemistry revealed an average increase of +85 mV for various distal ligands bound to the heme iron when the 1Leu-Pro-Gly3 peptide was removed. However, 1H NMR and EPR spectroscopy show that the electronic properties of the FeIII cofactor are similar in both wild-type NP7 and NP7(Delta1-3). Further, thermal denaturation that revealed a higher stability of wild-type NP7 compared to NP7(Delta1-3), in combination with a homology model based on the NP2 crystal structure (rmsd = 0.39 A), suggests that interaction of the 1Leu-Pro-Gly3 peptide with the A-B and/or G-H loops is key for proper protein function.

Solution NMR Studies of Iron(II) Spin-Crossover Complexes

The 1H NMR spectra of a series of mono- and dinuclear pyridine complexes [FeL1(R1/R2)(py)2] and [Fe2L2(R1/R2)(py)4] have been investigated in a mixed toluene-d8/pyridine-d5 solution. The equatorial tetradentade Schiff base like ligands L1(R1/R2) and L2(R1/R2) with a N2O22- coordination sphere for each metal center have been obtained by condensation of a substituted malonodialdehyde (R1/R2 are Me/COOEt, Me/COMe, or OEt/COOEt) with o-phenylenediamine (L1(R1/R2)) or 1,2,4,5-tetraaminobenzene (L2(R1/R2)). The 1H NMR resonances were assigned by comparison of differently substituted complexes in combination with a line-width comparison. The 1H NMR shifts from 188 to 358 K show a strong influence of the spin state of the iron center. The behavior of the pure high-spin iron(II) complexes is close to ideal Curie behavior. Analysis of the resonance shifts of the spin-transition complexes can be used for determining the high-spin mole fraction of the complex in solution at different temperatures. Magnetic susceptibility measurements in solution using the Evans method were made for all six complexes. Significant differences between the spin-transition behavior of the complexes in solution of those in the solid state were found. However, the plots of microeff as a function of temperature obtained using the Evans method and those obtained by interpretation of the NMR shifts were virtually identical. The isotropic shifts of protons in the complexes proved to be suitable tools for following a spin transition in solution. Comparison of the microeff plots of the mono- and dinuclear complexes in solution reveals slight differences between the steepness of the curves that may be attributable to cooperative interactions between the metal centers in the case of the dinuclear complexes.

Effect of the N-terminus on heme cavity structure, ligand equilibrium, rate constants, and reduction potentials of nitrophorin 2 from Rhodnius prolixus

The D1A mutant of recombinant NP2 has been prepared and shown to have the expression-initiation methionine-0 cleaved during expression in E. coli, as is the case for recombinant NP4, where Ala is the first amino acid for the recombinant protein as well as for the mature native protein. The heme substituent 1H NMR chemical shifts of NP2-D1A and those of its imidazole, N-methylimidazole, and cyanide complexes are rather different from those of NP2-M0D1. This difference is likely due to the much smaller size of the N-terminal amino acid (A) of NP2-D1A, which allows the formation of the closed loop form of this protein, as it does for NP4 (Weichsel, A., Andersen, J. F., Roberts, S. A., and Montfort, W. R. (2000) Nature Struct. Biol. 7, 551-554). The ratio of the two hemin rotational isomers A and B is different for the two proteins, and the rate at which the A:B ratio reaches equilibrium is strikingly different (NP2-M0D1 t1/2 for heme rotation approximately 2 h, NP2-D1A t1/2 approximately 43 h). This difference is consistent with the high stability of the closed loop form of the NP2-D1A protein and infrequent opening of the loops that could allow heme to at least partially exit the binding pocket in order to rotate about its alpha,gamma-meso axis. Consistent with this, the rates of histamine binding and release to/from NP2-D1A are significantly slower than those for NP2-M0D1 at pH 7.5. This work suggests that care must be taken in interpreting data obtained from proteins that carry the expression-initiation M0.

Models of the cytochromes: crystal structures and EPR spectral characterization of low-spin bis-imidazole complexes of (OETPP)Fe(III) having intermediate ligand plane dihedral angles

The preparation, EPR spectra, and crystal structures of octaethyltetraphenylporphyrinatoiron(III) having two imidazole, N-benzylimidazole, and N-methylimidazole axial ligands are reported, [(OETPP)Fe(HIm)2]Cl, [(OETPP)Fe(N-BzIm)2]Cl, and [(OETPP)Fe(N-MeIm)2]Cl. Despite large variation in axial ligand size, the unit cell parameters for all complexes are very similar; each structure has the same basic motif, with large voids formed by the extended porphyrin framework (filled by ordered or disordered axial ligands and disordered solvent), which allows differently sized ligands to fit within the same cell dimensions. Each porphyrin core adopts a saddled conformation with absolute value(deltaC(beta)) = 1.13-1.15 A. The dihedral angles between axial ligand planes, delta phi, are far from being either ideal parallel or perpendicular: 30.1 degrees, 57.2 degrees for [(OETPP)Fe(HIm)2]Cl (molecules 1 and 2), 56.8 degrees for [(OETPP)Fe(N-BzIm)(2)]Cl, and 16.0 degrees, 44.6 degrees, 59.6 degrees, and 88.1 degrees for [(OETPP)Fe(N-MeIm)2]Cl, which has disordered axial ligands. Among the complexes of this study, an axial ligand delta phi of 56.8 degrees is found to be the largest "parallel" angle (as defined by the observation of a normal rhombic or Type II EPR signal (N-BzIm, g = 3.08, 2.19, 1.31)), while 57.2 degrees is found to be the smallest "perpendicular" delta phi (as defined by the observation of a "large gmax" or Type I EPR signal (HIm, gmax = 3.24)). From the results of this study, it is clear that the size of the largest g for Types I and II complexes varies continuously, with no break between the two. While the switch in EPR signal type, from Type II to Type I, appears to be very sharp in this study, this may be somewhat artificial based upon limited numbers of examples and the required saddle distortion of the (OETPP)Fe(III) complexes. However, in comparison to several proteins with dihedral angles near 60 degrees and Type II EPR spectra, we may conclude that the switch in EPR signal type occurs near 57 degrees +/- 3-5 degrees.

Overexpression in Escherichia coli and functional reconstitution of the liposome binding ferriheme protein nitrophorin 7 from the bloodsucking bug Rhodnius prolixus

A number of ferriheme proteins, termed nitrophorins (NPs), occur in the saliva of the bloodsucking insect Rhodnius prolixus ('kissing bug'), which is a vector for Chagas' disease. Nitrophorins bind the heme b cofactor in the beta-barrel of their lipocalin fold, which is further anchored through a proximal histidine-Fe(III) bond. The distal Fe(III) coordination site then binds nitric oxide (NO) for delivery into a host's tissues during blood feeding, where, upon NO release, the distal Fe(III) site acts as a histamine trap to delay the victim's immune response. Previously, four nitrophorins from R. prolixus, NP1 to NP4, have been extensively characterized. Recently, another nitrophorin, NP7, was discovered in a cDNA library derived from the same insect. Among the R. prolixus nitrophorins, NP7 was found to be unique in its ability to bind to negatively charged cell surfaces. However, the yield of functional recombinant NP7 was rather low when the established protocol for NP1-4 was followed. Here, we report on a novel expression and reconstitution method for NP7 that yields sufficient amounts of pure protein for extensive characterization (28-fold increase). This method may prove useful for the reconstitution of other proteins with a lipocalin fold.

Assignment of the ferriheme resonances of the low-spin complexes of nitrophorins 1 and 4 by (1)H and (13)C NMR spectroscopy: comparison to structural data obtained from X-ray crystallography

In this work we report the assignment of the majority of the ferriheme resonances of low-spin nitrophorins (NP) 1 and 4 and compare them to those of NP2, published previously. It is found that the structure of the ferriheme complexes of NP1 and NP4, in terms of the orientation of the ligand(s), can be determined with good accuracy by NMR techniques in the low-spin forms and that angle plots proposed previously (Shokhirev, N. V.; Walker, F. A. J. Biol. Inorg. Chem. 1998, 3, 581-594) describe the angle of the effective nodal plane of the axial ligands in solution. The effective nodal plane of low-spin NP1, NP4, and NP2 complexes is in all cases of imidazole and histamine complexes quite similar to the average of the His-59 or -57 and the exogenous ligand angles seen in the X-ray crystal structures. For the cyanide complexes of the nitrophorins, however, the effective nodal plane of the axial ligand does not coincide with the actual histidine-imidazole plane orientation. This appears to be a result of the contribution of an additional source of asymmetry, the orientation of one of the zero-ruffling lines of the heme. Probably this effect exists for the imidazole and histamine complexes as well, but because the effect of asymmetry that occurs from planar exogenous axial ligands is much larger than the effect of heme ruffling the effect of the zero-ruffling line can only be detected for the cyanide complexes, where the only ligand plane is that of the proximal histidine. The three-dimensional structures of the three NP-CN complexes, including that of NP2-CN reported herein, confirm the high degree of ruffling of these complexes. There is an equilibrium between the two heme orientations (A and B) that depends on the heme cavity shape and changes somewhat with exogenous axial ligand. The A:B ratio can be much more accurately measured by NMR spectroscopy than by X-ray crystallography.

Fe L-edge X-ray absorption spectroscopy of low-spin heme relative to non-heme Fe complexes: delocalization of Fe d-electrons into the porphyrin ligand

Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin pi-->pi(*) transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P-->Fe sigma and pi-donation and Fe-->P pi back-bonding. We find that pi-donation to Fe(III) is much larger than pi back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry.

Assignment of the ferriheme resonances of the high-spin forms of nitrophorins 1 and 4 by 1H NMR spectroscopy: comparison to structural data obtained from X-ray crystallography

In this work, we report the assignment of the majority of the ferriheme resonances of high-spin nitrophorins (NPs) 1 and 4 and compare them to those of NP2, published previously. It is found that the structures of the ferriheme complexes of NP1 and NP4, in terms of the orientation of the histidine imidazole ligand, can be described with good accuracy by NMR techniques and that the angle plot proposed previously for the high-spin form of the NPs (Shokhireva, T. Kh.; Shokhirev, N. V.; Walker, F. A. Biochemistry 2003, 42, 679-693) describes the angle of the effective nodal plane of the axial histidine imidazole in solution. There is an equilibrium between the two heme orientations (A and B), which depends on the heme cavity shape, which can be altered by mutation of amino acids with side chains (phenyl vs tyrosyl) near the potential position where a heme vinyl group would be in one of the isomers. The A:B ratio can be much more accurately measured by NMR spectroscopy than by X-ray crystallography.

NMR and structural investigations of a nonplanar iron corrolate: modified patterns of spin delocalization and coupling in a slightly saddled chloroiron(III) corrolate radical

An undecasubstituted chloroiron corrolate, octamethyltriphenylcorrolatoiron chloride, (OMTPCorr)FeCl, has been synthesized and studied by X-ray crystallography and (1)H and (13)C NMR spectroscopy. It is found that, although the structure is slightly saddled, the average methyl out-of-plane distance is only 0.63 Angstroms, while it is much greater for the dodecasubstituted porphyrinate analogue (OMTPP)FeCl (1.19 Angstroms) (Cheng, R.-J.; Chen, P.-Y.; Gau, P.-R.; Chen, C.-C.; Peng, S.-M. J. Am. Chem. Soc. 1997, 119, 2563-2569). In addition, the distance of iron from the mean plane of the four macrocycle nitrogens is also smaller for (OMTPCorr)FeCl (0.387 Angstroms) than for (OMTPP)FeCl (0.46 Angstroms). The (1)H and (13)C NMR spectra of (OMTPCorr)FeCl, as well as the chloroiron complexes of triphenylcorrolate, (TPCorr)FeCl; 7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrolate, (DMHECorr)FeCl; 7,8,12,13-tetramethyl-2,3,17,18-tetraethylcorrolate, (TMTECorr)FeCl; and the phenyliron complex of 7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrolate, (DMHECorr)FePh, have been assigned, and the spin densities at the carbons that are part of the aromatic ring of the corrole macrocycle have been divided into the part due to spin delocalization by corrole --> Fe pi donation and the part due to the unpaired electron present on the corrole ring. It is found that although the spin density at the beta-pyrrole positions is fairly similar to that of (TPCorr)FeCl, the meso-phenyl-carbon shift differences delta(m) - delta(p) are opposite in sign of those of (TPCorr)FeCl. This finding suggests that the radical electron is ferromagnetically coupled to the unpaired electrons on iron, rather than antiferromagnetically coupled, as in all of the other chloroiron corrolates. The solution magnetic moment was measured for (OMTPCorr)FeCl and found to be mu(eff) = 4.7 +/- 0.5 micro(B), consistent with S = 2 and ferromagnetic coupling. From this study, two conclusions may be reached about iron corrolates: (1) the spin states of chloroiron corrolates are extremely sensitive to the out-of-plane distance of iron, and (2) pyrrole-H or -C shifts are not useful in delineating the spin state and electron configuration of (anion)iron corrolates.

NMR and EPR studies of chloroiron(III) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities

The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)2]+ (L = Im-d4, 2-MeHIm, 4-Me2NPy, Py, and 4-CNPy), have been investigated. The full resonance assignments of the [(TPC)Fe(L)2]+ complexes of this study have been made from correlation spectroscopy (COSY) and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF) calculations. Unlike the [(OEC)Fe(L)2]+ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)2]+ complexes of this study indicate that the ground state is S = 1/2 for each bisligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)2]+ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me2NPyH+ pKa = 9.7, H2Im+ pKa = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me2NPy bisligand complexes studied over the temperature range of the NMR investigations shows that they have mixed (dxy)2(dxz,dyz)3/(dxzdyz)4(dxy)1 electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate that it has a pure (dxzdyz)4(dxy)1 ground state and that it is ruffled, because ruffling mixes the a(2u)(pi)-like orbital of the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift of the pyrroline-H (-6.5 ppm at -40 degrees C) and thus the negative spin density at the pyrroline-alpha-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)2]+, whose pyrroline-H chemical shift is -36 ppm at 25 degrees C (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct.

Models of the membrane-bound cytochromes: mössbauer spectra of crystalline low-spin ferriheme complexes having axial ligand plane dihedral angles ranging from 0 degree to 90 degrees

Crystalline samples of four low-spin Fe(III) octaalkyltetraphenylporphyrinate and two low-spin Fe(III) tetramesitylporphyrinate complexes, all of which are models of the bis-histidine-coordinated cytochromes of mitochondrial complexes II, III, and IV and chloroplast complex b(6)f, and whose molecular structures and EPR spectra have been reported previously, have been investigated in detail by Mössbauer spectroscopy. The six complexes and the dihedral angles between axial ligand planes of each are [(TMP)Fe(1-MeIm)(2)]ClO(4) (0 degree), paral-[(OMTPP)Fe(1-MeIm)(2)]Cl (19.5 degrees), paral-[(TMP)Fe(5-MeHIm)(2)]ClO(4) (26 degrees, 30 degrees for two molecules in the unit cell whose EPR spectra overlap), [(OETPP)Fe(4-Me(2)NPy)(2)]Cl (70 degrees), perp-[(OETPP)Fe(1-MeIm)(2)]Cl (73 degrees), and perp-[(OMTPP)Fe(1-MeIm)(2)]Cl (90 degrees). Of these, the first three have been shown to exhibit normal rhombic EPR spectra, each with three clearly resolved g-values, while the last three have been shown to exhibit "large g(max)" EPR spectra at 4.2 K. It is found that the hyperfine coupling constants of the complexes are consistent with those reported previously for low-spin ferriheme systems, with the largest-magnitude hyperfine coupling constant, A(zz), being considerably smaller for the "parallel" complexes (400-540 kG) than for the strictly perpendicular complex (902 kG), A(xx) being negative for all six complexes, and A(zz) and A(xx) being of similar magnitude for the "parallel" complexes (for example, for [(TMP)Fe(1-MeIm)(2)]Cl, A(zz) = 400 kG, A(xx) = -400 kG). In all cases, A(yy) is small but difficult to estimate with accuracy. With results for six structurally characterized model systems, we find for the first time qualitative correlations of g(zz), A(zz), and DeltaE(Q) with axial ligand plane dihedral angle Deltavarphi.

The heme environment of mouse neuroglobin: histidine imidazole plane orientations obtained from solution NMR and EPR spectroscopy as compared with X-ray crystallography

The 1H NMR chemical shifts of the heme methyl groups of the ferriheme complex of metneuroglobin (Du et al. in J. Am. Chem. Soc. 125:8080-8081, 2003) predict orientations of the axial histidine ligands (Shokhirev and Walker in J. Biol. Inorg. Chem. 3:581-594, 1998) that are not consistent with the X-ray data (Vallone et al. in Proteins Struct. Funct. Bioinf. 56:85-94, 2004), and the EPR spectrum (Vinck et al. in J. Am. Chem. Soc. 126:4516-4517, 2004) is only marginally consistent with these data. The reasons for these inconsistencies appear to be rooted in the high degree of aqueous solution exposure of the heme group and the fact that there are no strong hydrogen-bond acceptors for the histidine imidazole N-H protons provided by the protein. Similar inconsistencies may exist for other water-soluble heme proteins, and 1H NMR spectroscopy provides a simple means to verify whether the solution structure of the heme center is the same as or different from that in the crystalline state.

Iron corrolates: Unambiguous chloroiron(III) (corrolate)2− π-cation radicals

The structures, electron configurations, magnetic susceptibilities, spectroscopic properties, molecular orbital energies and spin density distributions, redox properties and reactivities of iron corrolates having chloride, phenyl, pyridine, NO and other ligands are reviewed. It is shown that with one very strong donor ligand such as phenyl anion the electron configuration of the metal is d(4)S=1 Fe(IV) coordinated to a (corrolate)(3-) anion, while with one weaker donor ligand such as chloride or other halide, the electron configuration is d(5)S=3/2 Fe(III) coordinated to a (corrolate)(2-.) pi-cation radical, with antiferromagnetic coupling between the metal and corrolate radical electron. Many of these complexes have been studied by electrochemical techniques and have rich redox reactivity, in most cases involving two 1-electron oxidations and two 1-electron reductions, and it is not possible to tell, from the shapes of cyclic voltammetric waves, whether the electron is added or removed from the metal or the macrocycle; often infrared, UV-Vis, or EPR spectroscopy can provide this information. (1)H and (13)C NMR spectroscopic methods are most useful in delineating the spin state and pattern of spin density distribution of the complexes listed above, as would also be expected to be the case for the recently-reported formal Fe(V)O corrolate, if this complex were stable enough for characterization by NMR spectroscopy. Iron, manganese and chromium corrolates can be oxidized by iodosylbenzene and other common oxidants used previously with metalloporphyrinates to effect efficient oxidation of substrates. Whether the "resting state" form of these complexes, most generally in the case of iron [FeCl(Corr)], actually has the electron configuration Fe(IV)(Corr)(3-) or Fe(III)(Corr)(2-.) is not relevant to the high-valent reactivity of the complex.

Structure and stereochemistry of products of hydroxylation of human steroid hormones by a housefly cytochrome P450 (CYP6A1)

The structure and stereochemistry of nine steroid metabolites isolated in quantities ranging from 0.15 to 1.8 mg were determined using a variety of NMR techniques, including heteronuclear multiple bond correlation (HMBC) using broadband adiabatic 13C pulses and phase-sensitive data presentation. Testosterone, androstenedione and progesterone were oxidized with housefly cytochrome P450 6A1 enzyme reconstituted in vitro with housefly NADPH cytochrome P450 reductase and cytochrome b5. NMR analysis in CD3OD using a modified HMBC sequence as well as 2D heteronuclear single quantum correlation (HSQC), COSY and nuclear Overhauser and exchange spectroscopy (NOESY), combined with a detailed analysis of J couplings showed that hydroxylation occurs exclusively on the beta-face of the steroids, at positions 2, 12, and 15.

NMR and EPR Studies of Low-Spin Fe(III) Complexes of meso-Tetra-(2,6-Disubstituted Phenyl)Porphyrinates Complexed to Imidazoles and Pyridines of Widely Differing Basicities

A series of bis-axially ligated complexes of iron(III) tetramesitylporphyrin, TMPFe(III), tetra-(2,6-dibromophenyl)porphyrin, (2,6-Br2)4TPPFe(III), tetra-(2,6-dichlorophenyl)porphyrin, (2,6-Cl2)4TPPFe(III), tetra-(2,6-difluorophenyl)porphyrin, (2,6-F2)4TPPFe(III), and tetra-(2,6-dimethoxyphenyl)porphyrin, (2,6-(OMe)2)4TPPFe(III), where the axial ligands are 1-methylimidazole, 2-methylimidazole, and a series of nine substituted pyridines ranging in basicity from 4-(dimethylamino)pyridine (pK(a)(PyH(+)) = 9.70) to 3- and 4-cyanopyridine (pKa(PyH+) = 1.45 and 1.1, respectively), have been prepared and characterized by EPR and 1H NMR spectroscopy. The EPR spectra, recorded at 4.2 K, show "large g(max)", rhombic, or axial signals, depending on the iron porphyrinate and axial ligand, with the g(max) value decreasing as the basicity of the pyridine decreases, thus indicating a change in electron configuration from (d(xy))2(d(xz),d(yz)3 to (d(xz),d(yz))4(d(xy))1 through each series at this low temperature. Over the temperature range of the NMR investigations (183-313 K), most of the high-basicity pyridine complexes of all five iron(III) porphyrinates exhibit simple Curie temperature dependence of their pyrrole-H paramagnetic shifts and beta-pyrrole spin densities, rho(C) approximately 0.015-0.017, that are indicative of the S = 1/2 (d(xy))(2)(d(xz),d(yz))(3) electron configuration, while the temperature dependences of the pyrrole-H resonances of the lower-basicity pyridine complexes (pK(a)(PyH(+)) < 6.00) show significant deviations from simple Curie behavior which could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in this laboratory that includes consideration of a thermally accessible excited state. In most cases, the ground state of the lower-basicity pyridine complexes is an S = 1/2 state with a mixed (d(xy))2(d(xz),d(yz))3/(d(xz),d(yz))4(d(xy))1 electron configuration, indicating that these two are so close in energy that they cannot be separated by analysis of the NMR shifts; however, for the TMPFe(III) complexes with 3- and 4-CNPy, the ground states were found to be fairly pure (d(xz),d(yz))4(d(xy))1 electron configurations. In all but one case of the intermediate- to low-basicity pyridine complexes of the five iron(III) porphyrinates, the excited state is found to be S = 3/2, with a (d(xz),d(yz))3(d(xy))1(d(z)2)1 electron configuration, lying some 120-680 cm(-1) higher in energy, depending on the particular porphyrinate and axial ligand. Full analysis of the paramagnetic shifts to allow separation of the contact and pseudocontact contributions could be achieved only for the [TMPFe(L)2]+ series of complexes.

Nitric oxide interaction with insect nitrophorins and thoughts on the electron configuration of the {FeNO}6 complex

The nitrophorins are NO-carrying heme proteins that are found in the saliva of two species of blood-sucking insects, the kissing bug (Rhodnius prolixus) and the bedbug (Cimex lectularius). In both insects the NO is bound to the ferric form of the protein, which gives rise to Kds in the micromolar to nanomolar range, and thus upon injection of the saliva into the tissues of the victim the NO can dissociate to cause vasodilation and inhibition of platelet aggregation. The structures of the proteins from each of these insects are unique, and each has a large component of beta-sheet structure, which is unusual for heme proteins. While the Rhodnius nitrophorins increase the effectiveness of their NO-heme proteins by also binding histamine, secreted by the victim in response to the bite, to the heme, the Cimex nitrophorin does not bind histamine but rather binds two molecules of NO reversibly, one to the heme and the other to the cysteine thiolate which serves as the heme ligand in the absence of NO. This requires homolytic cleavage of the Fe-S-Cys bond, which produces an EPR-active Fe(II)-NO complex having the {FeNO}7 electron configuration. For the Rhodnius nitrophorins, the heme of the {FeNO}6 stable NO complex could have the limiting electron configurations Fe(III)-NO+ or Fe(II)-NO+. While vibrational spectroscopy suggests the latter and Mossbauer spectroscopy cannot differentiate between a purely diamagnetic Fe(II) center and a strongly antiferromagnetically coupled Fe(III)-NO* center, the strong ruffling of the heme (with alternate meso-carbons shifted significantly above and below the mean plane of the porphyrin, and concomitant shifts of the beta-pyrrole carbons above and below the mean plane of the porphyrin ring, to produce a very nonplanar porphyrin macrocycle) may suggest at least an important contribution of the latter. The strong ruffling would help to stabilize the (dxz, dyz)4(dxy)1 electron configuration of low-spin Fe(III) (but not low-spin Fe(II)), and the dxy orbital does not have correct symmetry for overlap with the half-filled pi* orbital of NO. This Fe(III)-NO* electron configuration would facilitate reversible dissociation of NO.

Magnetic Resonance Spectroscopic Investigations of the Electronic Ground and Excited States in Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins

A series of axially ligated complexes of iron(III) octamethyltetraphenylporphyrin, (OMTPP)Fe(III), octaethyltetraphenylporphyrin, (OETPP)Fe(III), its perfluorinated phenyl analogue, (F(20)OETPP)Fe(III), and tetra-(beta,beta'-tetramethylene)tetraphenylporphyrin, (TC(6)TPP)Fe(III), have been prepared and characterized by (1)H NMR spectroscopy: chloride, perchlorate, bis-4-(dimethylamino)pyridine, bis-1-methylimidazole, and bis-cyanide. Complete spectral assignments have been made using 1D and 2D techniques. The temperature dependences of the proton resonances of the complexes show significant deviations from simple Curie behavior and evidence of ligand exchange, ligand rotation, and porphyrin ring inversion at ambient temperatures. At temperatures below the point where dynamics effects contribute, the temperature dependences of the proton chemical shifts of the complexes could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in our laboratory that includes consideration of a thermally accessible excited state. The results show that, although the ground state differs for various axial ligand complexes and is usually fully consistent with that observed by EPR spectroscopy at 4.2 K, the excited state often has S = (3)/(2) (or S = (5)/(2) in the cases where the ground state has S = (3)/(2)). The EPR spectra (4.2 K) of bis-4-(dimethylamino)pyridine and bis-1-methylimidazole complexes show "large-g(max)" signals with g(max) = 3.20 and 3.12, respectively, and the latter also shows a normal rhombic EPR signal, indicating the presence of low-spin (LS) (d(xy))(2)(d(xz),d(yz))(3) ground states for both. The bis-cyanide complex also yields a large-g(max) EPR spectrum with g = 3.49 and other features that could suggest that some molecules have the (d(xz),d(yz))(4)(d(xy))(1) ground state. The EPR spectra of all five-coordinate chloride complexes have characteristic features of predominantly S = (5)/(2) ground-state systems with admixture of 1-10% of S = (3)/(2) character.

Kinetics of Ring Inversion in Strongly Nonplanar Iron(III) Octaalkyltetraphenylporphyrinates

The dynamics of porphyrin ring inversion of a number of Fe(III) complexes of octamethyltetraphenylporphyrin, (OMTPP)Fe(III); octaethyltetraphenylporphyrin, (OETPP)Fe(III); octaethyltetra(perfluorophenyl)porphyrin, (F(20)OETPP)Fe(III); and tetra-beta,beta'-tetramethylenetetraphenyl-porphyrin, (TC(6)TPP)Fe(III), having either one (Cl(-), ClO(4-)) or two [4-(dimethylamino)pyridine, 4-Me(2)NPy; 1-methylimidazole, 1-MeIm; tert-butylisocyanide, t-BuNC; or cyanide, CN(-)] axial ligands have been characterized by 1D dynamic NMR (DNMR) and 2D (1)H NOESY/EXSY spectroscopies as a function of temperature. The activation parameters, Delta H++, Delta S++, and Delta G++(298), and the extrapolated rate constants at 298 K for three chloride, one perchlorate, and three bis-(4-Me(2)NPy) complexes as well as [FeOETPP(1-MeIm)(2)]Cl, [FeOETPP(t-BuNC)(2)]ClO(4), and Na[FeOETPP(CN)(2)] have been determined. The results indicate that there is a wide range of flexibility for the porphyrin core (k(ex)(298) = 10-10(7) s(-1)) that decreases in the order TC(6)TPP > OMTPP > F(20)OETPP > or = OETPP, which correlates with increasing porphyrin nonplanarity. To determine the effect of axial ligands, we calculated the free energy of activation, Delta G++(298) for OETPPFe(III) bis-ligated with 4-Me(2)NPy, 1-MeIm, or 4-CNPy (approximately 59 kJ mol(-1)), and for complexes with small cylindrical ligands (t-BuNC and CN(-)) (approximately 37 kJ mol(-1)). These data suggest that the Delta G++(298) for planar ligand rotation is roughly 20-25 kJ mol(-1).

Characterization of the copper(II) binding site in the pink copper binding protein CusF by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy has been used to structurally characterize the copper-binding site in CusF protein from Escherichia coli. The EPR spectra indicate a single type II copper center with parameters typical for nitrogen and oxygen ligands (A(parallel) approximately 200 G, g(parallel) approximately 2.186, g(perpendicular) approximately 2.051). The pulsed EPR data show that one of the ligands to Cu2+ is an imidazole ring of a histidine residue. The remote amino nitrogen of this imidazole ring is readily observed by electron spin-echo envelope modulation spectroscopy, while the imino nitrogen that is directly coordinated to the Cu2+ ion is observed by pulsed electron-nuclear double resonance (ENDOR). In addition, the ENDOR spectra reveal the presence of one more nitrogen ligand that was assigned to be a deprotonated peptide nitrogen. Apart from the two nitrogen ligands, it has been established that there are two nearby hydroxyl protons, although whether these belong to a single equatorial water ligand or two equatorial hydroxide ligands is not known.

NMR and EPR Studies of the Bis(pyridine) and Bis(tert-butyl isocyanide) Complexes of Iron(III) Octaethylchlorin

The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))2 (d(xz)d(yz))3 to (d(xz)d(yz))4 (d(xy))1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large pi spin density on the adjacent carbons which are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (d(xz)d(yz))4 (d(xy))1 and (d(xy))2 (d(xz),d(yz))3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the alpha-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.

Magnetic Resonance and Structural Investigations of (Monooxooctaethylchlorinato)iron(III) Chloride and Its Bis(imidazole) Complex

(Monooxooctaethylchlorinato)iron(III) chloride, (oxo-OEC)FeCl, 1, has been investigated by X-ray crystallography and by 1H NMR spectroscopy. Its bis(imidazole-d4) complex has been studied by multidimensional 1H NMR and EPR spectroscopies, and the results are compared to those for the bis(Im-d4) complex of (octaethylchlorinato)iron(III) chloride, (OEC)FeCl, 2. EPR and NMR results show that both [(oxo-OEC)Fe(Im-d4)2]Cl and [(OEC)Fe(Im-d4)2]Cl are low-spin Fe(III) complexes with (d(xy))2 (d(xz),d(yz))3 electronic ground states, both at 4.2 K (EPR spectra) and at ambient temperatures utilized for solution NMR studies. The pattern of chemical shifts of the pyrrole-CH2 and meso protons are similar, with the 8,17-carbons having the largest and the 12,13-carbons having the smallest spin densities in each case, except that [(OEC)Fe(Im-d4)2]Cl has a slightly wider range of pyrrole-CH2 chemical shifts and more resonances are observed for [(oxo-OEC)Fe(Im-d4)2]Cl due to its lower symmetry. Full proton resonance assignments for both complexes have been made from COSY, NOESY, and NOE difference experiments.

Heme-assisted S-nitrosation of a proximal thiolate in a nitric oxide transport protein

Certain bloodsucking insects deliver nitric oxide (NO) while feeding, to induce vasodilation and inhibit blood coagulation. We have expressed, characterized, and determined the crystal structure of the Cimex lectularius (bedbug) nitrophorin, the protein responsible for NO storage and delivery, to understand how the insect successfully handles this reactive molecule. Surprisingly, NO binds not only to the ferric nitrophorin heme, but it can also be stored as an S-nitroso (SNO) conjugate of the proximal heme cysteine (Cys-60) when present at higher concentrations. EPR- and UV-visible spectroscopies, and a crystallographic structure determination to 1.75-A resolution, reveal SNO formation to proceed with reduction of the heme iron, yielding an Fe-NO complex. Stopped-flow kinetic measurements indicate that an ordered reaction mechanism takes place: initial NO binding occurs at the ferric heme and is followed by heme reduction, Cys-60 release from the heme iron, and SNO formation. Release of NO occurs through a reversal of these steps. These data provide, to our knowledge, the first view of reversible metal-assisted SNO formation in a protein and suggest a mechanism for its role in NO release from ferrous heme. This mechanism and Cimex nitrophorin structure are completely unlike those of the nitrophorins from Rhodnius prolixus, where NO protection is provided by a large conformational change that buries the heme nitrosyl complex, highlighting the remarkable evolution of proteins that assist insects in bloodfeeding.

Drosophila melanogaster CYP6A8, an insect P450 that catalyzes lauric acid (ω-1)-hydroxylation

Only a handful of P450 genes have been functionally characterized from the approximately 90 recently identified in the genome of Drosophila melanogaster. Cyp6a8 encodes a 506-amino acid protein with 53.6% amino acid identity with CYP6A2. CYP6A2 has been shown to catalyze the metabolism of several insecticides including aldrin and heptachlor. CYP6A8 is expressed at many developmental stages as well as in adult life. CYP6A8 was produced in Saccharomyces cerevisiae and enzymatically characterized after catalytic activity was reconstituted with D. melanogaster P450 reductase and NADPH. Although several saturated or non-saturated fatty acids were not metabolized by CYP6A8, lauric acid (C12:0), a short-chain unsaturated fatty acid, was oxidized by CYP6A8 to produce 11-hydroxylauric acid with an apparent V(max) of 25 nmol/min/nmol P450. This is the first report showing that a member of the CYP6 family catalyzes the hydroxylation of lauric acid. Our data open new prospects for the CYP6 P450 enzymes, which could be involved in important physiological functions through fatty acid metabolism.

Chimeragenesis of the fatty acid binding site of cytochrome P450BM3. Replacement of residues 73-84 with the homologous residues from the insect cytochrome P450 CYP4C7

A protein fragment of P450BM3 (residues 73-84) which participates in palmitoleate binding was subjected to scanning chimeragenesis. Amino acids 73-84, 73-78, 75-80, and 78-82 were replaced with the homologous fragments of the insect terpenoid hydroxylase CYP4C7. The four chimeric proteins, C(73-84), C(73-78), C(75-80), and C(78-82), were expressed, purified, and characterized. All the chimeric proteins contained all the cofactors and catalyzed monooxygenation of palmitate and of the sesquiterpene farnesol. Chimeragenesis altered substrate binding as shown by the changes in the amplitude of the palmitate-induced type I spectral shift. C(78-82) had monooxygenase activities close to those of P450BM3, while the rest of the chimeric proteins had monooxygenase activities that were inhibited relative to that of wild-type P450BM3. The extent of inhibition of the chimeric proteins varied depending on the substrate, and in the case of C(73-84), farnesol and palmitate oxidation was inhibited by 1 and 4 orders of magnitude, respectively. (1)H NMR spectroscopy and GC-MS were used to identify products of farnesol and palmitate oxidation. Wild-type P450BM3 and all chimeric proteins catalyzed oxidation of farnesol with formation of 9-hydroxyfarnesol and farnesol 10,11- and 2,3-epoxides. Three of the four chimeric proteins also formed a new compound, 5-hydroxyfarnesol, which was the major product in the case of C(73-78). In addition to hydroxylation of the C13-C15 atoms, the chimeric enzymes catalyze significant hydroxylation of the C10-C12 atoms of palmitate. In the case of C(78-82), the rates of formation of 11- and 12-hydroxypalmitates increased 7-fold compared to that of wild-type P450BM3 to 106 and 212 min(-)(1), respectively, while the rate of 10-hydroxypalmitate synthesis increased from zero to 106 min(-)(1). Thus, chimeragenesis of the region of residues 73-84 of the substrate binding site shifted the regiospecificity of substrate oxidation toward the center of the farnesol and palmitate molecules.

NMR and EPR Spectroscopic and Structural Studies of Low-Spin, (dxz,dyz)4(dxy)1 Ground State Fe(III) Bis-tert-Butylisocyanide Complexes of Dodecasubstituted Porphyrins

The bis-(1,1-dimethylethylisocyanide) (tert-butylisocyanide) complexes of three iron porphyrinates (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, OETPP; 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin, OMTPP; and 2,3,7,8,12,13,17,18-tetra-beta,beta'-tetramethylene-5,10,15,20-tetraphenylporphyrin, TC(6)TPP) have been prepared and studied by EPR and (1)H NMR spectroscopy. From EPR and NMR spectroscopic results it has been found that the ground states of the bis-(t-BuNC) complexes of OETPP, OMTPP, and TC(6)TPP are represented mainly (99.1-99.4%) as (d(xz,)d(yz))(4)(d(xy))(1) electron configurations, with an excited state lying 700 cm(-)(1) to higher energy for the OMTPP complex, and probably at lower and higher energies, respectively, for the OETPP and TC(6)TPP complexes. In the (1)H NMR spectra the (d(xz,)d(yz))(4)(d(xy))(1) electron configurations of all three complexes are indicated by the large and positive meso-phenyl-H shift differences, delta(m)-delta(o) and delta(m)-delta(p), and close to the diamagnetic shifts of groups (CH(3) or CH(2)) directly attached to the beta-carbons. However, in comparison to meso-only substituted porphyrinates such as [FeTPP(t-BuNC)(2)]ClO(4), the meso-phenyl shift differences are much smaller, especially for the OETPP complex. 2D NOESY spectra show that the flexibility of the porphyrin core decreases with increasing nonplanar distortion in the order TC(6)TPP > OMTPP > OETPP and in the same order the stability of the binding to t-BuNC ligands decreases. In addition, the structures of two crystalline forms of [FeOMTPP(t-BuNC)(2)]ClO(4) have been determined by X-ray crystallography. Both structures showed purely saddled porphyrin cores and somewhat off-axis binding of the isocyanide ligands. To our knowledge, this is the first example of a porphyrin complex with a purely saddled conformation that adopts the (d(xz,)d(yz))(4)(d(xy))(1) ground state. All structurally-characterized complexes of this electron configuration reported previously are ruffled. Therefore, we conclude that a ruffled geometry stabilizes the (d(xz,)d(yz))(4)(d(xy))(1) ground state, but is not necessary for its existence.