Ligand binding processes in hemoglobin. Chemical reactivity of iron studied by XANES spectroscopy

How Nanoparticles Modify Adsorbed Proteins: Impact of Silica Nanoparticles on the Hemoglobin Active Site

The adsorption of proteins on surfaces has been studied for a long time, but the relationship between the structural and functional properties of the adsorbed protein and the adsorption mechanism remains unclear. Using hemoglobin adsorbed on silica nanoparticles, we have previously shown that hemoglobin's affinity towards oxygen increases with adsorption. Nevertheless, it was also shown that there were no significant changes in the quaternary and secondary structures. In order to understand the change in activity, we decided in this work to focus on the active sites of hemoglobin, the heme and its iron. After measuring adsorption isotherms of porcine hemoglobin on Ludox silica nanoparticles, we analyzed the structural modifications of adsorbed hemoglobin by X-ray absorption spectroscopy and circular dichroism spectra in the Soret region. It was found that upon adsorption, there were modifications in the heme pocket environment due to changes in the angles of the heme vinyl functions. These alterations can explain the greater affinity observed.

X-ray snapshots reveal conformational influence on active site ligation during metalloprotein folding

Cytochrome c (cyt c) has long been utilized as a model system to study metalloprotein folding dynamics and the interplay between active site ligation and tertiary structure. However, recent reports regarding the weakness of the native Fe(ii)-S bond (Fe-Met80) call into question the role of the active site ligation in the protein folding process. In order to investigate the interplay between protein conformation and active site structures, we directly tracked the evolution of both during a photolysis-induced folding reaction using X-ray transient absorption spectroscopy and time-resolved X-ray solution scattering techniques. We observe an intermediate Fe-Met80 species appearing on ∼2 μs timescale, which should not be sustained without stabilization from the folded protein structure. We also observe the appearance of a new active site intermediate: a weakly interacting Fe-H2O state. As both intermediates require stabilization of weak metal-ligand interactions, we surmise the existence of a local structure within the unfolded protein that protects and limits the movement of the ligands, similar to the entatic state found in the native cyt c fold. Furthermore, we observe that in some of the unfolded ensemble, the local stabilizing structure is lost, leading to expansion of the unfolded protein structure and misligation to His26/His33 residues.

X-ray absorption spectroscopic studies of a transient intermediate in the reaction of cyanide metmyoglobin with dithionite by using rapid freezing

The reaction of cyanide metmyoglobin (Mb+CN-) with dithionite produces a transient intermediate, supposed to be cyanide-ligated ferrous myoglobin. The Fe K-edge X-ray absorption spectrum of the intermediate has been measured by using rapid freezing and compared with those of Mb+CN- and deoxymyoglobin (deoxyMb). The shapes of the XANES (X-ray Absorption Near Edge Structure) spectra of Mb+CN- and the intermediate are very similar, including the intensity ratios of the peak C1 to D. This indicates that CN- remains bound with a linear Fe-C-N configuration in the intermediate. The absorption edge of the intermediate is shifted to 1.2 eV lower energy than that of Mb+CN-, reflecting a valence change in the heme iron. The EXAFS (Extended X-ray Absorption Fine Structure) spectrum of the intermediate closely resembles that of Mb+CN- but significantly differs from that of deoxyMb. Analysis shows that the average iron-nearest neighbor atom distance is 1.99 +/- 0.01 A for both Mb+CN- and the intermediate and 2.05 +/- 0.01 A for deoxyMb. These results imply that the local structure around the heme iron of Mb+CN- does not change upon reduction until the cyanide ligand is released.

X-ray absorption spectroscopy of carbonyl basket handle Fe(II) porphyrins: the distortion of the tetrapyrrolic macrocycle

The distortion of the tetrapyrrolic macrocycle, the Fe-C-O bond angle and the Fe local electronic structure of carbonyl basket-handle Fe2+ porphyrins as a function of the basket-handle chain length have been studied by X-ray absorption near edge structure (XANES) spectroscopy, both in the solid state and in toluene solution. The Fe-C-O bond angle has been found to be linear in all compounds while the increasing distortion of the macrocycle with shortening of the chain length is indicated by the multiple scattering resonance in the heme plane appearing at 22 eV in the XANES spectrum.

Structural and electronic characterization of heme moiety in oxygenated hemoproteins by using XANES spectroscopy

Iron K-edge X-ray absorption near edge structure (XANES) spectra were measured for oxy-forms of cytochrome P-450cam (P-450cam), horseradish peroxidase (HRP) and myoglobin (Mb) by using Synchrotoron Radiation of Photon Factory (Tsukuba). A pronounced 1s-4p transition and some fine structures were well-resolved in the spectra obtained. Comparing the spectra, the features at the fine structures termed P, C and D, were similar among the three hemoproteins, suggesting a similar site-symmetry around the heme iron and the same Fe-O-O bond angle (about 115 degrees). On the other hand, absorption features at the edge region (7115-7135 eV) were slightly but significantly different from one another; the absorption intensity at 7115-7125 eV region increased in the order of Mb, HRP and P-450cam, while that at 7125-7135 eV decreased in the same order. A similar absorption feature was also obtained with their deoxy (ferrous high spin) forms. We assumed that the absorption at the lower energy region (7115-7125 eV) reflects the pi-character in the Fe-ligand bond, whereas that at the higher energy region (7125-7135 eV) does the sigma-character, on the basis of the previous and comprehensive studies of the XANES spectroscopy of the adsorbed molecules on the metal surface (McGovern et al. (1989) Handbook on Synchrotoron Radiation, Vol. 2, pp. 467-539). According to our assumption, our XANES results indicated that the pi-character of the Fe-ligand bond increases in the order of Mb, HRP and P-450cam, and that the pi-electron of the thiolate S- in P-450cam is donated to the Fe-O-O moiety, most probably to the antibonding pi* orbital of O2. Such an interpretation is consistent with the experimental findings or data accumulated so far by other methods, such as the resonance Raman spectroscopy.

Alteration of heme axial ligands in hemoglobin by organic solvents analyzed by CD, FTIR, and XANES techniques

The effects of mixed solvents on the ligand binding site in hemoglobin have been investigated though three spectroscopic techniques. Two classes of organic solvents (amides and alcohols) known to increase or decrease the hemoglobin affinity have been chosen for this study. The analysis of the iron CO stretching band shows that the ligand binding sites of alpha CO and beta CO subunits inside the alpha 2 beta 2 hemoglobin tetramer exhibit multiple conformations. From the circular dichroism and X-ray absorption near-edge structure data, it appears that no core deformation or heme reorientation occur with the affinity changes. The iron-ligand average bond angle is the sole parameter that depends on the external solvent. Since cosolvents seem to affect the dynamics rather than the hindrance of the heme cavity, we suggest that the protein affinity could be associated with a hierarchy of subtle dynamic states.

Heterogeneity of the isolated subunits of the fetal and adult human hemoglobin in solution, detected by XANES spectroscopy

Differences in the local structure of the heme in the isolated alpha-, beta- and gamma-chains of the adult and fetal human hemoglobin are detected by XANES (X-ray absorption near-edge structure) spectroscopy. The ligand bonding angle to the iron ion in the ligated forms and the displacement of the Fe respect to the porphyrin plane in the deoxy forms are found to be different for each chain.

XANES spectroscopy sensitivity to small electronic changes. Case of carp azidomethemoglobin

Spin states equilibrium of hemoglobin-iron varies with external conditions: pH, allosteric effectors, temperature. The small electronic reorganization of the iron caused by the spin state changes has been detected by X-ray absorption near edge structure (XANES) spectroscopy at room temperature. The iron K-edge region which is sensitive to spin state is located in 7110-7130 eV. Here are presented the 100% high spin and 100% low spin XANES spectra of carp azido ferric hemoglobin.

Linear Fe-C-O configuration in carbonyl 1-methylimidazole iron(II) porphyrin detected by XANES in dispersive mode

The linear Fe-C-O configuration has been determined in the carbonyl 1-methylimidazole hindered iron(II) porphyrin derived from 'basket handle' complexes in which there are no constraints on the proximal imidazole. The structure at the iron site has been determined in toluene solution by fast measurements of XANES spectra, using the dispersive X-ray absorption method.

Oxygen bonding in human hemoglobin and its isolated subunits: a XANES study

The X-ray absorption near edge structure (XANES) spectra of the human adult and foetal hemoglobin, of the isolated alpha and beta chains, in the oxygenated forms, and of the oxymyoglobin and carp oxyhemoglobin have been measured at the wiggler beam line of the Frascati Synchrotron radiation facility. The bonding angle of oxygen molecule at the iron site in these hemoproteins in solution, has been measured using the multiple scattering theory for data analysis.

XANES spectroscopy of carp hemoglobin-iron in correlation with the affinity changes of the protein for ligand

The strong variation of ligand-binding properties with pH for carp hemoglobin is not reflected in the electronic distribution of the heme-iron. Thus, we can suppose that hemoglobin affinity is directly controlled by the protein and by some particular changes of the iron atom.