Exploring the Fe(III) binding sites of human serum transferrin with EPR at 275 GHz

Exploring the Specific Role of Iron Center in the Catalytic Activity of Human Serum Transferrin: CTAB-Induced Conformational Changes and Sequestration by Mixed Micelles

Conformational changes play a seminal role in modulating the activity of proteins. This concept becomes all the more relevant in the context of metalloproteins, owing to the formation of specific conformation(s) induced by internal perturbations (like a change in pH, ligand binding, or receptor binding), which may carry out the binding and release of the metal ion/ions from the metal binding center of the protein. Herein, we investigated the conformational changes of an iron-binding protein, monoferric human serum transferrin (Fe-hTF), using several spectroscopic approaches. We could reversibly tune the cetyltrimethylammonium bromide (CTAB)-induced conformation of the protein, exploiting the concept of mixed micelles formed by three sequestrating agents: (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) hydrate (CHAPS) and two bile salts, namely, sodium cholate (NaC) and sodium deoxycholate (NaDC). The formation of mixed micelles between CTAB and these reagents (CHAPS/NaC/NaDC) results in the sequestration of CTAB molecules from the protein environment and aids the protein in reattaining its native-like structure. However, the guanidinium hydrochloride-induced denatured Fe-hTF did not acquire its native-like structure using these sequestrating agents, which substantiates the exclusive role of mixed micelles in the present study. Apart from this, we found that the conformation of transferrin (adopted in the presence of CTAB) displays pronounced esterase-like activity toward the para-nitrophenyl acetate (PNPA) substrate as compared to native transferrin. We also outlined the impact of the iron center and amino acids surrounding the iron center on the effective catalytic activity in the CTAB medium. We estimated ∼3 times higher specific catalytic efficiency for the iron-depleted Apo-hTF compared to the fully iron-saturated Fe2-hTF in the presence of CTAB.

Fe(III) T1 MRI Probes Containing Phenolate or Hydroxypyridine-Appended Triamine Chelates and a Coordination Site for Bound Water

Fe(III) complexes containing a triamine framework and phenolate or hydroxypyridine donors are characterized and studied as T1 MRI probes. In contrast to most Fe(III) MRI probes of linear chelates reported to date, the ligands reported here are pentadentate to give six-coordinate complexes with a coordination site for inner-sphere water. The crystal structure of the complex containing unsubstituted phenolate donors, Fe(L1)Cl, shows a six-coordinate iron center and contains a chloride ligand that is displaced in water. Two additional derivatives are sufficiently water-soluble for study as MRI probes, including a complex with a hydroxypyridine group, Fe(L2), and a hydroxybenzoic acid group, Fe(L3). The pH potentiometric titrations give protonation constants of 7.2 and 7.5 for Fe(L2) and Fe(L3), respectively, which are assigned to deprotonation of the bound water. Changes in the electronic absorbance spectra of the complexes as a function of pH are consistent with the deprotonation of phenol pendants at acidic pH values. However, the inner-sphere water ligand of Fe(L2) and Fe(L3) does not exchange rapidly on the NMR timescale at pH 6.0 or 7.4, as shown by variable-temperature 17O NMR spectroscopy. The pH-dependent proton relaxivity profiles show a maximum in relaxivity at a near-neutral pH, suggesting that exchange of the protons of the bound water is an important contribution. Competitive binding studies with ethylenediaminetetraacetic acid (EDTA) show effective stability constants for Fe(L2) and Fe(L3) at pH 7.4 with log K values of 21.1 and 20.5, respectively. These two complexes are kinetically inert in carbonate phosphate buffer at 37 °C for several hours but transfer iron to transferrin. Fe(L2) and Fe(L3) show enhanced contrast in T1-weighted imaging analyses in BALB/c mice. These studies show that Fe(L2) clears through mixed renal and hepatobiliary routes, while Fe(L3) has a similar pharmacokinetic clearance profile to a macrocyclic Gd(III) contrast agent.

Iron-Induced Oxidative Stress in Human Diseases

Iron is responsible for the regulation of several cell functions. However, iron ions are catalytic and dangerous for cells, so the cells sequester such redox-active irons in the transport and storage proteins. In systemic iron overload and local pathological conditions, redox-active iron increases in the human body and induces oxidative stress through the formation of reactive oxygen species. Non-transferrin bound iron is a candidate for the redox-active iron in extracellular space. Cells take iron by the uptake machinery such as transferrin receptor and divalent metal transporter 1. These irons are delivered to places where they are needed by poly(rC)-binding proteins 1/2 and excess irons are stored in ferritin or released out of the cell by ferroportin 1. We can imagine transit iron pool in the cell from iron import to the export. Since the iron in the transit pool is another candidate for the redox-active iron, the size of the pool may be kept minimally. When a large amount of iron enters cells and overflows the capacity of iron binding proteins, the iron behaves as a redox-active iron in the cell. This review focuses on redox-active iron in extracellular and intracellular spaces through a biophysical and chemical point of view.

Many-Body Study of Iron(III)-Bound Human Serum Transferrin

We present the very first density functional theory and dynamical mean field theory calculations of iron-bound human serum transferrin. Peaks in the optical conductivity at 250, 300, and 450 nm were observed, in line with experimental measurements. Spin multiplet analysis suggests that the ground state is a mixed state with high entropy, indicating the importance of strong electronic correlation in this system's chemistry.

EPR Spectroscopy of Iron- and Nickel-Doped [ZnAl]-Layered Double Hydroxides: Modeling Active Sites in Heterogeneous Water Oxidation Catalysts

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to interspin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into nonmagnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites was solvent-exposed (i.e., at an edge, corner, or defect site). These solvent-exposed sites featured an axial ZFS of 0.21 cm^-1 when hydrated. The ZFS increased dramatically upon dehydration (to -1.5 cm^-1), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other ("inert") Fe(III) site maintained an axial ZFS of 0.19-0.20 cm^-1 under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (±0.30 cm^-1) in hydrated material, whereas two distinct Ni(II) ZFS values (±0.30 and ±1.1 cm^-1) were observed in the dehydrated samples. Although the magnetically dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g., by simple vacuum drying) is an important step toward identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials.

Analysis of the EPR spectra of transferrin: the importance of a zero-field-splitting distribution and 4th-order terms

Multi-frequency EPR spectroscopy can provide high-level structural information on high-spin Fe³⁺ sites in proteins and enzymes. Unfortunately, analysis of the EPR spectra of these spin systems is hindered by the presence of broad distributions in the zero-field-splitting (ZFS) parameters, which reflect conformational heterogeneity of the iron sites. We present the analysis of EPR spectra of high-spin Fe³⁺ bound to human serum transferrin. We apply a method termed the grid-of-errors to extract the distributions of the individual ZFS parameters from EPR spectra recorded in the high-field limit at a microwave frequency of 275 GHz. Study of a series of transferrin variants shows that the ZFS distributions are as characteristic of the structure of a high-spin Fe³⁺ site as the ZFS parameters themselves. Simulations based on the extracted ZFS distributions reproduce spectra recorded at 34 GHz (Q band) and 9.7 GHz (X band), including subtle variations that were previously difficult to quantify. The X-band spectrum of transferrin shows a characteristic double peak, which has puzzled researchers for decades. We show that the double peak is uniquely related to the term B₄^-3O₄^-3(S) in the spin Hamiltonian. Our method is generally applicable in the analysis of spectra that arise from a broad distribution of parameters.

Fe-transferrins or their homologues in ex-vivo mushrooms as identified by ESR spectroscopy and quantum chemical calculations: A full spin-Hamiltonian approach for the ferric sextet state with intermediate zero-field splitting parameters

Fe-transferrins/their homologues in ex-vivo mushrooms were identified by ESR spectroscopy at liquid helium temperature, 4 K. The ESR fine-structure signals from Grifola frondosa were analyzed by spectral simulation with a full spin-Hamiltonian approach, determining the spin Hamiltonian parameters of the ferric iron species bound in the biological environment: S = 5/2, g = (2.045, 2.01, 2.235), |D| = 0.28 cm^-1, |E/D| = 0.05. The zero-field splitting (ZFS) parameters, D- and E-values, are very close to the reported values, |D| = 0.25 cm^-1 and |E/D| = 0.06, for an Fe-transferrin with oxalate anion, and to |D| = 0.25 cm^-1 and |E/D| = 0.04 for one with malonate anion in human sera, suggesting that the Fe³⁺ species are from Fe-transferrins or their homologues. Quantum chemical calculations of the ZFS tensors for Fe-transferrins were carried out. Fe-transferrins/homologues have been identified for all the mushrooms under study, suggesting that such Fe³⁺ enzymes are widely distributed in mushrooms.

Molecular cloning and expression analysis of Megalobrama amblycephala transferrin gene and effects of exposure to iron and infection with Aeromonas hydrophila

Transferrin (Tf) plays an important function in iron homeostasis and metabolism of organisms. In this study, we identified and characterized the Tf gene in Megalobrama amblycephala and evaluated its expression in basal conditions as well as after iron overload and experimental infection with Aeromonas hydrophila. Furthermore, we studied the iron binding properties of recombinant Tf. The full-length M. amblycephala Tf complementary DNA (cDNA) (GenBank accession no.: KX698308) of 2245 bp was cloned and contained a 1953 bp open reading frame (ORF) encoding 650 amino acid residues and flanked by a 68 bp 5' and a 204 bp 3' untranslated regions (UTR). Predicted conservative structure illustrated that M. amblycephala Tf consisted of two conservative Tf domains. Amino acid sequence alignment revealed that M. amblycephala Tf had high similarity with that of cyprinids deposited in Genbank, and phylogenetic analysis showed that M. amblycephala Tf clustered with Ctenopharyngodon idella and Hypophthalmichthys molitrix. Tissue expression pattern analyses demonstrated that the liver was the main Tf mRNA expressing organ, being significantly higher than other tissues (p < 0.05). In the liver, Tf mRNA expression in fish artificially injected with the pathogenic bacteria A. hydrophila was significantly upregulated, reaching a peak at 12 h post injection (hpi) and then decreasing afterward. The expression in FeCl₃-injected fish showed a similar tendency, but reached a peak at 8 hpi. Meanwhile, fish serum iron significantly decreased following A. hydrophila injection, but increased to peak at 4 hpi and then decreased in FeCl₃-injected fish. The recombinant M. amblycephala Tf showed iron binding capacity using CAS analysis. These results are helpful to understand the structure and regulation of expression of Tf, as well as the specific function of Tf for both immune responses and iron homeostasis.

Interactions and accumulation differences of metal(loid)s in three sea cucumber species collected from the Northern Mediterranean Sea

This study was conducted on Holothuria polii, Holothuria tubulosa, and Holothuria mammata collected from five stations with different depths in the Northern Mediterranean Sea. The body walls and guts of these holothurians were examined in terms of interactions of 10 metals (iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), chromium (Cr), cobalt (Co), vanadium (V), nickel (Ni), cadmium (Cd), and lead (Pb)) and one metalloid (arsenic (As)) using a multivariate analysis, and interspecies differences were determined. The multivariate analysis of variance (MANOVA) revealed significant differences between the species in terms of metal(loid) accumulations. The principal component analysis (PCA) showed a more association between H. tubulosa and H. polii with regard to the accumulation. The cluster analysis (CA) located Pb concentrations of the guts to the farthest place from all elements regardless of the species. A correlation analysis displayed that the element concentrations of the guts were more closely related to each other compared with those of the walls. The most inconsistent element in terms of correlations was the gut Fe contents. Accordingly, while Fe concentrations of H. mammata and H. tubulosa were correlated with all elements (except Pb) in divalent metal transporter 1 (DMT1) (divalent cation transporter 1 (DCT1) or natural resistance-associated macrophage protein 2 (NRAMP2)) belonging to the NRAM protein family, this was not the case in H. polii. Consequently, significant relationships between accumulated metal(loid)s that changed by tissues and sea cucumber species were observed.