X-ray structure of chromium(III)-containing transferrin: First structure of a physiological Cr(III)-binding protein

Cisplatin Binding to Human Serum Transferrin: A Crystallographic Study

The molecular mechanism of how human serum transferrin (hTF) recognizes cisplatin at the atomic level is still unclear. Here, we report the molecular structure of the adduct formed upon the reaction of hTF with cisplatin. Pt binds the side chain of Met256 (at the N-lobe), without altering the protein overall conformation.

Quantifying In Situ Structural Stabilities of Human Blood Plasma Proteins Using a Novel Iodination Protein Stability Assay

Many of the diseases that plague society today are driven by a loss of protein quality. One method to quantify protein quality is to measure the protein folding stability (PFS). Here, we present a novel mass spectrometry (MS)-based approach for PFS measurement, iodination protein stability assay (IPSA). IPSA quantifies the PFS by tracking the surface-accessibility differences of tyrosine, histidine, methionine, and cysteine under denaturing conditions. Relative to current methods, IPSA increases protein coverage and granularity to track the PFS changes of a protein along its sequence. To our knowledge, this study is the first time the PFS of human serum proteins has been measured in the context of the blood serum (in situ). We show that IPSA can quantify the PFS differences between different transferrin iron-binding states in near in vivo conditions. We also show that the direction of the denaturation curve reflects the in vivo surface accessibility of the amino acid residue and reproducibly reports a residue-specific PFS. Along with IPSA, we introduce an analysis tool Chalf that provides a simple workflow to calculate the residue-specific PFS. The introduction of IPSA increases the potential to use protein structural stability as a structural quality metric in understanding the etiology and progression of human disease. Data is openly available at Chorusproject.org (project ID 1771).

Holo-chromodulin: competition between the native Cr3+ and other biogenic cations (Fe3+, Fe2+, Mg2+, and Zn2+) for the binding sites

Chromodulin is an oligopeptide that has an essential role for the flawless functioning of insulin. Although the precise sequence of the constituent amino acid residues and the 3D structure of the molecule has not yet been deciphered, it is known that chromodulin contains only four amino acids in the ratio of Glu-: Gly: Cys: Asp- = 4: 2: 2: 2. An indispensable part for the integrity of the molecule in its active (holo-) form are four chromium cations (hence the name) in the oxidation state of 3+, positioned in two metal binding sites containing one and three Cr3+ ions. Experimental works provide some hints/clues concerning the structure of the metal centers, although their exact composition, type, and arrangement of metal ligating entities remain enigmatic. In the current study, we endeavor to unveil possible structure(s) of the Cr3+ loaded binding sites by strictly following the evidence provided by the experimental data. Well-calibrated in silico methodology for optimization and evaluation of Gibbs free energies is applied and gives strong premises for reliably deciphering the composition/structure of chromodulin metal binding sites. Additional computations reveal the advantage of choosing Cr3+ over other tri- (Fe3+) and divalent (Fe2+, Mg2+, and Zn2+) biogenic ions for securing maximum stability of the metal-occupied binding sites.

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.

Urea Gel Electrophoresis in Studies of Conformational Changes of Transferrin on Binding and Transport of Non-Ferric Metal Ions

Transferrin (Tf) is a crucial transporter protein for Fe(III), but its biological role in binding other metal ions and their delivery into cells remain highly controversial. The first systematic exploration of the effect of non-Fe(III) metal ion binding on Tf conformation has been performed by urea-polyacrylamide gel electrophoresis (urea-PAGE), which is commonly used for nucleic acids but rarely for proteins. Closed Tf conformation, similar to that caused by Fe(III)-Tf binding, was formed for In(III), V(III) or Cr(III) binding to Tf. In all these cases, metal distribution between Tf lobes and/or the rate of metal release under acidic conditions differed from that of Fe(III)-Tf. By contrast, Ga(III) and V(IV) did not form closed Tf conformation under urea-PAGE conditions. Apart from Fe(III), only In(III) was able to increase the proportion of closed Tf conformation in whole serum. These results suggest that Tf is unlikely to act as a natural carrier of any metal ion, except Fe(III), into cells but can reduce toxicity of exogenous metal ions by binding them in serum and preventing their entry into cells.

Ionic Liquid Extraction Behavior of Cr(VI) Absorbed on Humic Acid-Vermiculite

Cr(VI) can be released into soil as a result of mining, electroplating, and smelting operations. Due to the high toxicity of Cr(VI), its removal is necessary in order to protect ecosystems. Vermiculite is applied in situations where there is a high degree of metal pollution, as it is helpful during the remediation process due to its high cation exchange capacity. The Cr(VI) contained in the vermiculite should be extracted in order to recover it and to reduce the impact on the environment. In this work, adsorption equilibrium data for Cr(VI) in a simulated sorbent for soil remediation (a mixture that included both humic acid (HA) and vermiculite) were a good fit with the Langmuir isotherm model. The simulated sorbent for soil remediation was a favorable sorbent for Cr(VI) when it was in the test soil. An ionic liquid, [C₄mim]Cl (1-butyl-3-methylimidazolium chloride), was studied to determine its efficiency in extracting Cr(VI) from the Cr- contaminated simulated sorbent in soil remediation. At 298 K and within 30 min, approximately 33.48 ± 0.79% of Cr(VI) in the simulated sorbent in soil remediation was extracted into [C₄mim]Cl. Using FTIR spectroscopy, the absorbance intensities of the bands at 1032 and 1010 cm⁻¹, which were attributed to C-O bond stretching in the polysaccharides of HA, were used to detect the changes in HA in the Cr-contaminated simulated sorbent for soil remediation before and after extraction. The results showed that Cr(VI) that has been absorbed on HA can be extracted into [C₄mim]Cl. Using ¹H NMR, it was observed that the 1-methylimizadole of [C₄mim] Cl played an important role in the extraction of Cr(VI), which bonded with HA on vermiculite and was able to be transformed into the [C₄mim]Cl phase.

Low-molecular-weight chromium-binding substance (LMWCr) may bind and carry Cr(III) from the endosome

Trivalent chromium has been proposed to be transported in vivo from the bloodstream to the tissues via endocytosis by transferrin (Tf), the major iron transport protein in the blood. While Cr(III) loss from the Tf/Tf receptor complex after acidification to pH 5.5 has recently been shown to be sufficiently rapid to be physiologically relevant, the released Cr(III) still must exit the endosome during the time of the endocytosis cycle (circa 15 min). Cr(III) binds too slowly to small ligands such as citrate or ascorbate, or even EDTA, for such complexes to form and be transported from the endosome, while no trivalent ion transporters are known. However, the apo form of the peptide low-molecular-weight chromium-binding substance (LMWCr) can remove Cr(III) from Cr(III)₂-Tf at neutral pH, albeit slowly, and LMWCr is known to be transported from cells after binding Cr(III), although the transporter is not known. LMWCr subsequently carries Cr(III) to the bloodstream ultimately for removal from the body in the urine. The rate of binding of Cr(III) to apoLMWCr was significantly enhanced in the presence of the Tf/Tf receptor complex. These results suggest that apoLMWCr may function to bind Cr(III) released in the endosomes for ultimate removal from the body as part of a Cr(III) detoxification process.

Exploring Serum Transferrin Regulation of Nonferric Metal Therapeutic Function and Toxicity

Serum transferrin (sTf) plays a pivotal role in regulating iron biodistribution and homeostasis within the body. The molecular details of sTf Fe(III) binding blood transport, and cellular delivery through transferrin receptor-mediated endocytosis are generally well-understood. Emerging interest exists in exploring sTf complexation of nonferric metals as it facilitates the therapeutic potential and toxicity of several of them. This review explores recent X-ray structural and physiologically relevant metal speciation studies to understand how sTf partakes in the bioactivity of key non-redox active hard Lewis acidic metals. It challenges preconceived notions of sTf structure function correlations that were based exclusively on the Fe(III) model by revealing distinct coordination modalities that nonferric metal ions can adopt and different modes of binding to metal-free and Fe(III)-bound sTf that can directly influence how they enter into cells and, ultimately, how they may impact human health. This knowledge informs on biomedical strategies to engineer sTf as a delivery vehicle for metal-based diagnostic and therapeutic agents in the cancer field. It is the intention of this work to open new avenues for characterizing the functionality and medical utility of nonferric-bound sTf and to expand the significance of this protein in the context of bioinorganic chemistry.