Crystal structures of two mutants (K206Q, H207E) of the N-lobe of human transferrin with increased affinity for iron

Probing the interaction of fisetin with human serum transferrin via spectroscopic and molecular docking approaches

The binding of fisetin to human serum transferrin (HST) was investigated by spectroscopic (steady-state fluorescence, synchronous fluorescence, Förster resonance energy transfer) and molecular docking approaches. HST fluorescence is quenched by fisetin by a static process. The binding takes place with a moderate affinity and it is driven by hydrogen bonding and van der Waals forces. Synchronous fluorescence study indicates that Trp is more involved in the fluorescent quenching of HST by fisetin than Tyr. The energy transfer between HST and fisetin occurs at a distance of 2.31 nm confirming the results obtained by fluorescence. The binding of fisetin to HST favors thermal denaturation of HST conformation. The transition temperature for HST was obtained at 53.81 °C while the presence of the fisetin led to its change to 49.06 °C. The molecular docking of fisetin to HST confirms the results obtained by the spectroscopic experiments showing a moderate affinity of fisetin for HST.Communicated by Ramaswamy H. Sarma.

Protein Desorption Kinetics Depends on the Timescale of Observation

The presence of so-called reversible and irreversible protein adsorption on solid surfaces is well documented in the literature and represents the basis for the development of nanoparticles and implant materials to control interactions in physiological environments. Here, using a series of complementary single-molecule tracking approaches appropriate for different timescales, we show that protein desorption kinetics is much more complex than the traditional reversible-irreversible binary picture. Instead, we find that the surface residence time distribution of adsorbed proteins transitions from power law to exponential behavior when measured over a large range of timescales (10^-2-10⁶ s). A comparison with macroscopic results obtained using a quartz crystal microbalance suggested that macroscopic measurements have generally failed to observe such nonequilibrium phenomena because they are obscured by ensemble-averaging effects. These findings provide new insights into the complex phenomena associated with protein adsorption and desorption.

In situ analysis of nanoparticle soft corona and dynamic evolution

How soft corona, the protein corona’s outer layer, contributes to biological identity of nanomaterials is largely because capturing protein composition of the soft corona in situ remains challenging. We herein develop an in situ Fishing method that can monitor the dynamic formation of protein corona on ultra-small chiral Cu₂S nanoparticles (NPs) allowing us to directly separate and identify the corona protein composition. Our method detects spatiotemporal processes in the evolution of hard and soft coronas on chiral NPs, revealing subtle differences in NP − protein interactions even within several minutes. This study highlights the importance of in situ and dynamic analysis of soft/hard corona, provides insights into the role of soft corona in mediating biological responses of NPs, and offers a universal strategy to characterize soft corona to guide the rational design of biomedical nanomaterials.

Characterizing the soft protein corona on nanoparticles i.e. the outer layer of the corona, remains a longstanding challenge. Here, the authors develop an in situ method to monitor the dynamic processes of multilayered corona formation and evolution that offers a universal strategy to characterize the soft corona proteome.

Intracellular Iron Binding and Antioxidant Activity of Phytochelators

Phytochelators have been studied as templates for designing new drugs for chelation therapy. This work evaluated key chemical and biological properties of five candidate phytochelators for iron overload diseases: maltol, mimosine, morin, tropolone, and esculetin. Intra- and extracellular iron affinity and antioxidant activity, as well as the ability to scavenge iron from holo-transferrin, were studied in physiologically relevant settings. Tropolone and mimosine (and, to a lesser extent, maltol) presented good binding capacity for iron, removing it from calcein, a high-affinity fluorescent probe. Tropolone and mimosine arrested iron-mediated oxidation of ascorbate with the same efficiency as the standard iron chelator DFO. Also, both were cell permeant and able to access labile pools of iron in HeLa and HepG2 cells. Mimosine was an effective antioxidant in cells stressed by iron and peroxide, being as efficient as the cell-permeant iron chelator deferiprone. These results reinforce the potential of those molecules, especially mimosine, as adjuvants in treatments for iron overload.

Interactions of Chemically Synthesized Ferrihydrite Nanoparticles with Human Serum Transferrin: Insights from Fluorescence Spectroscopic Studies

Human serum transferrin (HST) is a glycoprotein involved in iron transport that may be a candidate for functionalized nanoparticles to bind and target cancer cells. In this study, the effects of the simple and doped with cobalt (Co) and copper (Cu) ferrihydrite nanoparticles (Fh-NPs, Cu-Fh-NPs, and Co-Fh-NPs) were studied by spectroscopic and molecular approaches. Fluorescence spectroscopy revealed a static quenching mechanism for all three types of Fh-NPs. All Fh-NPs interacted with HST with low affinity, and the binding was driven by hydrogen bonding and van der Waals forces for simple Fh-NPs and by hydrophobic interactions for Cu-Fh-NPs and Co-Fh-NPs binding, respectively. Of all samples, simple Fh-NPs bound the most to the HST binding site. Fluorescence resonance energy transfer (FRET) allowed the efficient determination of the energy transfer between HST and NPs and the distance at which the transfer takes place and confirmed the mechanism of quenching. The denaturation of the HST is an endothermic process, both in the case of apo HST and HST in the presence of the three types of Fh-NPs. Molecular docking studies revealed that Fh binds with a low affinity to HST (K_a = 9.17 × 10³ M⁻¹) in accord with the fluorescence results, where the interaction between simple Fh-NPs and HST was described by a binding constant of 9.54 × 10³ M⁻¹.

Structural and molecular aspects of flavonoids as ligands for serum transferrin

Human serum transferrin (HST) acts as a carrier for Fe³⁺ and other ions. Binding of flavonoids to HST produces changes in the protein structure with direct implication on iron delivery into cells. We investigate the binding mechanism and affinity towards HST of three flavonoids: rutin, luteolin, and apigenin by different techniques: UV-Vis, fluorescence, fluorescence resonance energy transfer (FRET) combined with molecular docking. UV-Vis results indicate an interaction between flavonoids and HST. It was observed that HST fluorescence was quenched by these three flavonoids via a static process. All the interactions were moderate and the main driving forces are hydrophobic (ΔH > 0 and ΔS > 0) for rutin and luteolin binding or electrostatic (ΔH < 0 and ΔS > 0) for apigenin binding. FRET and molecular docking studies confirm the fluorescence static quenching mechanism by flavonoid binding. The binding of all three flavonoids increases HST stability. These results present the potential use of HST in target-oriented delivery of flavonoids and possibly other drugs into cells.

A quantitative view on multivalent nanomedicine targeting

Although the concept of selective delivery has been postulated over 100 years ago, no targeted nanomedicine has been clinically approved so far. Nanoparticles modified with targeting ligands to promote the selective delivery of therapeutics towards a specific cell population have been extensively reported. However, the rational design of selective particles is still challenging. One of the main reasons for this is the lack of quantitative theoretical and experimental understanding of the interactions involved in cell targeting. In this review, we discuss new theoretical models and experimental methods that provide a quantitative view of targeting. We show the new advancements in multivalency theory enabling the rational design of super-selective nanoparticles. Furthermore, we present the innovative approaches to obtain key targeting parameters at the single-cell and single molecule level and their role in the design of targeting nanoparticles. We believe that the combination of new theoretical multivalent design and experimental methods to quantify receptors and ligands aids in the rational design and clinical translation of targeted nanomedicines.

A mechanistic examination of salting out in protein-polymer membrane interactions

Developing a mechanistic understanding of protein dynamics and conformational changes at polymer interfaces is critical for a range of processes including industrial protein separations. Salting out is one example of a procedure that is ubiquitous in protein separations yet is optimized empirically because there is no mechanistic description of the underlying interactions that would allow predictive modeling. Here, we investigate peak narrowing in a model transferrin-nylon system under salting out conditions using a combination of single-molecule tracking and ensemble separations. Distinct surface transport modes and protein conformational changes at the negatively charged nylon interface are quantified as a function of salt concentration. Single-molecule kinetics relate macroscale improvements in chromatographic peak broadening with microscale distributions of surface interaction mechanisms such as continuous-time random walks and simple adsorption-desorption. Monte Carlo simulations underpinned by the stochastic theory of chromatography are performed using kinetic data extracted from single-molecule observations. Simulations agree with experiment, revealing a decrease in peak broadening as the salt concentration increases. The results suggest that chemical modifications to membranes that decrease the probability of surface random walks could reduce peak broadening in full-scale protein separations. More broadly, this work represents a proof of concept for combining single-molecule experiments and a mechanistic theory to improve costly and time-consuming empirical methods of optimization.

Structure of the saxiphilin:saxitoxin (STX) complex reveals a convergent molecular recognition strategy for paralytic toxins

Dinoflagelates and cyanobacteria produce saxitoxin (STX), a lethal bis-guanidinium neurotoxin causing paralytic shellfish poisoning. A number of metazoans have soluble STX-binding proteins that may prevent STX intoxication. However, their STX molecular recognition mechanisms remain unknown. Here, we present structures of saxiphilin (Sxph), a bullfrog high-affinity STX-binding protein, alone and bound to STX. The structures reveal a novel high-affinity STX-binding site built from a "proto-pocket" on a transferrin scaffold that also bears thyroglobulin domain protease inhibitor repeats. Comparison of Sxph and voltage-gated sodium channel STX-binding sites reveals a convergent toxin recognition strategy comprising a largely rigid binding site where acidic side chains and a cation-π interaction engage STX. These studies reveal molecular rules for STX recognition, outline how a toxin-binding site can be built on a naïve scaffold, and open a path to developing protein sensors for environmental STX monitoring and new biologics for STX intoxication mitigation.

Functional characterisation of two ferric-ion coordination modes of TtFbpA, the periplasmic subunit of an ABC-type iron transporter from Thermus thermophilus HB8

The ferric ion binding protein A of Thermus thermophilus HB8 (TtFbpA) is the periplasmic subunit of an ABC-type iron transporter.

Computational approaches for deciphering the equilibrium and kinetic properties of iron transport proteins

With the advances in three-dimensional structure determination techniques, high quality structures of the iron transport proteins transferrin and the bacterial ferric binding protein (FbpA) have been deposited in the past decade. These are proteins of relatively large size, and developments in hardware and software have only recently made it possible to study their dynamics using standard computational resources. We review computational techniques towards understanding the equilibrium and kinetic properties of iron transport proteins under different environmental conditions. At the level of detail that requires quantum chemical treatments, the octahedral geometry around iron has been scrutinized and it has been established that the iron coordinating tyrosines are in an unusual deprotonated state. At the atomistic level, both the N-lobe and the full bilobal structure of transferrin have been studied under varying conditions of pH, ionic strength and binding of other metal ions by molecular dynamics (MD) simulations. These studies have allowed questions to be answered, among others, on the function of second shell residues in iron release, the role of synergistic anions in preparing the active site for iron binding, and the differences between the kinetics of the N- and the C-lobe. MD simulations on FbpA have led to the detailed observation of the binding kinetics of phosphate to the apo form, and to the conformational preferences of the holo form under conditions mimicking the environmental niches provided by the periplasmic space. To study the dynamics of these proteins with their receptors, one must resort to coarse-grained methodologies, since these systems are prohibitively large for atomistic simulations. A study of the complex of human transferrin (hTf) with its pathogenic receptor by such methods has revealed a potential mechanistic explanation for the defense mechanism that arises in evolutionary warfare. Meanwhile, the motions in the transferrin receptor bound hTf have been shown to disfavor apo hTf dissociation, explaining why the two proteins remain in complex during the recycling process from the endosome to the cell surface. Open problems and possible technological applications related to metal ion binding-release in iron transport proteins that may be handled by hybrid use of quantum mechanical, MD and coarse-grained approaches are discussed.

Raman detection of localized transferrin-coated gold nanoparticles inside a single cell

We investigated the cellular uptake behavior of non-fluorescent metal nanoparticles (NPs) by use of surface-enhanced Raman scattering (SERS) combined with dark-field microscopy (DFM). The uptake of Au NPs inside a single cell could also be identified by DFM first and then confirmed by z-depth-dependent SERS at micrometer resolution. Guided by DFM for the location of Au NPs, an intracellular distribution assay was possible using Raman dyes with unique vibrational marker bands in order to identify the three-dimensional location inside the single cell by obtaining specific spectral features. Au NPs modified by 4-mercaptobenzoic acid (MBA) bearing its -COOH surface functional group were used to conjugate transferrin (Tf) protein using the 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) reaction. The protein conjugation reaction on Au surfaces was examined by means of color change, absorption spectroscopy, and SERS. Our results demonstrate that DFM techniques combined with SERS may have great potential for monitoring biological processes with protein conjugation at the single-cell level.

Recombinant human transferrin: beyond iron binding and transport

Iron is indispensible for life and essential for such processes as oxygen transport, electron transfer and DNA synthesis. Transferrin (Tf) is a ubiquitous protein with a central role in iron transport and metabolism. There is evidence, however, that Tf has many other biological roles in addition to its primary function of facilitating iron transport and metabolism, such as its profound effect on mammalian cell growth and productivity. The multiple functions of Tf can be exploited to develop many novel applications. Indeed, over the past several years, considerable efforts have been directed towards exploring human serum Tf (hTf), especially the use of recombinant native hTf and recombinant Tf fusion proteins, for various applications within biotechnology and medicine. Here, we review some of the remarkable progress that has been made towards the application of hTf in these diverse areas and discuss some of the exciting future prospects for hTf.

Structural biology of bacterial iron uptake

To fulfill their nutritional requirement for iron, bacteria utilize various iron sources which include the host proteins transferrin and lactoferrin, heme, and low molecular weight iron chelators termed siderophores. The iron sources are transported into the Gram-negative bacterial cell via specific uptake pathways which include an outer membrane receptor, a periplasmic binding protein (PBP), and an inner membrane ATP-binding cassette (ABC) transporter. Over the past two decades, structures for the proteins involved in bacterial iron uptake have not only been solved, but their functions have begun to be understood at the molecular level. However, the elucidation of the three dimensional structures of all components of the iron uptake pathways is currently limited. Despite the low sequence homology between different bacterial species, the available three-dimensional structures of homologous proteins are strikingly similar. Examination of the current three-dimensional structures of the outer membrane receptors, PBPs, and ABC transporters provides an overview of the structural biology of iron uptake in bacteria.

fac-{Ru(CO)3}2+-Core Complexes and Design of Metal-Based Drugs. Synthesis, Structure, and Reactivity of Ru−Thiazole Derivative with Serum Proteins and Absorption−Release Studies with Acryloyl and Silica Hydrogels as Carriers in Physiological Media

The reaction of [Ru2(CO)6Cl4], 1, with excess THZ (1,3-thiazole) in absolute ethanol at 55 degrees C produces fac-[Ru(CO)3Cl2(THZ)], 2, in high yield. [Ru(CO)2Cl2(THZ)2], 3, is formed at higher temperature (ca 70 degrees C) and higher concentration of THZ. The X-ray structures of the new compounds have been determined, and density functional studies performed at the hybrid B3LYP/(Lanl2DZ, Ru; 6-311+G**, CHClNOS) level allowed the estimation of the structures of several conformers as well as that of their relative total electronic energies. Compound 2 is soluble (slowly) in aqueous media, where it reacts with the transport proteins bovine serum albumin (BSA) and human apotransferrin (HTF), and at a lower extent with calf thymus DNA (CT-DNA) and with guanosine-5'-monophosphate (GMP). The complex molecule is adsorbed by certain synthetic acryloyl polymers that have terminal carboxylate functions and is embedded in silica gels when these latter are prepared in the presence of a solution of 2. Ruthenium species are slowly released from the loaded gels into physiological solutions at pH 7.4. The reactivity of 2 with biomolecules and synthetic hydrogels makes it a compound of interest for anticancer and antimetastases tests.

A computational study of the open and closed forms of the N-lobe human serum transferrin apoprotein

Human serum transferrin tightly binds ferric ions in the blood stream but is able to release them in cells by a process involving receptor-mediated endocytosis and decrease in pH. Iron binding and release are accompanied by a large conformation change. In this study, we investigate theoretically the open and closed forms of the N-lobe human serum transferrin apoprotein by performing pKa calculations and molecular dynamics and free-energy simulations. In agreement with the hypothesis based on the x-ray crystal structures, our calculations show that there is a shift in the pKa values of the lysines forming the dilysine trigger when the conformation changes. We argue, however, that simple electrostatic repulsion between the lysines is not sufficient to trigger domain opening and, instead, propose an alternative explanation for the dilysine-trigger effect. Analysis of the molecular dynamics and free-energy results indicate that the open form is more mobile than the closed form and is much more stable at pH 5.3, in large part due to entropic effects. Despite a lower free energy, the dynamics simulation of the open form shows that it is flexible enough to sample conformations that are consistent with iron binding.

Identification of possible kinetically significant anion-binding sites in human serum transferrin using molecular modeling strategies

Certain anions have been shown experimentally to influence the rate of iron release from human serum transferrin (HST), implying the existence of one or more allosteric kinetically significant anion-binding (KISAB) sites on or near the surface of the protein. A rank-ordered selection of potential HST KISAB sites has been obtained using a novel three-stage molecular modeling strategy. The crystal structure of HST (1A8E.pdb) was first subjected to a heuristic analysis, in which positively charged and hydrogen-bonding residues on or near the surface of the protein were identified. In this stage, a preliminary electrostatic potential map was also calculated, yielding six preliminary sites. Next, energy-grid calculations were conducted in order to identify anion-protein interaction energy minima, which resulted in the inclusion of three additional sites. Finally, three anions already shown experimentally to demonstrate varied effects on HST iron-release kinetics were placed at each potential site; molecular dynamics and molecular mechanics calculations were performed in order to elucidate the hydrogen-bonding environment around each anion of the protein as well as to calculate anion-protein-binding energies.

Crystal structure of Pasteurella haemolytica ferric ion-binding protein A reveals a novel class of bacterial iron-binding proteins

Pasteurellosis caused by the Gram-negative pathogen Pasteurella haemolytica is a serious disease leading to death in cattle. To scavenge growth-limiting iron from the host, the pathogen utilizes the periplasmic ferric ion-binding protein A (PhFbpA) as a component of an ATP-binding cassette transport pathway. We report the 1.2-A structure of the iron-free (apo) form of PhFbpA, which is a member of the transferrin structural superfamily. The protein structure adopts a closed conformation, allowing us to reliably assign putative iron-coordinating residues. Based on our analysis, PhFbpA utilizes a unique constellation of binding site residues and anions to octahedrally coordinate an iron atom. A surprising finding in the structure is the presence of two formate anions on opposite sides of the iron-binding pocket. The formate ions tether the N- and C-terminal domains of the protein and stabilize the closed structure, also providing clues as to probable candidates for synergistic anions in the iron-loaded state. PhFbpA represents a new class of bacterial iron-binding proteins.