Transferrin, a mechanism for iron release

Substitution of carbonate by non-physiological synergistic anion modulates the stability and iron release kinetics of serum transferrin

Serum transferrin (sTf) is a bi-lobal protein. Each lobe of sTf binds one Fe³⁺ ion in the presence of a synergistic anion. Physiologically, carbonate is the main synergistic anion but other anions such as oxalate, malonate, glycolate, maleate, glycine, etc. can substitute for carbonate in vitro. The present work provides the possible pathways by which the substitution of carbonate with oxalate affects the structural, kinetic, thermodynamic, and functional properties of blood plasma sTf. Analysis of equilibrium experiments measuring iron release and structural unfolding of carbonate and oxalate bound diferric-sTf (Fe₂sTf) as a function of pH, urea concentration, and temperature reveal that the structural and iron-centers stability of Fe₂sTf increase by substitution of carbonate with oxalate. Analysis of isothermal titration calorimetry (ITC) scans showed that the affinity of Fe³⁺ with apo-sTf is enhanced by substituting carbonate with oxalate. Analysis of kinetic and thermodynamic parameters measured for the iron release from the carbonate and oxalate bound monoferric-N-lobe of sTf (Fe_NsTf) and Fe₂sTf at pH 7.4 and pH 5.6 reveals that the substitution of carbonate with oxalate inhibits/retards the iron release via increasing the enthalpic barriers.

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.

Predicting long term cooperativity and specific modulators of receptor interactions in human transferrin from dynamics within a single microstate

PRS identifies regions contacting rapidly evolving residues that mechanically manipulate dissociation from the pathogen in the human transferrin–bacterial receptor complex.

Transferrin receptor-1 iron-acquisition pathway - synthesis, kinetics, thermodynamics and rapid cellular internalization of a holotransferrin-maghemite nanoparticle construct

BACKGROUND

Targeting nanoobjects via the iron-acquisition pathway is always reported slower than the transferrin/receptor endocytosis. Is there a remedy?

METHODS

Maghemite superparamagnetic and theragnostic nanoparticles (diameter 8.6nm) were synthesized, coated with 3-aminopropyltriethoxysilane (NP) and coupled to four holotransferrin (TFe2) by amide bonds (TFe2-NP). The constructs were characterized by X-ray diffraction, transmission electron microscopy, FTIR, X-ray Electron Spectroscopy, Inductively Coupled Plasma with Atomic Emission Spectrometry. The in-vitro protein/protein interaction of TFe2-NP with transferrin receptor-1 (R1) and endocytosis in HeLa cells were investigated spectrophotometrically, by fast T-jump kinetics and confocal microscopy.

RESULTS

In-vitro, R1 interacts with TFe2-NP with an overall dissociation constant KD=11nM. This interaction occurs in two steps: in the first, the C-lobe of the TFe2-NP interacts with R1 in 50μs: second-order rate constant, k1=6×10(10)M(-1)s(-1); first-order rate constant, k-1=9×10(4)s(-1); dissociation constant, K1d=1.5μM. In the second step, the protein/protein adduct undergoes a slow (10,000s) change in conformation to reach equilibrium. This mechanism is identical to that occurring with the free TFe2. In HeLa cells, TFe2-NP is internalized in the cytosol in less than 15min.

CONCLUSION

This is the first time that a nanoparticle-transferrin construct is shown to interact with R1 and is internalized in time scales similar to those of the free holotransferrin.

GENERAL SIGNIFICANCE

TFe2-NP behaves as free TFe2 and constitutes a model for rapidly targeting theragnostic devices via the main iron-acquisition pathway.

Transferrin-mediated cellular iron delivery

Essential to iron homeostasis is the transport of iron by the bilobal protein human serum transferrin (hTF). Each lobe (N- and C-lobe) of hTF forms a deep cleft which binds a single Fe(3+). Iron-bearing hTF in the blood binds tightly to the specific transferrin receptor (TFR), a homodimeric transmembrane protein. After undergoing endocytosis, acidification of the endosome initiates the release of Fe(3+) from hTF in a TFR-mediated process. Iron-free hTF remains tightly bound to the TFR at acidic pH; following recycling back to the cell surface, it is released to sequester more iron. Efficient delivery of iron is critically dependent on hTF/TFR interactions. Therefore, identification of the pH-specific contacts between hTF and the TFR is crucial. Recombinant protein production has enabled deconvolution of this complex system. The studies reviewed herein support a model in which pH-induced interrelated events control receptor-stimulated iron release from each lobe of hTF.

Uptake and release of metal ions by transferrin and interaction with receptor 1

BACKGROUND

For a metal to follow the iron acquisition pathway, four conditions are required: 1-complex formation with transferrin; 2-interaction with receptor 1; 3-metal release in the endosome; and 4-metal transport to cytosol.

SCOPE OF THE REVIEW

This review deals with the mechanisms of aluminum(III), cobalt(III), uranium(VI), gallium(III) and bismuth(III) uptake by transferrin and interaction with receptor 1.

MAJOR CONCLUSIONS

The interaction of the metal-loaded transferrin with receptor 1 takes place in one or two steps: a very fast first step (μs to ms) between the C-lobe and the helical domain of the receptor, and a second slow step (2-6h) between the N-lobe and the protease-like domain. In transferrin loaded with metals other than iron, the dissociation constants for the interaction of the C-lobe with TFR are in a comparable range of magnitudes 10 to 0.5μM, whereas those of the interaction of the N-lobe are several orders of magnitudes lower or not detected. Endocytosis occurs in minutes, which implies a possible internalization of the metal-loaded transferrin with only the C-lobe interacting with the receptor.

GENERAL SIGNIFICANCE

A competition with iron is possible and implies that metal internalization is more related to kinetics than thermodynamics. As for metal release in the endosome, it is faster than the recycling time of transferrin, which implies its possible liberation in the cell. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Physiological roles of ovotransferrin

BACKGROUND

Ovotransferrin is an iron-binding glycoprotein, found in avian egg white and in avian serum, belonging to the family of transferrin iron-binding glycoproteins. All transferrins show high sequence homology. In mammals are presents two different soluble glycoproteins with different functions: i) serum transferrin that is present in plasma and committed to iron transport and iron delivery to cells and ii) lactoferrin that is present in extracellular fluids and in specific granules of polymorphonuclear lymphocytes and committed to the so-called natural immunity. To the contrary, in birds, ovotransferrin remained the only soluble glycoprotein of the transferrin family present both in plasma and egg white.

SCOPE OF REVIEW

Substantial experimental evidences are summarized, illustrating the multiple physiological roles of ovotransferrin in an attempt to overcome the common belief that ovotransferrin is a protein dedicated only to iron transport and to iron withholding antibacterial activity.

MAJOR CONCLUSIONS

Similarly to the better known family member protein lactoferrin, ovotransferrin appears to be a multi-functional protein with a major role in avian natural immunity.

GENERAL SIGNIFICANCE

Biotechnological applications of ovotransferrin and ovotransferrin-related peptides could be considered in the near future, stimulating further research on this remarkable protein. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Kinetics of iron release from transferrin bound to the transferrin receptor at endosomal pH

BACKGROUND

Human serum transferrin (hTF) is a bilobal glycoprotein that reversibly binds Fe(3+) and delivers it to cells by the process of receptor-mediated endocytosis. Despite decades of research, the precise events resulting in iron release from each lobe of hTF within the endosome have not been fully delineated.

SCOPE OF REVIEW

We provide an overview of the kinetics of iron release from hTF±the transferrin receptor (TFR) at endosomal pH (5.6). A critical evaluation of the array of biophysical techniques used to determine accurate rate constants is provided.

GENERAL SIGNIFICANCE

Delivery of Fe(3+)to actively dividing cells by hTF is essential; too much or too little Fe(3+) directly impacts the well-being of an individual. Because the interaction of hTF with the TFR controls iron distribution in the body, an understanding of this process at the molecular level is essential.

MAJOR CONCLUSIONS

Not only does TFR direct the delivery of iron to the cell through the binding of hTF, kinetic data demonstrate that it also modulates iron release from the N- and C-lobes of hTF. Specifically, the TFR balances the rate of iron release from each lobe, resulting in efficient Fe(3+) release within a physiologically relevant time frame. This article is part of a Special Issue entitled Molecular Mechanisms of Iron Transport and Disorders.

Absolute quantification of transferrin in blood samples of harbour seals using HPLC-ICP-MS

Harbour seals (Phoca vitulina) are bio-indicators for the assessment of their habitat and environmental changes. Besides population parameters and trends (survival, age structure, sex ratio), the individual health status represents a further important parameter for this assessment. The health status of seals is a complex and vague term, determined by a wide range of diagnostic parameters. Quantities of important blood proteins such as transferrin (Tf), as well as altered distribution patterns of its glycoforms, are frequently used as biomarkers in clinical diagnosis. Within this context Tf quantities and a varying pattern of its glycoforms are used as indicator for e.g. certain liver diseases, which also represents one of the most frequently observed pathological indication in harbour seals of the North Sea. Currently, most assay based quantification methods for Tf are limited since they often provide only information regarding the total Tf concentration rather than information of its different glycoforms. Due to a lack of suitable seal Tf antibodies also the application of more specific antibody based approaches is not possible. Within this background a new approach for the absolute quantification of the iron-transport protein Tf in the blood of harbour seals using its characteristic iron content and HPLC-ICP-MS detection is described. Method validation was performed using a certified human serum reference material (ERM-DA470K/IFCC). A Tf concentration of 2.33 ± 0.03 g L(-1) (sum of all quantified glycoforms) has been calculated, which is in good agreement with the certified total Tf concentration of 2.35 ± 0.08 g L(-1), confirming the accuracy of the proposed analytical method. Finally, different seal samples were analysed to demonstrate the suitability of the procedure for the quantification of Tf in real samples as well as to observe modified glycoform patterns. Compared to our previous studies for the first time it was possible to quantify the serum Tf baseline reference range for male (1.42-2.35 g L(-1)) and female German North Sea seals (1.93-2.74 g L(-1)) as well as a CDT level of 0.00-0.10 g L(-1), respectively, which provides valuable further diagnostic information regarding the health status of these specific marine mammals. Compared to assay based quantification approaches the proposed technique indicates great potential to obtain comparable and traceable absolute quantitative results, which are in particular important for long term investigations. This absolute quantification is based on an accurate, traceable element standard, while assay based approaches often show variations depending on the kit quality or changing activities of the used antibodies.

The unique kinetics of iron release from transferrin: the role of receptor, lobe-lobe interactions, and salt at endosomal pH

Transferrins are a family of bilobal iron-binding proteins that play the crucial role of binding ferric iron and keeping it in solution, thereby controlling the levels of this important metal. Human serum transferrin (hTF) carries one iron in each of two similar lobes. Understanding the detailed mechanism of iron release from each lobe of hTF during receptor-mediated endocytosis has been extremely challenging because of the active participation of the transferrin receptor (TFR), salt, a chelator, lobe-lobe interactions, and the low pH within the endosome. Our use of authentic monoferric hTF (unable to bind iron in one lobe) or diferric hTF (with iron locked in one lobe) provided distinct kinetic end points, allowing us to bypass many of the previous difficulties. The capture and unambiguous assignment of all kinetic events associated with iron release by stopped-flow spectrofluorimetry, in the presence and in the absence of the TFR, unequivocally establish the decisive role of the TFR in promoting efficient and balanced iron release from both lobes of hTF during one endocytic cycle. For the first time, the four microscopic rate constants required to accurately describe the kinetics of iron removal are reported for hTF with and without the TFR. Specifically, at pH 5.6, the TFR enhances the rate of iron release from the C-lobe (7-fold to 11-fold) and slows the rate of iron release from the N-lobe (6-fold to 15-fold), making them more equivalent and producing an increase in the net rate of iron removal from Fe(2)hTF. Calculated cooperativity factors, in addition to plots of time-dependent species distributions in the absence and in the presence of the TFR, clearly illustrate the differences. Accurate rate constants for the pH and salt-induced conformational changes in each lobe precisely delineate how delivery of iron within the physiologically relevant time frame of 2 min might be accomplished.

Inequivalent contribution of the five tryptophan residues in the C-lobe of human serum transferrin to the fluorescence increase when iron is released

Human serum transferrin (hTF), with two Fe3+ binding lobes, transports iron into cells. Diferric hTF preferentially binds to a specific receptor (TFR) on the surface of cells, and the complex undergoes clathrin dependent receptor-mediated endocytosis. The clathrin-coated vesicle fuses with an endosome where the pH is lowered, facilitating iron release from hTF. On a biologically relevant time scale (2-3 min), the factors critical to iron release include pH, anions, a chelator, and the interaction of hTF with the TFR. Previous work, in which the increase in the intrinsic fluorescence signal was used to monitor iron release from the hTF/TFR complex, established that the TFR significantly enhances the rate of iron release from the C-lobe of hTF. In the current study, the role of the five C-lobe Trp residues in reporting the fluorescence change has been evaluated (+/-sTFR). Only four of the five recombinant Trp --> Phe mutants produced well. A single slow rate constant for iron release is found for the monoferric C-lobe (FeC hTF) and the four Trp mutants in the FeC hTF background. The three Trp residues equivalent to those in the N-lobe differed from the N-lobe and each other in their contributions to the fluorescent signal. Two rate constants are observed for the FeC hTF control and the four Trp mutants in complex with the TFR: k(obsC1) reports conformational changes in the C-lobe initiated by the TFR, and k(obsC2) is ascribed to iron release. Excitation at 295 nm (Trp only) and at 280 nm (Trp and Tyr) reveals interesting and significant differences in the rate constants for the complex.

Human serum transferrin: a tale of two lobes. Urea gel and steady state fluorescence analysis of recombinant transferrins as a function of pH, time, and the soluble portion of the transferrin receptor

Iron release from human serum transferrin (hTF) has been studied extensively; however, the molecular details of the mechanism(s) remain incomplete. This is in part due to the complexity of this process, which is influenced by lobe-lobe interactions, the transferrin receptor (TFR), the salt effect, the presence of a chelator, and acidification within the endosome, resulting in iron release. The present work brings together many of the concepts and assertions derived from previous studies in a methodical, uniform, and visual manner. Examination of earlier work reveals some uncertainty due to sample and technical limitations. We have used a combination of steady-state fluorescence and urea gels to evaluate the effect of conformation, pH, time, and the soluble portion of the TFR (sTFR) on iron release from each lobe of hTF. The use of authentic recombinant monoferric and locked species removes any possibility of cross-contamination by acquisition of iron. Elimination of detergent by use of the sTFR provides a further technical advantage. We find that iron release from the N-lobe is very sensitive to the conformation of the C-lobe, but is insensitive to the presence of the sTFR or to changes in pH (between 5.6 and 6.4). Specifically, when the cleft of the C-lobe is locked, the urea gels indicate that only about half of the iron is completely removed from the cleft of the N-lobe. Iron release from the C-lobe is most affected by the presence of the sTFR and changes in pH, but is unaffected by the conformation of the N-lobe. A model for iron release from diferric hTF is provided to delineate our findings.

A general map of iron metabolism and tissue-specific subnetworks

Iron is required for survival of mammalian cells. Recently, understanding of iron metabolism and trafficking has increased dramatically, revealing a complex, interacting network largely unknown just a few years ago. This provides an excellent model for systems biology development and analysis. The first step in such an analysis is the construction of a structural network of iron metabolism, which we present here. This network was created using CellDesigner version 3.5.2 and includes reactions occurring in mammalian cells of numerous tissue types. The iron metabolic network contains 151 chemical species and 107 reactions and transport steps. Starting from this general model, we construct iron networks for specific tissues and cells that are fundamental to maintaining body iron homeostasis. We include subnetworks for cells of the intestine and liver, tissues important in iron uptake and storage, respectively, as well as the reticulocyte and macrophage, key cells in iron utilization and recycling. The addition of kinetic information to our structural network will permit the simulation of iron metabolism in different tissues as well as in health and disease.

Nonheme-iron histochemistry for light and electron microscopy: a historical, theoretical and technical review

We reviewed the methods of nonheme-iron histochemistry with special focus on the underlying chemical principles. The term nonheme-iron includes heterogeneous species of iron complexes where iron is more loosely bound to low-molecular weight organic bases and proteins than that of heme (iron-protoporphyrin complex). Nonheme-iron is liberated in dilute acid solutions and available for conventional histochemistry by the Perls and Turnbull and other methods using iron chelators, which depend on the production of insoluble iron compounds. Treatment with strong oxidative agents is required for the liberation of heme-iron, which therefore is not stained by conventional histochemistry. The Perls method most commonly used in laboratory investigations largely stains ferric iron, but stains some ferrous iron as well, while the Turnbull method is specific for the latter. Although the Turnbull method performed on sections fails in staining ferrous iron or stains only such parts of the tissue where iron is heavily accumulated, an in vivo perfusion-Turnbull method demonstrated the ubiquitous distribution of ferrous iron, particularly in lysosomes. The Perls or Turnbull reaction is enhanced by DAB/silver/gold methods for electron microscopy. The iron sulfide method and the staining of redox-active iron with H(2)O(2) and DAB are also applicable for electron microscopy. Although the above histochemical methods have advantages for visualizing iron by conventional light and electron microscopy, the quantitative estimation of iron is not easy. Recent methods depending on the quenching of fluorescent divalent metal indicators by Fe(2+) and dequenching by divalent metal chelators have enabled the quantitative estimation of chelatable Fe(2+) in isolated viable cells.

The mechanism of iron release from the transferrin-receptor 1 adduct

We report the determination in cell-free assays of the mechanism of iron release from the N-lobe and C-lobe of human serum transferrin in interaction with intact transferrin receptor 1 at 4.3< or =pH< or =6.5. Iron is first released from the N-lobe in the tens of milliseconds range and then from the C-lobe in the hundreds of seconds range. In both cases, iron loss is rate-controlled by slow proton transfers, rate constant for the N-lobe k(1)=1.20(+/-0.05)x10(6)M(-1)s(-1) and for the C-lobe k(2)=1.6(+/-0.1)x10(3)M(-1)s(-1). This iron loss is subsequent to a fast proton-driven decarbonation and is followed by two proton gains, (pK(1a))/2=5.28 per proton for the N-lobe and (pK(2a))/2=5.10 per proton for the C-lobe. Under similar experimental conditions, iron loss is about 17-fold faster from the N-lobe and is at least 200-fold faster from the C-lobe when compared to holotransferrin in the absence of receptor 1. After iron release, the apotransferrin-receptor adduct undergoes a slow partial dissociation controlled by a change in the conformation of the receptor; rate constant k(3)=1.7(+/-0.1)x10(-3)s(-1). At endosomic pH, the final equilibrated state is attained in about 1000 s, after which the free apotransferrin, two prototropic species of the acidic form of the receptor and apotransferrin interacting with the receptor coexist simultaneously. However, since recycling of the vesicle containing the receptor to the cell surface takes a few minutes, the major part of transferrin will still be forwarded to the biological fluid in the form of the apotransferrin-receptor protein-protein adduct.

Large cooperativity in the removal of iron from transferrin at physiological temperature and chloride ion concentration

Iron removal from serum transferrin by various chelators has been studied by gel electrophoresis, which allows direct quantitation of all four forms of transferrin (diferric, C-monoferric, N-monoferric, and apotransferrin). Large cooperativity between the two lobes of serum transferrin is found for iron removal by several different chelators near physiological conditions (pH 7.4, 37 degrees C, 150 mM NaCl, 20 mM NaHCO(3)). This cooperativity is manifested in a dramatic decrease in the rate of iron removal from the N-monoferric transferrin as compared with iron removal from the other forms of ferric transferrin. Cooperativity is diminished as the pH is decreased; it is also very sensitive to changes in chloride ion concentration, with a maximum cooperativity at 150 mM NaCl. A mechanism is proposed that requires closure of the C-lobe before iron removal from the N-lobe can be effected; the "open" conformation of the C-lobe blocks a kinetically significant anion-binding site of the N-lobe, preventing its opening. Physiological implications of this cooperativity are discussed.

Structure of the human transferrin receptor-transferrin complex

Iron, insoluble as free Fe(3+) and toxic as free Fe(2+), is distributed through the body as Fe(3+) bound to transferrin (Tf) for delivery to cells by endocytosis of its complex with transferrin receptor (TfR). Although much is understood of the transferrin endocytotic cycle, little has been uncovered of the molecular details underlying the formation of the receptor-transferrin complex. Using cryo-electron microscopy, we have produced a density map of the TfR-Tf complex at subnanometer resolution. An atomic model, obtained by fitting crystal structures of diferric Tf and the receptor ectodomain into the map, shows that the Tf N-lobe is sandwiched between the membrane and the TfR ectodomain and that the C-lobe abuts the receptor helical domain. When Tf binds receptor, its N-lobe moves by about 9 A with respect to its C-lobe. The structure of TfR-Tf complex helps account for known differences in the iron-release properties of free and receptor bound Tf.

Kinetics and mechanism of iron release from the bacterial ferric binding protein nFbp: exogenous anion influence and comparison with mammalian transferrin

Ferric binding protein, Fbp, serves an essential biological function in shuttling naked (hydrated) Fe(3+) across the periplasmic space of many Gram-negative bacteria. In this process, iron must be released at the cytoplasmic membrane to a permease. How iron is released from Fbp has yet to be resolved. Consequently, understanding the dynamics of iron release from Fbp is of both biological and chemical interest. Fbp requires an exogenous anion, e.g. phosphate when isolated from cell lysates, for tight iron sequestration. To address the role of exogenous anion identity and lability on Fe(aq)(3+) dissociation from Fbp, the kinetics of PO(4)(3-) exchange in Fe(3+) nFbp(PO(4)) ( nFbp=recombinant Fbp from Neisseria meningitidis) were investigated by dynamic (31)P NMR and the kinetics of Fe(3+) dissociation from Fe(3+) nFbp(X) (X=PO(4)(3-), citrate anion) were investigated by stopped-flow pH-jump measurements. We justify the use of non-physiological low-pH conditions because a high [H(+)] will drive the Fe(aq)(3+) dissociation reaction to completion without using competing chelators, whose presence may complicate or influence the dissociation mechanism. For perspective, these studies of nFbp (which has been referred to as a bacterial transferrin) are compared to new and previously published kinetic and thermodynamic data for mammalian transferrin. Significantly, we address the lability of the Fe(3+) coordination shell in nFbp, Fe(3+) nFbp(X) (X=PO(4)(3-), citrate), with respect to exogenous anion (X(n-)) exchange and dissociation, and ultimately complete dissociation of the protein to yield naked (hydrated) Fe(aq)(3+). These findings are a first step in understanding the process of iron donation to the bacterial permease for transport across the cytoplasmic membrane.

Spectral studies on aluminum ion binding to the ligands with phenolic group(s): implications for the differences between N- and C-terminal binding sites of human serum apotransferrin

Both the binding and releasing of ferric ions in C-, and N-terminal binding sites of human serum transferrin are different. To understand the difference here the interactions of aluminum with the ligands containing phenolic group(s), including 8-hydroxyquinoline, salicylic acid, N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid, N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine], and human serum apotransferrin, respectively, are investigated by using UV difference and fluorescence spectra methods in 0.1 M N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid at pH 7.4. Aluminum binding produces a UV difference peak near 235 nm that is characteristic of phenolic groups binding to aluminum. The peak at 235 nm has been used to determine conditional binding constants of log K(Al-HBED)=8.88+/-0.74 and log K(Al-EHPG)=9.38+/-0.03. However, the effects of aluminum binding on the fluorescence intensity of N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine], salicylic acid and N,N'-di(2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid, 8-hydroxyquinoline are disparate, the former showing a decrease and the latter an increase. At pH 7.4, there is N cdots, three dots, centered H-O type intramolecular hydrogen bond in 8-hydroxyquinoline, N,N'-di(2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid and O cdots, three dots, centered H-O type intramolecular hydrogen bond in salicylic acid, N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine]. The effects of salts on the fluorescence intensity of the ligands containing phenolic group(s) show that fluorescence emission increases with the breaking of an N cdots, three dots, centered H-O type intramolecular hydrogen bond and fluorescence emission decreases with the breaking of an O cdots, three dots, centered H-O type intramolecular hydrogen bond. Fluorescence titrations of apotransferrin and both forms of monoferric transferrin with aluminum indicated that there is O cdots, three dots, centered H-O type intramolecular hydrogen bonds for the phenolic groups of Tyr426 and Tyr517 in the C-terminal binding site. While N cdots, three dots, centered H-O type intramolecular hydrogen bonds are found for the phenolic groups of Tyr95 and Tyr188 in the N-terminal binding site.

Dealing with iron: common structural principles in proteins that transport iron and heme

Iron is essential to life, but poses severe problems because of its toxicity and the insolubility of hydrated ferric ions at neutral pH. In animals, a family of proteins called transferrins are responsible for the sequestration, transport, and distribution of free iron. Comparison of the structure and function of transferrins with a completely unrelated protein hemopexin, which carries out the same function for heme, identifies molecular features that contribute to a successful protein system for iron acquisition, transport, and release. These include a two-domain protein structure with flexible hinges that allow these domains to enclose the bound ligand and provide suitable chemistry for stable binding and an appropriate trigger for release.

Iron metabolism

The understanding of iron metabolism at the molecular level has been enormously expanded in recent years by new findings about the functioning of transferrin, the transferrin receptor and ferritin. Other recent developments include the discovery of the hemochromatosis gene HFE, identification of previously unknown proteins involved in iron transport, divalent metal transporter 1 and stimulator of Fe transport, and expanded insights into the regulation and expression of proteins involved in iron metabolism. Interactions among principal participants in iron transport have been uncovered, although the complexity of such interactions is still incompletely understood. Correlated efforts involving techniques and concepts of crystallography, spectroscopy and molecular biology applied to cellular processes have been, and should continue to be, particularly revealing.

The pH-induced release of iron from transferrin investigated with a continuum electrostatic model

A reduction in pH induces the release of iron from transferrin in a process that involves a conformational change in the protein from a closed to an open form. Experimental evidence suggests that there must be changes in the protonation states of certain, as yet not clearly identified, residues in the protein accompanying this conformational change. Such changes in protonation states of residues and the consequent changes in electrostatic interactions are assumed to play a large part in the mechanism of release of iron from transferrin. Using the x-ray crystal structures of human ferri- and apo-lactoferrin, we calculated the pKa values of the titratable residues in both the closed (iron-loaded) and open (iron-free) conformations with a continuum electrostatic model. With the knowledge of a residue's pKa value, its most probable protonation state at any specified pH may be determined. The preliminary results presented here are in good agreement with the experimental observation that the binding of ferric iron and the synergistic anion bicarbonate/carbonate results in the release of approximately three H+ ions. It is suggested that the release of these three H+ ions may be accounted for, in most part, by the deprotonation of the bicarbonate and residues Tyr-92, Lys-243, Lys-282, and Lys-285 together with the protonation of residues Asp-217 and Lys-277.

Tertiary structural changes and iron release from human serum transferrin

Iron release from human serum transferrin was investigated by comparison of the extent of bound iron, measured by charge transfer absorption band intensity (465 nm), with changes observed by small-angle solution X-ray scattering (SAXS) for a series of equilibrated samples between pH 5.69 and 7.77. The phosphate buffers used in this study promote iron release at relatively high pH values, with an empirical pK of 6.9 for the convolved release from the two sites. The spectral data reveal that the N-lobe release is nearly complete by pH 7.0, while the C-lobe remains primarily metal-laden. Conversely, the radius of gyration, Rg, determined from the SAXS data remains constant between pH 7.77 and 7.05, and the evolution of Rg between its value observed for the diferric protein at pH 7.77 (31.2+/-0.2 A) and that of the apo protein at pH 5.69 (33.9+/-0.4 A) exhibits an empirical pK of 6.6. While Rg is effectively constant in the pH range associated with iron release from the N-lobe, the radius of gyration of cross-section, Rc, increases from 16.9+/-0.2 A to 17.6+/-0.2 A. Model simulations suggest that two different rotations of the NII domain relative to the NI domain about a hinge deep in the iron-binding cleft of the N-lobe, one parallel with and one perpendicular to the plane of the iron-binding site, can be significantly advanced relative to their holo protein positions while yielding constant Rg and increased Rc values consistent with the scattering data. Rotation of the CII domain parallel with the C-lobe iron-binding site plane can partially account for the increased Rg values measured at low pH; however, no reasonable combined repositioning of the NII and CII domains yields the experimentally observed increase in Rg.