Inequivalence of the two tyrosine ligands in the N-lobe of human serum transferrin

A conserved asparagine residue stabilizes iron binding in Manduca sexta transferrin-1

Transferrin 1 (Tsf1) is an insect-specific iron-binding protein that is abundant in hemolymph and other extracellular fluids. It binds iron tightly at neutral pH and releases iron under acidic conditions. Tsf1 influences the distribution of iron in the body and protects against infection. Elucidating the mechanisms by which Tsf1 achieves these functions will require an understanding of how Tsf1 binds and releases iron. Previously, crystallized Tsf1 from Manduca sexta was shown to have a novel type of iron coordination that involves four iron-binding ligands: two tyrosine residues (Tyr90 and Tyr204), a buried carbonate anion, and a solvent-exposed carbonate anion. The solvent-exposed carbonate anion was bound by a single amino acid residue, a highly conserved asparagine at position 121 (Asn121); thus, we predicted that Asn121 would be essential for high-affinity iron binding. To test this hypothesis, we analyzed the iron-binding and -release properties of five forms of recombinant Tsf1: wild-type, a Y90F/Y204F double mutant (negative control), and three Asn121 mutants (N121A, N121D and N121S). Each of the Asn121 mutants exhibited altered spectral properties, confirming that Asn121 contributes to iron coordination. The N121D and N121S mutations resulted in slightly lower affinity for iron, especially at acidic pH, while iron binding and release by the N121A mutant was indistinguishable from that of the wild-type protein. The surprisingly minor consequences of mutating Asn121, despite its high degree of conservation in diverse insect species, suggest that Asn121 may play a role that is essential in vivo but non-essential for high affinity iron binding in vitro.

Functional Divergence of the N-Lobe and C-Lobe of Transferrin Gene in Pungitius sinensis (Amur Stickleback)

Transferrin is an important iron-binding glycosylated protein and plays key roles in iron-binding and immune response. Here, a 2037-bp open reading frame was obtained from our previous transcriptome sequencing data of Amur stickleback, which encoded a 679 amino acid putative transferrin protein harbored obvious N-lobe and C-lobe domains. The tissue-specific expression pattern showed that the transcript was detected in a variety of tissues, with the highest signal in liver. Moreover, Streptococcus iniae pathogen stimulation can increase the expression level of this transcript, implying important immune properties for organisms. Next, N-lobes and C-lobes were obtained from 45 fish species. The phylogenetic tree showed that N-lobes and C-lobes were in two different evolutionary branches, and they had different motif composition. Functional divergence indicated a higher evolutionary rate or site-specific alteration among the N-lobe and C-lobe groups. Ka/Ks value of C-lobe group was relatively higher than that of N-lobe group, indicating a faster change rate of C-lobe sequences in evolution. Moreover, some sites experiencing positive selection were also found, which may be involved in the iron- or anion-binding, pathogen resistance and diversification of transferrin protein. Differential iron-binding activity was also detected between N-lobe and C-lobe of Amur stickleback transferrin protein with Chrome Azurol S assay. Compared with the C-lobe, the N-lobe showed stronger growth inhibitory activity of Escherichia coli, implying their potential antibacterial properties. This study will give a reference for subsequent research of transferrin proteins.

Iron binding and release properties of transferrin-1 from Drosophila melanogaster and Manduca sexta: Implications for insect iron homeostasis

Transferrins belong to an ancient family of extracellular proteins. The best-characterized transferrins are mammalian proteins that function in iron sequestration or iron transport; they accomplish these functions by having a high-affinity iron-binding site in each of their two homologous lobes. Insect hemolymph transferrins (Tsf1s) also function in iron sequestration and transport; however, sequence-based predictions of their iron-binding residues have suggested that most Tsf1s have a single, lower-affinity iron-binding site. To reconcile the apparent contradiction between the known physiological functions and predicted biochemical properties of Tsf1s, we purified and characterized the iron-binding properties of Drosophila melanogaster Tsf1 (DmTsf1), Manduca sexta Tsf1 (MsTsf1), and the amino-lobe of DmTsf1 (DmTsf1_N). Using UV-Vis spectroscopy, we found that these proteins bind iron, but they exhibit shifts in their spectra compared to mammalian transferrins. Through equilibrium dialysis experiments, we determined that DmTsf1 and MsTsf1 bind only one ferric ion; their affinity for iron is high (log K' = 18), but less than that of the well-characterized mammalian transferrins (log K' ~ 20); and they release iron under moderately acidic conditions (pH₅₀ = 5.5). Iron release analysis of DmTsf1_N suggested that iron binding in the amino-lobe is stabilized by the carboxyl-lobe. These findings will be critical for elucidating the mechanisms of Tsf1 function in iron sequestration and transport in insects.

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.

Stable nanoconjugates of transferrin with alloyed quaternary nanocrystals Ag-In-Zn-S as a biological entity for tumor recognition

One way to limit the negative effects of anti-tumor drugs on healthy cells is targeted therapy employing functionalized drug carriers. Here we present a biocompatible and stable nanoconjugate of transferrin anchored to Ag-In-Zn-S quantum dots modified with 11-mercaptoundecanoic acid (Tf-QD) as a drug carrier versus typical anticancer drug, doxorubicin. Detailed investigations of Tf-QD nanoconjugates without and with doxorubicin by fluorescence studies and cytotoxic measurements showed that the biological activity of both the transferrin and doxorubicin was fully retained in the nanoconjugate. In particular, the intercalation capabilities of free doxorubicin versus ctDNA remained essentially intact upon its binding to the nanoconjugate. In order to evaluate these capabilities, we studied the binding constant of doxorubicin attached to Tf-QDs with ctDNA as well as the binding site size on the ctDNA molecule. The binding constant slightly decreased compared to that of free doxorubicin while the binding site size, describing the number of consecutive DNA lattice residues involved in the binding, increased. It was also demonstrated that the QDs alone and in the form of a nanoconjugate with Tf were not cytotoxic towards human non-small cell lung carcinoma (H460 cell line) and the tumor cell sensitivity of the DOX-Tf-QD nanoconjugate was comparable to that of doxorubicin alone.

Incorporation of transuranium elements: coordination of Cm(iii) to human serum transferrin

Structure determination of Cm(iii)-transferrin by a combined spectroscopic and theoretical approach gives insight into the biochemical behaviour of incorporated actinides.

Phylogenetic analyses uncover a novel clade of transferrin in nonmammalian vertebrates

Transferrin is a protein super-family involved in iron transport, a central process in cellular homeostasis. Throughout the evolution of vertebrates, transferrin members have diversified into distinct subfamilies including serotransferrin, ovotransferrin, lactoferrin, melanotransferrin, the inhibitor of carbonic anhydrase, pacifastin, and the major yolk protein in sea urchin. Previous phylogenetic analyses have established the branching order of the diverse transferrin subfamilies but were mostly focused on the transferrin repertoire present in mammals. Here, we conduct a comprehensive phylogenetic analysis of transferrin protein sequences in sequenced vertebrates, placing a special focus on the less-studied nonmammalian vertebrates. Our analyses uncover a novel transferrin clade present across fish, sauropsid, and amphibian genomes but strikingly absent from mammals. Our reconstructed scenario implies that this novel class emerged through a duplication event at the vertebrate ancestor, and that it was subsequently lost in the lineage leading to mammals. We detect footprints of accelerated evolution following the duplication event, which suggest positive selection and early functional divergence of this novel clade. Interestingly, the loss of this novel class of transferrin in mammals coincided with the divergence by duplication of lactoferrin and serotransferrin in this lineage. Altogether, our results provide novel insights on the evolution of iron-binding proteins in the various vertebrate groups.

Melanotransferrin: search for a function

BACKGROUND

Melanotransferrin was discovered in the 1980s as one of the first melanoma tumour antigens. The molecule is a transferrin homologue that is found predominantly bound to the cell membrane by a glycosyl-phosphatidylinositol anchor. MTf was described as an oncofoetal antigen expressed in only small quantities in normal tissues, but in much larger amounts in neoplastic cells. Several diseases are associated with expression of melanotransferrin, including melanoma and Alzheimer's disease, although the significance of the protein to the pathogenesis of these conditions remains unclear.

SCOPE OF REVIEW

In this review, we discuss the roles of melanotransferrin in physiological and pathological processes and its potential use as an immunotherapy.

MAJOR CONCLUSIONS

Although the exact biological functions of melanotransferrin remain elusive, a growing number of roles have been attributed to the protein, including iron transport/metabolism, angiogenesis, proliferation, cellular migration and tumourigenesis.

GENERAL SIGNIFICANCE

The high expression of melanotransferrin in several disease states, particularly malignant melanoma, remains intriguing and may have clinical significance. Further studies on the biology of this protein may provide new insights as well as potential therapeutic avenues for cancer treatment. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

The role of vicinal tyrosine residues in the function of Haemophilus influenzae ferric-binding protein A

The periplasmic FbpA (ferric-binding protein A) from Haemophilus influenzae plays a critical role in acquiring iron from host transferrin, shuttling iron from the outer-membrane receptor complex to the inner-membrane transport complex responsible for transporting iron into the cytoplasm. In the present study, we report on the properties of a series of site-directed mutants of two adjacent tyrosine residues involved in iron co-ordination, and demonstrate that, in contrast with mutation of equivalent residues in the N-lobe of human transferrin, the mutant FbpAs retain significant iron-binding affinity regardless of the nature of the replacement amino acid. The Y195A and Y196A FbpAs are not only capable of binding iron, but are proficient in mediating periplasm-to-cytoplasm iron transport in a reconstituted FbpABC pathway in a specialized Escherichia coli reporter strain. This indicates that their inability to mediate iron acquisition from transferrin is due to their inability to compete for iron with receptor-bound transferrin. Wild-type iron-loaded FbpA could be crystalized in a closed or open state depending upon the crystallization conditions. The synergistic phosphate anion was not present in the iron-loaded open form, suggesting that initial anchoring of iron was mediated by the adjacent tyrosine residues and that alternate pathways for iron and anion binding and release may be considered. Collectively, these results demonstrate that the presence of a twin-tyrosine motif common to many periplasmic iron-binding proteins is critical for initially capturing the ferric ion released by the outer-membrane receptor complex.

Spectroscopic and metal-binding properties of DF3: an artificial protein able to accommodate different metal ions

The design, synthesis, and metal-binding properties of DF3, a new de novo designed di-iron protein model are described ("DF" represents due ferri, Italian for "two iron," "di-iron"). DF3 is the latest member of the DF family of synthetic proteins. They consist of helix-loop-helix hairpins, designed to dimerize and form an antiparallel four-helix bundle that encompasses a metal-binding site similar to those of non-heme carboxylate-bridged di-iron proteins. Unlike previous DF proteins, DF3 is highly soluble in water (up to 3 mM) and forms stable complexes with several metal ions (Zn, Co, and Mn), with the desired secondary structure and the expected stoichiometry of two ions per protein. UV-vis studies of Co(II) and Fe(III) complexes confirm a metal-binding environment similar to previous di-Co(II)- and di-Fe(III)-DF proteins, including the presence of a mu-oxo-di-Fe(III) unit. Interestingly, UV-vis, EPR, and resonance Raman studies suggest the interaction of a tyrosine adjacent to the di-Fe(III) center. The design of DF3 was aimed at increasing the accessibility of small molecules to the active site of the four-helix bundle. Indeed, binding of azide to the di-Fe(III) site demonstrates a more accessible metal site compared with previous DFs. In fact, fitting of the binding curve to the Hill equation allows us to quantify a 150% accessibility enhancement, with respect to DF2. All these results represent a significant step towards the development of a functional synthetic DF metalloprotein.

Structural and functional consequences of the substitution of glycine 65 with arginine in the N-lobe of human transferrin

The G65R mutation in the N-lobe of human transferrin was created to mimic a naturally occurring variant (G394R) found in the homologous C-lobe. Because Gly65 is hydrogen-bonded to the iron-binding ligand Asp63, it comprises part of the second-shell hydrogen bond network surrounding the iron within the metal-binding cleft of the protein. Substitution with an arginine residue at this position disrupts the network, resulting in much more facile removal of iron from the G65R mutant. As shown by UV-vis and EPR spectroscopy, and by kinetic assays measuring the release of iron, the G65R mutant can exist in three forms. Two of the forms (yellow and pink in color) are interconvertible. The yellow form predominates in 1 M bicarbonate; the pink form is generated from the yellow form upon exchange into 1 M HEPES buffer (pH 7.4). The third form (also pink in color) is produced by the addition of Fe(3+)-(nitrilotriacetate)(2) to apo-G65R. Hydrogen-deuterium exchange experiments are consistent with all forms of the G65R mutant assuming a more open conformation. Additionally, mass spectrometric analysis reveals the presence of nitrilotriacetate in the third form. The inability to obtain crystals of the G65R mutant led to development of a novel crystallization strategy in which the G65R/K206E double mutation stabilizes a single closed pink conformer and captures Arg65 in a single position. Collectively, these studies highlight the importance of the hydrogen bond network in the cleft, as well as the inherent flexibility of the N-lobe which, although able to adapt to accommodate the large arginine substitution, exists in multiple conformations.

Differential regulation of transferrin 1 and 2 in Aedes aegypti

Available evidence has shown that transferrins are involved in iron metabolism, immunity and development in eukaryotic organisms including insects. Here we characterize the gene and message expression profile of Aedes aegypti transferrin 2 (AaTf2) in response to iron, bacterial challenge and life stage. We show that AaTf2 shares a low similarity with A. aegypti transferrin 1 (AaTf1), but higher similarity with mammalian transferrins and avian ovotransferrin. Iron-binding pocket analysis indicates that AaTf2 has residue substitutions of Y188F, T120S, and R124S in the N lobe, and Y517N, H585N, T452S, and R456T in the C lobe, which could alter or reduce iron-binding activity. In vivo studies of message expression reveal that AaTf2 message is expressed at higher levels in larva and pupa, as well as adult female ovaries 72h post blood meal (PBM) and support that AaTf2 could play a role in larval and pupal development and in late physiological events of the gonotrophic cycle. Bacterial challenge significantly increases AaTf1 expression in ovaries at 0 and 24h PBM, but decreases AaTf2 expression in ovaries at 72h PBM, suggesting that AaTf1 and AaTf2 play different roles in immunity of female adults during a gonotrophic cycle.

Evolution reversed: the ability to bind iron restored to the N-lobe of the murine inhibitor of carbonic anhydrase by strategic mutagenesis

The murine inhibitor of carbonic anhydrase (mICA) is a member of the superfamily related to the bilobal iron transport protein transferrin (TF), which binds a ferric ion within a cleft in each lobe. Although the gene encoding ICA in humans is classified as a pseudogene, an apparently functional ICA gene has been annotated in mice, rats, cows, pigs, and dogs. All ICAs lack one (or more) of the amino acid ligands in each lobe essential for high-affinity coordination of iron and the requisite synergistic anion, carbonate. The reason why ICA family members have lost the ability to bind iron is potentially related to acquiring a new function(s), one of which is inhibition of certain carbonic anhydrase (CA) isoforms. A recombinant mutant of the mICA (W124R/S188Y) was created with the goal of restoring the ligands required for both anion (Arg124) and iron (Tyr188) binding in the N-lobe. Absorption and fluorescence spectra definitively show that the mutant binds ferric iron in the N-lobe. Electrospray ionization mass spectrometry confirms the presence of both ferric iron and carbonate. At the putative endosomal pH of 5.6, iron is released by two slow processes indicative of high-affinity coordination. Induction of specific iron binding implies that (1) the structure of mICA resembles those of other TF family members and (2) the N-lobe can adopt a conformation in which the cleft closes when iron binds. Because the conformational change in the N-lobe indicated by metal binding does not impact the inhibitory activity of mICA, inhibition of CA was tentatively assigned to the C-lobe. Proof of this assignment is provided by limited trypsin proteolysis of porcine ICA.

A novel murine protein with no effect on iron homoeostasis is homologous with transferrin and is the putative inhibitor of carbonic anhydrase

In a search for genes that modify iron homoeostasis, a gene (1300017J02Rik) was located immediately upstream of the murine TF (transferrin) gene. However, expression of the 1300017J02Rik gene product was not responsive to a number of modulators of iron metabolism. Specifically, expression was not altered in mouse models of iron disorders including mice with deficiencies in the haemochromatosis protein Hfe, the recombination-activating protein, Rag, beta2-microglobulin, TF, ceruloplasmin or Hb, or in mice with microcytic anaemia. Additionally, neither lipopolysaccharide nor hypoxia treatment resulted in any significant changes in the 1300017J02Rik expression level. The genomic DNA sequence suggested that the 1300017J02Rik gene product might be a protein equivalent to the pICA {porcine ICA [inhibitor of CA (carbonic anhydrase)]}. The coding region for the murine 1300017J02Rik gene was placed into the pNUT expression vector. Transformed BHK cells (baby-hamster kidney cells) were transfected with this plasmid, resulting in secretion of recombinant mICA (murine ICA) into the tissue culture medium. Following purification to homogeneity, the yield of mICA from the BHK cells was found to be considerably greater (at least 4-fold) than the yield of pICA from a previously reported Pichia pastoris (yeast) expression system. MS showed that the recombinant mICA was a glycoprotein that associated with CA in a 1:1 stoichiometry. Despite its high sequence similarity to TF, titration experiments showed that mICA was unable to bind iron specifically. Although enzymatic assays revealed that mICA was able to inhibit CA, it is unclear if this is its sole or even its major function since, to date, humans and other primates appear to lack functional ICA. Lastly, we note that this member of the TF superfamily is a relatively recent addition resulting from a tandem duplication event.

Evolution of the transferrin family: Conservation of residues associated with iron and anion binding

The transferrin family spans both vertebrates and invertebrates. It includes serum transferrin, ovotransferrin, lactoferrin, melanotransferrin, inhibitor of carbonic anhydrase, saxiphilin, the major yolk protein in sea urchins, the crayfish protein, pacifastin, and a protein from green algae. Most (but not all) contain two domains of around 340 residues, thought to have evolved from an ancient duplication event. For serum transferrin, ovotransferrin and lactoferrin each of the duplicated lobes binds one atom of Fe (III) and one carbonate anion. With a few notable exceptions each iron atom is coordinated to four conserved amino acid residues: an aspartic acid, two tyrosines, and a histidine, while anion binding is associated with an arginine and a threonine in close proximity. These six residues in each lobe were examined for their evolutionary conservation in the homologous N- and C-lobes of 82 complete transferrin sequences from 61 different species. Of the ligands in the N-lobe, the histidine ligand shows the most variability in sequence. Also, of note, four of the twelve insect transferrins have glutamic acid substituted for aspartic acid in the N-lobe (as seen in the bacterial ferric binding proteins). In addition, there is a wide spread substitution of lysine for the anion binding arginine in the N-lobe in many organisms including all of the fish, the sea squirt and many of the unusual family members i.e., saxiphilin and the green alga protein. It is hoped that this short analysis will provide the impetus to establish the true function of some of the TF family members that clearly lack the ability to bind iron in one or both lobes and additionally clarify the evolutionary history of this important family of proteins.

Transferrin: structure, function and potential therapeutic actions

There are many proteins that can multi-task. Transferrin, widely known as an iron-binding protein, is one such example of a multi-tasking protein. In this review, the multiple biological actions of transferrin, including its growth and cytoprotective activities, are discussed with the view of highlighting the potential therapeutic applications of this protein.

Expression, purification, and characterization of authentic monoferric and apo-human serum transferrins

Transferrin is a bilobal protein with the ability to bind iron in two binding sites situated at the bottom of a cleft in each lobe. We have previously described the production of recombinant non-glycosylated human serum transferrins (hTF-NG), containing a factor Xa cleavage site and a hexa-His tag at the amino-terminus. Constructs in this background that contain strategic mutations to completely prevent iron binding in each lobe or in both lobes have now been produced. These monoferric hTFs will allow dissection of the contribution of each lobe to transferrin function. In addition, the construct completely lacking in the ability to bind iron in either lobe provides an opportunity to assess whether hTF has any other functions in addition to iron transport. Following insertion of the His-tagged hTF molecules into the pNUT vector, transfection into baby hamster kidney cells and selection with methotrexate, the secreted recombinant proteins were isolated from the tissue culture medium and characterized with regard to their iron binding properties. Significant improvements over our previous protocol include: (1) addition of butyric acid at a level of 1mM which leads to a substantial increase in protein production (as much as a 65% increase compared to control cells); and (2) elimination of an anion exchange column prior to isolation on a Qiagen Ni-NTA column which makes purification of the His-tagged constructs faster and therefore more efficient. These improvements should be applicable to expression of other recombinant proteins in mammalian cells.

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.

The low pKa value of iron-binding ligand Tyr188 and its implication in iron release and anion binding of human transferrin

2D NMR-pH titrations were used to determine pKa values for four conserved tyrosine residues, Tyr45, Tyr85, Tyr96 and Tyr188, in human transferrin. The low pKa of Tyr188 is due to the fact that the iron-binding ligand interacts with Lys206 in open-form and with Lys296 in the closed-form of the protein. Our current results also confirm the anion binding of sulfate and arsenate to transferrin and further suggest that Tyr188 is the actual binding site for the anions in solution. These data indicate that Tyr188 is a critical residue not only for iron binding but also for chelator binding and iron release in transferrin.

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.

A Density Functional Theory Study of the Iron-Binding Site of Human Serum Transferrin

Human serum transferrin binds ferric ions with high affinity in the blood stream and releases them into cells by a process involving receptor-mediated endocytosis and a decrease in pH. The iron-release mechanism is unclear but protonation events and conformational changes are known to be important. In this study, we investigate properties of the iron-binding site theoretically. Our results suggest that an equatorial histidine could be in its histidinate form when bound to iron at neutral and high pH and that protonation of an axial tyrosine is a key event in iron release. Support for this mechanism from other metal-binding enzymes is also presented.

Oxo-iron clusters in a bacterial iron-trafficking protein: new roles for a conserved motif

We report a set of three 1.8-1.9 A resolution X-ray crystal structures of Neisseria gonorrhoeae Fbp (ferric-ion binding protein): (i) open-cleft apo-Fbp containing bound phosphate, (ii) open-cleft mono-Fe Fbp capped by nitrilotriacetate, and (iii) open-cleft trinuclear oxo-iron Fbp, the first structure of an iron-cluster adduct of a transferrin. The nine independent molecules in the unit cells provide 'snapshots' of the versatile dynamic structural roles of the conserved dityrosyl iron-binding motif (Tyr195-Tyr196) which control the capture and, possibly, processing of iron. These findings have implications for understanding bacterial iron acquisition and dissimilation, and organic/mineral interfaces.

Anion exchange in human serum transferrin N-lobe: a model study with variant His249Ala

The removal of Fe(III) from human serum transferrin by chelators is thought to proceed through intermediate species in which the chelator becomes associated with the metal center of the protein. The visible spectral shifts associated with the formation of such intermediates in the wild-type (WT) protein are too small for reliable kinetic data to be obtained. Therefore, studies were undertaken with the recombinant N-terminal lobe variant H249A, a variant showing more pronounced spectral changes. The kinetics of the synergistic anion-exchange reaction between nitrilotriacetate (NTA) and carbonate in variant H249A was studied by stopped-flow spectrophotometry as a model for this process in the WT protein. Anion exchange occurs by two pathways at pH 7.4 and 25 degrees C: an NTA-independent dissociative pathway to form a carbonate-free intermediate Fe-H249A (Eq. 1) that subsequently reacts with NTA (Eq. 2):and an NTA-dependent associative pathway (the major pathway) in which a quaternary Fe-H249A-(CO(3))(NTA) intermediate is formed (Eq. 3), which then decays to product (Eq. 4):The reverse reaction, where HCO(3)(-) exchanges for NTA, likewise follows these two pathways. The overall apparent equilibrium constant for formation of Fe-H249A-NTA from Fe-H249A-CO(3) is K'=442 at pH 7.4. The NTA complex is favored over the carbonate complex both kinetically and thermodynamically in the pH range 7.4-8.2.

Electron paramagnetic resonance analysis of transferrin-bound iron in animal models of blunt trauma

BACKGROUND

Blood iron sequestration is known to be implicated in the systemic acute-phase response to trauma injury. The objective of the present research was to assess the effect of iron sequestration in animal models of blunt trauma by means of electron paramagnetic resonance spectroscopy of iron in complex with transferrin, a main iron-transporting protein in blood, and to correlate this effect with the extent of induced injury.

METHODS

Two animal models of blunt trauma were explored in the present study. Blunt trauma in the rat model was produced by exposure of 14 animals to blast overpressure (BOP) (at peak BOP of either 86 +/- 5 kPa or 112 +/- 2 kPa) generated in a shock tube. Blunt trauma in the porcine model was produced by impact of high-speed projectiles made from a rubber-tipped, plastic composite weighing 28.64 +/- 0.12 g (mean +/- SEM, n = 8) with a length of approximately 6 cm and a diameter of approximately 4 cm. The projectiles were propelled by compressed helium onto eight animals at a velocity of 101.8 +/- 3.8 m/s (mean +/- SEM, n = 8) at the point of impact. Each experiment was accompanied by a pathology assessment using an injury scoring system developed for blunt trauma injuries to derive a severity score for whole-body involvement. Amounts of transferrin-bound iron (TRF-[Fe3+]) in whole blood and blood plasma samples were measured using quantitative electron paramagnetic resonance spectroscopy. The observed alterations in the amounts of blood TRF-[Fe3+] were correlated with estimated injury score ratios in each animal.

RESULTS

Blunt trauma produced by either BOP exposure of rats or projectile impacts in pigs was accompanied by TRF-[Fe3+] sequestration observed in both blood and blood plasma. The amount of TRF-[Fe3+] in blood was shown to have inverse correlation with the extent of injury (Pearson r = -0.90 in the rat model and r = -0.93 in the porcine model) estimated by injury score ratios and was not dependent on location of the injury (lung, liver, spleen, or jejunum).

CONCLUSION

The presented data suggest that assessment of TRF-[Fe3+] in blunt trauma can provide a good deal of information on severity of injury. The response of TRF-[Fe3+] can be considered as a potential surrogate marker of the systemic alterations in blunt trauma and, therefore, warrants further investigation in a human pilot study.

Mapping of stereoselective recognition sites on human serum transferrin by capillary electrophoresis and molecular modelling

Stereoselective recognition of chiral compounds can be used for mapping of surface interaction sites on proteins. Iron-free human serum transferrin is a suitable chiral selector in capillary electrophoresis used in native form in solution. Separation of optical isomers of tryptophan-methylester, tryptophan-ethylester and tryptophan-butylester and various drugs were studied in capillary zone electrophoresis applying a distinct transferrin zone prior to sample injection. Changes in the electrophoretic patterns (i.e., in the migration properties) of the molecules reflected the possible interactions with the protein. The tryptophan derivatives and eight drugs possessed stereoselective interactions, seven drugs showed interactions without appreciable chiral separation, and the others did not present any direct complexation with the protein molecules. Molecular modelling was performed to characterize the binding areas at the iron binding site of iron-free transferrin. The docking of tryptophan derivatives on transferrin showed that the R-enantiomers possess a stronger complexation with transferrin, whereas the S-enantiomers are bound by weaker interactions, which is in excellent agreement with the capillary electrophoresis results, where the R-enantiomers were always retarded stronger by transferrin. A ranking of drugs by the lipo score parameter of the docking shows an accordance with the stereoselective interactions by the protein.

Copper(II) complexes of novel N-alkylated derivatives of cis,cis-1,3,5-triaminocyclohexane. 1. Preparation and structure

Novel N,N',N' '-trialkylated derivatives of cis,cis-1,3,5-triaminocyclohexane (tach), designated tach-R(3), were prepared through alkylation of N-protected tach with subsequent acid deprotection, to afford N-methyl, N-ethyl, and N-n-propyl derivatives as their trihydrobromide salts. The tach-neopentyl(3) and tach-furan(3) derivatives were prepared by formation of the imine from tach and pivaldehyde or furan-2-carboxaldehyde, respectively, followed by reduction of the imine. Complexes [Cu(tach-R(3))Cl(2)] (R = Me, Et, n-Pr, CH(2)-2-thienyl, and CH(2)-2-furanyl) were prepared from CuCl(2) in MeOH or MeOH-Et(2)O solvent. Crystallographic characterization of [Cu(tach-Et(3))Br(0.8)Cl(1.2)] (Pnma, a = 8.2265(1) A, b = 12.5313(1) A, c = 15.3587(3) A, Z = 4) reveals a square-based pyramidal CuN(3)X(2) coordination sphere in which one nitrogen donor occupies the apical position at a slightly longer distance (Cu-N = 2.218(5) A) than those of the basal nitrogens (Cu-N = 2.053(2) A). The solution-phase (pH 7.4 buffered and methanol) and solid-phase structures of [Cu(tach-R(3))Cl(2)] have been studied extensively by EPR and visible-near-IR spectroscopies. The square-based pyramidal structure is retained in solution, according to correspondence of solution and solid-state data. In aqueous solution, halide is replaced by water, as indicated by the high-energy UV-vis spectral shifts and bonding parameters of [Cu(tach-Et(3))](2+)(aq) derived from EPR data. The proposed aqueous-phase species, in the pH range 7.4 to 10.1, is [Cu(tach-Et(3))(H(2)O)(2)](2+). The complex [Cu(tach-Me(3))](2+)(aq) does not appear to dimerize or form metal-hydroxo species at pH 7.4, in contrast to other Cu(II)-triamine complexes, e.g., [Cu(1,4,7-triazacyclononane)](2+) (aq) and [Cu(tach-H(3))](2+)(aq) (the complex of unalkylated tach). This difference is attributed to the steric effect of the N-alkyl groups in the tach-R(3) series.

The membrane-bound transferrin homologue melanotransferrin: roles other than iron transport?

Melanotransferrin (MTf) is a membrane-bound transferrin (Tf) homologue that is found at high levels in melanoma cells. MTf has many characteristics in common with serum Tf and previous studies have shown that it can bind Fe. This has led to speculation that MTf may be involved in Fe transport. Because Fe is required for a variety of metabolic reactions including ATP and DNA synthesis, MTf could play a role in proliferation. However, recently it has been shown that MTf plays very little role in Fe uptake by melanoma cells, and unlike other Fe transport molecules (e.g. the transferrin receptor), its expression is not controlled by Fe. In the present review the function of MTf is discussed in relation to data suggesting other roles apart from Fe uptake.

Anion-mediated iron release from transferrins. The kinetic and mechanistic model for N-lobe of ovotransferrin

Iron release process of ovotransferrin N-lobe (N-oTf) to anion/chelators has been resolved using kinetic and mechanistic approach. The iron release kinetics of N-oTf were measured at the endosomal pH of 5.6 with three different anions such as nitrilotriacetate, pyrophosphate, and sulfate using stopped flow spectrofluorimetric method, all yielding clear biphasic progress curves. The two observed rate constants and the corresponding amplitudes obtained from the double exponential curve fit to the biphasic curves varied depending on the type and concentration of anions. Several possible models for the iron release kinetic mechanism were examined on the basis of a newly introduced quantitative equation. Results from the curve fitting analyses were consistent with a dual pathway mechanism that includes the competitive iron release from two different protein states, namely, X and Y, with the respective first order rate constants of K(1) and K(2) (X, domain closed holo N-oTf; Y, anion induced different conformer of holo N-oTf). The reversible interconversions of X to Y and Y to X are driven by the second order rate constant k(3) and the first order rate constant K(4), respectively. The obtained rate constants were greatly variable for the three anions depending on the synergistic or nonsynergistic nature. In the light of the anion-binding sites of N-oTf located crystallographically, the compatible mechanistic model that includes competitive anion binding to the iron coordination sites and to a specific anion site is suggested for the dual pathway iron release mechanism.

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

The X-ray crystallographic structures of two mutants (K206Q and H207E) of the N-lobe of human transferrin (hTF/2N) have been determined to high resolution (1.8 and 2.0 A, respectively). Both mutant proteins bind iron with greater affinity than native hTF/2N. The structures of the K206Q and H207E mutants show interactions (both H-bonding and electrostatic) that stabilize the interaction of Lys296 in the closed conformation, thereby stabilizing the iron bound forms.

Mutations at nonliganding residues Tyr-85 and Glu-83 in the N-lobe of human serum transferrin. Functional second shell effects

The x-ray crystal structure of the N-lobe of human serum transferrin has shown that there is a hydrogen bond network, the so-called "second shell," around the transferrin iron binding site. Tyrosine at position 85 and glutamic acid at position 83 are two nonliganding residues in this network in the human serum transferrin N-lobe (hTF/2N). Mutation of each of these two amino acids has a profound effect on the metal binding properties of hTF/2N. When Tyr-85 is mutated to phenylalanine, iron release from the resulting mutant Y85F is much more facile than from the parent protein. Elimination of the hydrogen bond between Tyr-85 and Lys-296 appears to interfere with the "di-lysine (Lys-206-Lys-296) trigger," which affects the iron binding stability of the protein. Surprisingly, mutation of Glu-83 to alanine leads to the absence of one of the normal iron binding ligands; introduction of a monovalent anion is able to restore the normal first coordination sphere. The missing ligand appears to be His-249, as revealed by comparison of the metal binding behaviors of mutants H249Q and E83A and structural analysis. Glu-83 has a strong H bond linkage with His-249 in apo-hTF/2N, which helps to hold the His-249 in the proper position for iron binding. Disabling Glu-83 by mutation to an alanine seriously disturbs the H bond network, allowing His-249 to move away. A monovalent anion can help reestablish the normal network by providing a negative charge near the position of Glu-83 to reach charge balance, so that ligand His-249 is available again for iron binding.