Rapid and parallel formation of Fe3+ multimers, including a trimer, during H-type subunit ferritin mineralization

Supramolecular protein polymers using mini-ferritin Dps as the building block

Production of long polymer chains with iron oxidation and storage activity, built from protein nanocages using a click chemistry approach.

Direct Evidence for Ferrous Ion Oxidation and Incorporation in the Absence of Oxidants by Dps from Marinobacter hydrocarbonoclasticus

Dps proteins (DNA-binding protein from starved cells) are hollow-sphere-shaped, dodecameric enzymes found in bacteria and archaeal species. They can oxidize ferrous iron in a controlled manner using hydrogen peroxide or molecular oxygen as co-substrate, and most of them confer physical protection through DNA binding. Oxidized iron is stored, as a mineral core, in a central cavity. Direct evidence is now provided that, furthermore, Dps proteins containing small mineral cores can oxidize and mineralize toxic ferrous ions in anaerobic conditions and in the absence of any additional aqueous oxidant co-substrate. Dps proteins containing cores of 24 irons per dodecamer can oxidize about 5 ferrous irons per dodecamer, with that number approximately doubling for protein particles containing in average 96 irons per protein. This additional activity carries importance as it can be a detoxification mechanism present during anaerobic or oxygen-limited growth conditions.

About the use of 13C-13C NOESY in bioinorganic chemistry

Herein we present examples of the application of the ¹³C-¹³C Nuclear Overhauser Effect Spectroscopy (NOESY) experiment to the study of metalloproteins and we critically discuss the advantages and drawbacks of the method as a function of the molecular size of the investigated systems. The contribution is focused on a few case studies among the systems analyzed in the group of the corresponding author. The ¹³C-¹³C NOESY experiment represents the gold standard for the observation of NMR signals in the 480 kDa ferritin nanocage and for monitoring its interaction with iron. By decreasing the protein size, the experiment progressively loses its importance as a tool for the detection of the complete spin pattern of the amino acid side chains, as exemplified by nickel-dependent regulatory protein, NikR (molecular mass of the homo-tetramer ~80 kDa). In very small proteins, such as mitochondrial cytochrome c (12.3 kDa), we are only able to detect cross peaks between adjacent ¹³C nuclei; this feature turned out to be useful for the assignment of the ¹³C core resonances of the porphyrin in a uniformly enriched heme.

Spectroscopic evidence for the role of a site of the di-iron catalytic center of ferritins in tuning the kinetics of Fe(ii) oxidation

Ferritin is a nanocage protein made of 24 subunits. Its major role is to manage intracellular concentrations of free Fe(ii) and Fe(iii) ions, which is pivotal for iron homeostasis across all domains of life. This function of the protein is regulated by a conserved di-iron catalytic center and has been the subject of extensive studies over the past 50 years. Yet, it has not been fully understood how Fe(ii) is oxidized in the di-iron catalytic center and it is not known why eukaryotic and microbial ferritins oxidize Fe(ii) with different kinetics. In an attempt to obtain a new insight into the mechanism of Fe(ii) oxidation and understand the origin of the observed differences in the catalysis of Fe(ii) oxidation among ferritins we studied and compared the mechanism of Fe(ii) oxidation in the eukaryotic human H-type ferritin (HuHF) and the archaeal ferritin from Pyrococcus furiosus (PfFtn). The results show that the spectroscopic characteristics of the intermediate of Fe(ii) oxidation and the Fe(iii)-products are the same in these two ferritins supporting the proposal of unity in the mechanism of Fe(ii) oxidation among eukaryotic and microbial ferritins. Moreover, we observed that a site in the di-iron catalytic center controls the distribution of Fe(ii) among subunits of HuHF and PfFtn differently. This observation explains the reported differences between HuHF and PfFtn in the kinetics of Fe(ii) oxidation and the amount of O₂ consumed per Fe(ii) oxidized. These results provide a fresh understanding of the mechanism of Fe(ii) oxidation by ferritins.

Iron Oxidation and Core Formation in Recombinant Heteropolymeric Human Ferritins

In animals, the iron storage and detoxification protein, ferritin, is composed of two functionally and genetically distinct subunit types, H (heavy) and L (light), which co-assemble in various ratios with tissue specific distributions to form shell-like protein structures of 24 subunits within which a mineralized iron core is stored. The H-subunit possesses a ferroxidase center (FC) that catalyzes Fe(II) oxidation, whereas the L-subunit does not. To assess the role of the L-subunit in iron oxidation and core formation, two human recombinant heteropolymeric ferritins, designated H-rich and L-rich with ratios of ∼20H:4L and ∼22L:2H, respectively, were employed and compared to the human homopolymeric H-subunit ferritin (HuHF). These heteropolymeric ferritins have a composition similar to the composition of those found in hearts and brains (i.e., H-rich) and in livers and spleens (i.e., L-rich). As for HuHF, iron oxidation in H-rich ferritin was found to proceed with a 2:1 Fe(II):O₂ stoichiometry at an iron level of 2 Fe(II) atoms/H-subunit with the generation of H₂O₂. The H₂O₂ reacted with additional Fe(II) in a 2:1 Fe(II):H₂O₂ ratio, thus avoiding the production of hydroxyl radical. A μ-1,2-peroxo-diFe(III) intermediate was observed at the FC of H-rich ferritin as for HuHF. Importantly, the H-rich protein regenerated full ferroxidase activity more rapidly than HuHF did and additionally formed larger iron cores, indicating dual roles for the L-subunit in facilitating iron turnover at the FC and in mineralization of the core. The L-rich ferritin, while also facilitating iron oxidation at the FC, additionally promoted oxidation at the mineral surface once the iron binding capacity of the FC was exceeded.

Chemistry at the protein-mineral interface in L-ferritin assists the assembly of a functional (μ3-oxo)Tris[(μ2-peroxo)] triiron(III) cluster

X-ray structures of homopolymeric L-ferritin obtained by freezing protein crystals at increasing exposure times to a ferrous solution showed the progressive formation of a triiron cluster on the inner cage surface of each subunit. After 60 min exposure, a fully assembled (μ³-oxo)Tris[(μ²-peroxo)(μ²-glutamato-κO:κO')](glutamato-κO)(diaquo)triiron(III) anionic cluster appears in human L-ferritin. Glu60, Glu61, and Glu64 provide the anchoring of the cluster to the protein cage. Glu57 shuttles incoming iron ions toward the cluster. We observed a similar metallocluster in horse spleen L-ferritin, indicating that it represents a common feature of mammalian L-ferritins. The structures suggest a mechanism for iron mineral formation at the protein interface. The functional significance of the observed patch of carboxylate side chains and resulting metallocluster for biomineralization emerges from the lower iron oxidation rate measured in the E60AE61AE64A variant of human L-ferritin, leading to the proposal that the observed metallocluster corresponds to the suggested, but yet unobserved, nucleation site of L-ferritin.

The Ferritin Superfamily

Iron is very important in many biological processes and the ferritin protein family has evolved to store iron and to maintain cellular iron homeostasis. The deletion of the coding gene for the H subunit of ferritin leads to early embryonic death in mice and mutations in the gene for the L subunits in humans has been observed in neurodegenerative diseases, such as neuroferritinopathy. Thus, understanding how ferritin works is imperative and many studies have been conducted to delineate the molecular mechanism of ferritins and bacterioferritins. In the ferritin protein family, it is clear that a catalytic center for iron oxidation, the routes for iron to reach this center and the ability to nucleate an iron core, are common requirements for all ferritins. However, there are differences in the structural and mechanistic details of iron oxidation and mineralization. Although a common mechanism has been proposed for all ferritins, this mechanism needs to be further explored. There is a mechanistic diversity related to structural variation in the ferritin protein family. It is clear that other factors appear to affect the mechanism of iron oxidation and mineralization. This review focusses on the structural features of the ferritin protein family and its role in the mechanism of iron mineralization.

Mechanisms of iron mineralization in ferritins: one size does not fit all

Significant progress has been made in recent years toward understanding the processes by which an iron mineral is deposited within members of the ferritin family of 24mer iron storage proteins, enabled by high-resolution structures together with spectroscopic and kinetic studies. These suggest common characteristics that are shared between ferritins, namely, a highly symmetric arrangement of subunits that provides a protein coat around a central cavity in which the mineral is formed, channels through the coat that facilitate ingress and egress of ions, and catalytic sites, called ferroxidase centers, that drive Fe(2+) oxidation. They also reveal significant variations in both structure and mechanism amongst ferritins. Here, we describe three general types of structurally distinct ferroxidase center and the mechanisms of mineralization that they are associated with. The highlighted variation leads us to conclude that there is no universal mechanism by which ferritins function, but instead there exists several distinct mechanisms of ferritin iron mineralization.

Functionality of the three-site ferroxidase center of Escherichia coli bacterial ferritin (EcFtnA)

At least three ferritins are found in the bacterium Escherichia coli : the heme-containing bacterioferritin (EcBFR) and two nonheme bacterial ferritins (EcFtnA and EcFtnB). In addition to the conserved A and B sites of the diiron ferroxidase center, EcFtnA has a third iron-binding site (the C site) of unknown function that is nearby the diiron site. In the present work, the complex chemistry of iron oxidation and deposition in EcFtnA was further defined through a combination of oximetry, pH stat, stopped-flow and conventional kinetics, UV-vis, fluorescence, and EPR spectroscopic measurements on both the wild-type protein and site-directed variants of the A, B, and C sites. The data reveal that although H2O2 is a product of dioxygen reduction in EcFtnA and oxidation occurs with a stoichiometry of Fe(2+)/O2 ∼ 3:1 most of the H2O2 produced is consumed in subsequent reactions with a 2:1 Fe(2+)/H2O2 stoichiometry, thus suppressing hydroxyl-radical formation. Although the A and B sites are essential for rapid iron oxidation, the C site slows oxidation and suppresses iron turnover at the ferroxidase center. A tyrosyl radical, assigned to Tyr24 near the ferroxidase center, is formed during iron oxidation, and its possible significance to the function of the protein is discussed. Taken as a whole, the data indicate that there are multiple iron-oxidation pathways in EcFtnA with O2 and H2O2 as oxidants. Furthermore, our data do not support a universal mechanism for iron oxidation in all ferritins whereby the C site acts as transit site, as has been recently proposed.

Desulfovibrio vulgaris bacterioferritin uses H(2)O(2) as a co-substrate for iron oxidation and reveals DPS-like DNA protection and binding activities

A gene encoding Bfr (bacterioferritin) was identified and isolated from the genome of Desulfovibrio vulgaris cells, and overexpressed in Escherichia coli. In vitro, H(2)O(2) oxidizes Fe(2+) ions at much higher reaction rates than O(2). The H(2)O(2) oxidation of two Fe(2+) ions was proven by Mössbauer spectroscopy of rapid freeze-quenched samples. On the basis of the Mössbauer parameters of the intermediate species we propose that D. vulgaris Bfr follows a mineralization mechanism similar to the one reported for vertebrate H-type ferritins subunits, in which a diferrous centre at the ferroxidase site is oxidized to diferric intermediate species, that are subsequently translocated into the inner nanocavity. D. vulgaris recombinant Bfr oxidizes and stores up to 600 iron atoms per protein. This Bfr is able to bind DNA and protect it against hydroxyl radical and DNase deleterious effects. The use of H(2)O(2) as an oxidant, combined with the DNA binding and protection activities, seems to indicate a DPS (DNA-binding protein from starved cells)-like role for D. vulgaris Bfr.

The Mössbauer and magnetic properties of ferritin cores

BACKGROUND

Mössbauer and magnetization measurements, singly or in combination, extract detailed information on the microscopic or internal magnetism of iron-based materials and their macroscopic or bulk magnetization. The combination of the two techniques affords a powerful investigatory probe into spin relaxation processes of nanosize magnetic systems. The ferritin core constitutes a paradigm of such nano-magnetic system where Mössbauer and magnetization studies have been broadly combined in order to elucidate its composition, the initial steps of iron nucleation and biomineralization, particle growth and core-size distribution. In vivo produced and in vitro reconstituted wild-type and variant ferritins have been extensively studied in order to elucidate structure/function correlations and ferritin's role in iron overloading or neurodegenerative disorders.

SCOPE OF REVIEW

Studies on the initial stages of iron biomineralization, biomimetic synthetic analogues and ferrous ion retention within the ferritin core are presented. The dynamical magnetic properties of ferritin by Mössbauer and magnetization measurements are critically reviewed. The focus is on experiments that reveal the internal magnetic structure of the ferritin core. Novel magnetic measurements on individual ferritin molecules via AFM and nanoSQUID investigations are also mentioned.

MAJOR CONCLUSIONS

A complex two-phase spin system is revealed due to finite-size effects and non-compensated spins at the surface of the anti-ferromagnetic ferritin core. Below the blocking temperature surface spins participate in relaxation processes much faster than those associated with collective magnetic excitations of interior spins.

GENERAL SIGNIFICANCE

The studies reviewed contribute uniquely to the elucidation of the spin-structure and spin-dynamics of anti-ferromagnetic nanolattices and their possible applications to nano/bio-technology.

Effects of ascorbic acid, phytic acid and tannic acid on iron bioavailability from reconstituted ferritin measured by an in vitro digestion-Caco-2 cell model

The effects of ascorbic acid (AA), phytate and tannic acid (TA) on Fe bioavailability from Fe supplied as reconstituted ferritin were compared with FeSO4 using an in vitro digestion-Caco-2 cell model. Horse spleen apoferritin was chemically reconstituted into an animal-type ferritin (HSF) and a plant-type ferritin (P-HSF) according to the typical ratios of Fe:P found in these molecules. In the presence of AA (Fe:AA molar ratio of 1:20), significantly more Fe was absorbed from FeSO4 (about 303 %), HSF (about 454 %) and P-HSF (about 371 %) when compared with ferrous sulfate or ferritin without AA. Phytic acid (PA; Fe:PA molar ratio of 1:20) significantly reduced Fe bioavailability from FeSO4 (about 86 %), HSF (about 82 %) and P-HSF (about 93 %) relative to FeSO4 and the ferritin controls. Treatment with TA (Fe:TA molar ratio of 1:1) significantly decreased Fe bioavailability (about 97 %) from both FeSO4 and the ferritin samples. AA was able to partially reverse the negative effect of PA (Fe:PA:AA molar ratio of 1:20:20) on Fe bioavailability but did not reverse the inhibiting effect of TA (Fe:TA:AA molar ratio of 1:1:20) on Fe bioavailability from ferritin and FeSO4. Overall, there were no significant differences in bioavailable Fe between P-HSF, HSF or FeSO4. Furthermore, the addition of AA (a known promoter) or the inhibitors, PA and TA, or both, did not result in significant differences in bioavailable Fe from ferritin relative to FeSO4. The results suggest that Fe in the reconstituted ferritin molecule is easily released during in vitro digestion and interacts with known promoters and inhibitors.

Spectroscopic definition of the ferroxidase site in M ferritin: comparison of binuclear substrate vs cofactor active sites

Maxi ferritins, 24 subunit protein nanocages, are essential in humans, plants, bacteria, and other animals for the concentration and storage of iron as hydrated ferric oxide, while minimizing free radical generation or use by pathogens. Formation of the precursors to these ferric oxides is catalyzed at a nonheme biferrous substrate site, which has some parallels with the cofactor sites in other biferrous enzymes. A combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD (VTVH MCD) has been used to probe Fe(II) binding to the substrate active site in frog M ferritin. These data determined that the active site within each subunit consists of two inequivalent five-coordinate (5C) ferrous centers that are weakly antiferromagnetically coupled, consistent with a mu-1,3 carboxylate bridge. The active site ligand set is unusual and likely includes a terminal water bound to each Fe(II) center. The Fe(II) ions bind to the active sites in a concerted manner, and cooperativity among the sites in each subunit is observed, potentially providing a mechanism for the control of ferritin iron loading. Differences in geometric and electronic structure--including a weak ligand field, availability of two water ligands at the biferrous substrate site, and the single carboxylate bridge in ferritin--coincide with the divergent reaction pathways observed between this substrate site and the previously studied cofactor active sites.

Ferritins: iron/oxygen biominerals in protein nanocages

Ferritin protein nanocages that form iron oxy biominerals in the central nanometer cavity are nature's answer to managing iron and oxygen; gene deletions are lethal in mammals and render bacteria more vulnerable to host release of antipathogen oxidants. The multifunctional, multisubunit proteins couple iron with oxygen (maxi-ferritins) or hydrogen peroxide (mini-ferritins) at catalytic sites that are related to di-iron sites oxidases, ribonucleotide reductase, methane monooxygenase and fatty acid desaturases, and synthesize mineral precursors. Gated pores, distributed symmetrically around the ferritin cages, control removal of iron by reductants and chelators. Gene regulation of ferritin, long known to depend on iron and, in animals, on a noncoding messenger RNA (mRNA) structure linked in a combinatorial array to functionally related mRNA of iron transport, has recently been shown to be linked to an array of proteins for antioxidant responses such as thioredoxin and quinone reductases. Ferritin DNA responds more to oxygen signals, and ferritin mRNA responds more to iron signals. Ferritin genes (DNA and RNA) and protein function at the intersection of iron and oxygen chemistry in biology.

Iron absorption from soybean ferritin in nonanemic women

BACKGROUND

Dietary ferritin, a protein cage around an iron mineral, is an underestimated source of bioavailable iron. Plant ferritin, the most common dietary ferritin, has not been studied. Iron from animal ferritin is absorbed as well as is iron from FeSO4 in women.

OBJECTIVE

The objective was to examine iron absorption from purified soybean ferritin.

DESIGN

Healthy, nonanemic women (n = 16) were fed a standardized meal (bagel, cream cheese, and apple juice) containing 1 microCi 59Fe/meal as FeSO4 or (extrinsically labeled) as iron-free soybean ferritin reconstituted with the high phosphate characteristic of plant ferritin (iron:phosphorus = 4:1). Iron-free, apo-soybean ferritin was prepared (with the use of thioglycolic acid and extensive dialysis) from purified ferritin. In a randomized crossover design, the other labeled meal, which contained FeSO4 or ferritin, was given after 4 wk. The subjects received 140 microg Fe as ferritin (2.5 mg) or as FeSO4. After 28 d, whole-body 59Fe and 59Fe in red blood cells were measured before and after dosing.

RESULTS

There was no significant difference in whole-body iron absorption from soybean ferritin (29.9 +/- 19.8%) and that from FeSO4 (34.3 +/- 23.6%) or in iron absorption calculated from red blood cell incorporation (33.0 +/- 20.1% for soybean ferritin and 35.3 +/- 23.4% for FeSO4), which confirmed previous results with animal ferritin that was mineralized and labeled similarly. An inverse relation was observed between serum ferritin and iron absorption from both ferritin and FeSO4, which suggested that sensors regulating iron absorption respond similarly to iron provided as ferrous salts or as ferritin mineral.

CONCLUSION

Iron from soybean ferritin is well absorbed and may provide a model for novel, utilizable, plant-based forms of iron for populations with a low iron status.

The dinuclear iron-oxo ferroxidase center ofPyrococcus furiosusferritin is a stable prosthetic group with unexpectedly high reduction potentials

Recombinant ferritin from Pyrococcus furiosus expressed in Escherichia coli exhibits in EPR monitored redox titrations a mixed valence (Fe(3+)-Fe2+) S=1/2 signal at intermediate potentials that is a high-resolution homolog of the ferroxidase signal previously described for reconstituted horse spleen apo-ferritin. P. furiosus reconstituted apo-ferritin reduced to intermediate potentials exhibits the same mixed-valence signal, which integrates to close to one spin per subunit. The reduction potentials of +210 and +50 mV imply that the iron dimer is a stable prosthetic group with three redox states.

Rapid freeze-quench 57Fe Mössbauer spectroscopy: monitoring changes of an iron-containing active site during a biochemical reaction

Nuclear gamma resonance spectroscopy, also known as Mössbauer spectroscopy, is a technique that probes transitions between the nuclear ground state and a low-lying nuclear excited state. The nucleus most amenable to Mössbauer spectroscopy is 57Fe, and 57Fe Mössbauer spectroscopy provides detailed information about the chemical environment and electronic structure of iron. Iron is by far the most structurally and functionally diverse metal ion in biology, and 57Fe Mössbauer spectroscopy has played an important role in the elucidation of its biochemistry. In this article, we give a brief introduction to the technique and then focus on two recent exciting developments pertaining to the application of 57Fe Mössbauer spectroscopy in biochemistry. The first is the use of the rapid freeze-quench method in conjunction with Mössbauer spectroscopy to monitor changes at the Fe site during a biochemical reaction. This method has allowed for trapping and subsequent detailed spectroscopic characterization of reactive intermediates and thus has provided unique insight into the reaction mechanisms of Fe-containing enzymes. We outline the methodology using two examples: (1) oxygen activation by the non-heme diiron enzymes and (2) oxygen activation by taurine:alpha-ketoglutarate dioxygenase (TauD). The second development concerns the calculation of Mössbauer parameters using density functional theory (DFT) methods. By using the example of TauD, we show that comparison of experimental Mössbauer parameters with those obtained from calculations on model systems can be used to provide insight into the structure of a reaction intermediate.

Crystal Structures of a Tetrahedral Open Pore Ferritin from the Hyperthermophilic Archaeon Archaeoglobus fulgidus

Ferritins are known as important iron storage/detoxification proteins and are widely found in living organisms. This report details the 2.1 A resolution native and 2.7 A resolution iron bound structures of the ferritin from the hyperthermophilic Archaeon Archaeoglobus fulgidus, and represents the first structure of a ferritin from an archaeon, or a hyperthermophilic organism. The A. fulgidus ferritin (AfFtn) monomer has a high degree of structural similarity with archetypal ferritins from E. coli and humans, but the AfFtn quaternary structure is novel; 24 subunits assemble into a shell having tetrahedral (2-3) rather than the canonical octahedral (4-3-2) symmetry of archetypal ferritins. The difference in assembly opens four large (approximately 45 A) pores in the AfFtn shell. Two nonconservative amino acid substitutions may be critical for stabilizing the tetrahedral form.

A protein carboxylate coordinated oxo-centered tri-nuclear iron complex with possible implications for ferritin mineralization

The crystal structure of an oxo-centered tri-nuclear iron complex formed on a protein surface is presented. The cluster forms when crystals of the class Ib ribonucleotide reductase R2 protein from Corynebacterium ammoniagenes are subjected to iron soaking. The tri-iron-oxo complex is coordinated by protein-derived carboxylate ligands arranged in a motif similar to the one found on the inner surface of ferritins and may mimic an early stage in the mineralization of iron in ferritins. In addition, the structure adds to the very limited data on protein-mineral interfaces.

Ferritins, iron uptake and storage from the bacterioferritin viewpoint

Ferritins constitute a broad superfamily of iron storage proteins, widespread in all domains of life, in aerobic or anaerobic organisms. Ferritins isolated from bacteria may be haem-free or contain a haem. In the latter case they are called bacterioferritins. The primary function of ferritins inside cells is to store iron in the ferric form. A secondary function may be detoxification of iron or protection against O(2) and its radical products. Indeed, for bacterioferritins this is likely to be their primary function. Ferritins and bacteroferritins have essentially the same architecture, assembling in a 24mer cluster to form a hollow, roughly spherical construction. In this review, special emphasis is given to the structure of the ferroxidase centres with native iron-containing sites, since oxidation of ferrous iron by molecular oxygen takes place in these sites. Although present in other ferritins, a specific entry route for iron, coupled with the ferroxidase reaction, has been proposed and described in some structural studies. Electrostatic calculations on a few selected proteins indicate further ion channels assumed to be an entry route in the later mineralization processes of core formation.

mu-1,2-Peroxobridged di-iron(III) dimer formation in human H-chain ferritin

Biomineralization of the ferritin iron core involves a complex series of events in which H(2)O(2) is produced during iron oxidation by O(2) at a dinuclear centre, the 'ferroxidase site', located on the H-subunit of mammalian proteins. Rapid-freeze quench Mössbauer spectroscopy was used to probe the early events of iron oxidation and mineralization in recombinant human ferritin containing 24 H-subunits. The spectra reveal that a mu-1,2-peroxodiFe(III) intermediate (species P) with Mössbauer parameters delta (isomer shift)=0.58 mm/s and DeltaE(Q) (quadrupole splitting)=1.07 mm/s at 4.2 K is formed within 50 ms of mixing Fe(II) with the apoprotein. This intermediate accounts for almost all of the iron in the sample at 160 ms. It subsequently decays within 10 s to form a mu-oxodiFe(III)-protein complex (species D), which partially vacates the ferroxidase sites of the protein to generate Fe(III) clusters (species C) at a reaction time of 10 min. The intermediate peroxodiFe(III) complex does not decay under O(2)-limiting conditions, an observation suggesting inhibition of decay by unreacted Fe(II), or a possible role for O(2) in ferritin biomineralization in addition to that of direct oxidation of iron(II).

Iron uptake in ferritin is blocked by binding of [Cr(TREN)(H(2)O)(OH)](2+), a slow dissociating model for [Fe(H(2)O)(6)](2+)

Ferritin concentrates iron as a hydrous ferric oxide in a protein cavity (8 nm in diameter) by using eight pores along the threefold symmetry axes of the octahedral supramolecular structure. The role of ligand exchange in the entry of Fe(II) hexahydrate into ferritin protein has been studied with [Cr(TREN)(H(2)O)(OH)](2+) [TREN = N(CH(2)CH(2)NH(2))(3)], a model for Fe(H(2)O)(6)2+ with only two exchangeable ligands. The results show that five different ferritin proteins, varying in pore structure, oxidation sites, and nucleation sites, bind Cr(TREN) at functional protein sites, based on inhibition of iron mineralization and oxidation. Properties of Cr(TREN)-ferritin adducts include an increased isoelectric point, a shift in the Cr(TREN) UV/vis spectrum consistent with exchange of water for protein carboxylate or thiolate ligands, binding affinities of 50-250 microM, and a slow rate of dissociation (k = 4 x 10(-6) sec(-1)). The relationship of Cr(TREN) inhibition of iron oxidation and mineralization by Cr(TREN) to the known structures of the various ferritins tested suggests that Cr(TREN) plugs the ferritin pores, obstructing Fe(II) entry in folded and unfolded pores. Because only two exchangeable waters are sufficient for pore binding of Cr(TREN), the physiological Fe(II) donor must bind to the pore with few exchangeable ligands. These results show the advantage of using stable model complexes to explore properties of transient Fe-protein complexes during Fe mineralization in ferritin.

The high-resolution X-ray crystallographic structure of the ferritin (EcFtnA) of Escherichia coli; comparison with human H ferritin (HuHF) and the structures of the Fe(3+) and Zn(2+) derivatives

The high-resolution structure of the non-haem ferritin from Escherichia coli (EcFtnA) is presented together with those of its Fe(3+) and Zn(2+) derivatives, this being the first high-resolution X-ray analysis of the iron centres in any ferritin. The binding of both metals is accompanied by small changes in the amino acid ligand positions. Mean Fe(A)(3+)-Fe(B)(3+) and Zn(A)(2+)-Zn(B)(2+) distances are 3.24 A and 3.43 A, respectively. In both derivatives, metal ions at sites A and B are bridged by a glutamate side-chain (Glu50) in a syn-syn conformation. The Fe(3+) derivative alone shows a third metal site (Fe( C)( 3+)) joined to Fe(B)(3+) by a long anti-anti bidentate bridge through Glu130 (mean Fe(B)(3+)-Fe(C)(3+) distance 5.79 A). The third metal site is unique to the non-haem bacterial ferritins. The dinuclear site lies at the inner end of a hydrophobic channel connecting it to the outside surface of the protein shell, which may provide access for dioxygen and possibly for metal ions shielded by water. Models representing the possible binding mode of dioxygen to the dinuclear Fe(3+) pair suggest that a gauche micro-1,2 mode may be preferred stereochemically. Like those of other ferritins, the 24 subunits of EcFtnA are folded as four-helix bundles that assemble into hollow shells and both metals bind at dinuclear centres in the middle of the bundles. The structural similarity of EcFtnA to the human H chain ferritin (HuHF) is remarkable (r.m.s. deviation of main-chain atoms 0.66 A) given the low amino acid sequence identity (22 %). Many of the conserved residues are clustered at the dinuclear centre but there is very little conservation of residues making inter-subunit interactions.

The iron oxidation and hydrolysis chemistry of Escherichia coli bacterioferritin

Bacterioferritins are members of a class of spherical shell-like iron storage proteins that catalyze the oxidation and hydrolysis of iron at specific sites inside the protein shell, resulting in formation of a mineral core of hydrated ferric oxide within the protein cavity. Electrode oximetry/pH stat was used to study iron oxidation and hydrolysis chemistry in E. coli bacterioferritin. Consistent with previous UV-visible absorbance measurements, three distinct kinetic phases were detected, and the stoichiometric equations corresponding to each have been determined. The rapid phase 1 reaction corresponds to pairwise binding of 2 Fe(2+) ions at a dinuclear site, called the ferroxidase site, located within each of the 24 subunits, viz., 2Fe(2+) + P(Z) --> [Fe(2)-P](Z) + 4H(+), where P(Z) is the apoprotein of net charge Z and [Fe(2)-P](Z) represents a diferrous ferroxidase complex. The slower phase 2 reaction corresponds to the oxidation of this complex by molecular oxygen according to the net equation: [Fe(2)-P](Z) + (1)/(2)O(2) --> [Fe(2)O-P](Z) where [Fe(2)O-P](Z) represents an oxidized diferric ferroxidase complex, probably a mu-oxo-bridged species as suggested by UV-visible and EPR spectrometric titration data. The third phase corresponds to mineral core formation according to the net reaction: 4Fe(2+) + O(2) + 6H(2)O --> 4FeO(OH)((core)) + 8H(+). Iron oxidation is inhibited by the presence of Zn(2+) ions. The patterns of phase 2 and phase 3 inhibition are different, though inhibition of both phases is complete at 48 Zn(2+)per 24mer, i.e., 2 Zn(2+) per ferroxidase center.

Localized unfolding at the junction of three ferritin subunits. A mechanism for iron release?

How and where iron exits from ferritin for cellular use is unknown. Twenty-four protein subunits create a cavity in ferritin where iron is concentrated >10(11)-fold as a mineral. Proline substitution for conserved leucine 134 (L134P) allowed normal assembly but increased iron exit rates. X-ray crystallography of H-L134P ferritin revealed localized unfolding at the 3-fold axis, also iron entry sites, consistent with shared use sites for iron exit and entry. The junction of three ferritin subunits appears to be a dynamic aperture with a "shutter" that cytoplasmic factors might open or close to regulate iron release in vivo.

Reaction paths of iron oxidation and hydrolysis in horse spleen and recombinant human ferritins

UV-visible spectroscopy, electrode oximetry, and pH stat were used to study Fe(II) oxidation and hydrolysis in horse spleen ferritin (HoSF) and recombinant human H-chain and L-chain ferritins (HuHF and HuLF). Appropriate test reactions and electrode responses were measured, establishing the reliability of oxygen electrode/pH stat for kinetics studies of iron uptake by ferritin. Stoichiometric ratios, Fe(II)/O2 and H+/Fe(II), and rates of oxygen uptake and proton production were simultaneously measured as a function of iron loading of the protein. The data show a clear distinction between the diiron ferroxidase site and mineral surface catalyzed oxidation of Fe(II). The oxidation/hydrolysis reaction attributed to the ferroxidase site has been determined for the first time and is given by 2Fe2+ + O2 + 3H2O --> [Fe2O(OH)2]2+ + H2O2 + 2H+ where [Fe2O(OH)2]2+ represents the hydrolyzed dinuclear iron(III) center postulated to be a mu-oxo-bridged species from UV spectrometric titration data and absorption band maxima. The transfer of iron from the ferroxidase site to the mineral core has been now established to be [Fe2O(OH)2]2+ + H2O --> 2FeOOH(core) + 2H+. Regeneration of protein ferroxidase activity with time is observed for both HoSF and HuHF, consistent with their having enzymatic properties, and is facilitated by higher pH (7.0) and temperature (37 degreesC) and by the presence of L-subunit and is complete within 10 min. In accord with previous studies, the mineral surface reaction is given by 4Fe2+ + O2 + 6H2O --> 4FeOOH(core) + 8H+. As the protein progressively acquires iron, oxidation/hydrolysis increasingly shifts from a ferroxidase site to a mineral surface based mechanism, decreasing the production of H2O2.