Protein association and dissociation regulated by ferric ion: a novel pathway for oxidative deposition of iron in pea seed ferritin

Vertasile ferritin nanocages: Applications in detection and bioimaging

Food safety and human health remain significant concerns in the food industry. Detecting food contaminants and diagnosing diseases are critical aspects. Ferritin, an iron storage protein widely found in nature, offers unique advantages. Its hollow protein nanocage structure, distinct interfaces, hydrophobic or hydrophilic channels, and B-C loop regions recognized by transferrin receptor 1 make ferritin versatile for detecting heavy metals, free radicals, and bioimaging both in vitro and in vivo. This review summarizes ferritin's general characteristics, its specific properties as biosensors, and its applications in food safety and in vivo imaging. It emphasizes not only ferritin's role in detecting heavy metals like mercury and chemical hazards but also its potential in early diagnosing chronic diseases such as tumors, macrophages, and kidney diseases. Further research into ferritin promises advancements in enhancing food safety and improving human health diagnostics.

Possibility and challenge of plant-derived ferritin cages encapsulated polyphenols in the precise nutrition field

Polyphenols have attracted extensive attention due to their rich functional activities, such as antioxidant, anti-inflammatory and anti-tumor. However, the low solubility and poor stability limit their bioavailability and functional activities. Plant-derived ferritin cages have a unique hollow cage structure that can embed polyphenols to improve their unfavorable properties. Therefore, it is essential to adequately elaborate and summarize plant-derived ferritin cages to maximize their potential benefits in nutritional interventions. This review focuses on the fundamental properties of plant-derived ferritin cages, including the preparation process, purification technology, identification methods, and structural and functional properties. The relevant research on ferritin cages in polyphenol delivery has been summarized, including the delivery of water/lipid soluble polyphenols, modification of ferritin cages, and the interaction between polyphenols and ferritin cages. The research progress, shortcomings and prospects of plant-derived ferritin cages in precise nutrition are introduced. In addition, the relevant research on ferritin in immune response and protein engineering is also discussed to provide the theoretical basis for applying plant-derived ferritin cages in many frontier fields.

Exploring the evolutionary landscape and structural resonances of ferritin with insights into functional significance in plant

The mineral iron plays a crucial role in facilitating the optimal functioning of numerous biological processes within the cellular environment. These processes involve the transportation of oxygen, energy production, immune system functioning, cognitive abilities, and muscle function. However, it is crucial to note that excessive levels of iron can result in oxidative damage within cells, primarily through Fenton reactions. Iron availability and toxicity present significant challenges that have been addressed through evolution. Ferritin is an essential protein that stores iron and is divided into different subfamilies, including DNA-binding proteins under starvation (Dps), bacterioferritin, and classical ferritin. Ferritin plays a critical role in maintaining cellular balance and protecting against oxidative damage. This study delves into ferritin's evolutionary dynamics across diverse taxa, emphasizing structural features and regulatory mechanisms. Insights into ferritin's evolution and functional diversity are gained through phylogenetic and structural analysis in bacterial Dps, bacterioferritin, and classical ferritin proteins. Additionally, the involvement of ferritin in plant stress responses and development is explored. Analysis of ferritin gene expression across various developmental stages and stress conditions provides insights into its regulatory roles. This comprehensive exploration enhances our understanding of ferritin's significance in plant biology, offering insights into its evolutionary history, structural diversity, and protective mechanisms against oxidative stress.

Advancements of the Molecular Directed Design and Structure-Activity Relationship of Ferritin Nanocage

Ferritin nanocages possess remarkable structural properties and biological functions, making them highly attractive for applications in functional materials and biomedicine. This comprehensive review presents an overview of the molecular characteristics, extraction and identification of ferritin, ferritin receptors, as well as the advancements in the directional design of high-order assemblies of ferritin and the applications based on its unique structural properties. Specifically, this Review focuses on the regulation of ferritin assembly from one to three dimensions, leveraging the symmetry of ferritin and modifications on key interfaces. Furthermore, it discusses targeted delivery of nutrition and drugs through facile loading and functional modification of ferritin. The aim of this Review is to inspire the design of micro/nano functional materials using ferritin and the development of nanodelivery vehicles for nutritional fortification and disease treatment.

Multifaceted Applications of Ferritin Nanocages in Delivering Metal Ions, Bioactive Compounds, and Enzymes: A Comprehensive Review

Ferritin, a distinctive iron-storage protein, possesses a unique cage-like nanoscale structure that enables it to encapsulate and deliver a wide range of biomolecules. Recent advances prove that ferritin can serve as an efficient 8 nm diameter carrier for various bioinorganic nutrients, such as minerals, bioactive polyphenols, and enzymes. This review offers a comprehensive summary of ferritin's structural features from different sources and emphasizes its functions in iron supplementation, calcium delivery, single- and coencapsulation of polyphenols, and enzyme package. Additionally, the influence of innovative food processing technologies, including manothermosonication, pulsed electric field, and atmospheric cold plasma, on the structure and function of ferritin are examined. Furthermore, the limitations and prospects of ferritin in food and nutritional applications are discussed. The exploration of ferritin as a multifunctional protein with the capacity to load various biomolecules is crucial to fully harnessing its potential in food applications.

Extension Peptide of Plant Ferritin from Setaria italica Presents a Novel Fold

The extension peptide (EP) is the most distinctive feature of mature plant ferritin. Some EPs have exhibited serine-like protease activity, which is associated with iron uptake and release. EP forms a helix and a long loop, followed by a conserved core helical bundle. However, whether the EP adopts a stable or uniform folding pattern in all plants remains unclear. To clarify this, we investigated the crystal structure of ferritin-1 from Setaria italica (SiFer1), a type of monocotyledon. In our structure of SiFer1, the EP is different from other EPs in other solved structures of plant ferritins and consisted of a pair of β-sheets, a shorter helix, and two loops, which masks two hydrophobic pockets on the outer surface of every subunit. Furthermore, sequence analysis and structure comparison suggest that the EPs in ferritins from monocotyledons may adopt a novel fold pattern, and the conformations of EPs in ferritins are alterable among different plant species. Furthermore, additional eight iron atoms were first founded binding in the fourfold channels, demonstrating the vital function of fourfold channels in iron diffusion. In all, our structure provides new clues for understanding plant ferritins and the functions of the EP.

Interaction mechanism of ferritin protein with chlorogenic acid and iron ion: The structure, iron redox, and polymerization evaluation

Ferritin is an iron-containing protein and functions in the maintenance of iron balance in organisms. Currently the interaction among ferritin, ion iron, and food bioactive compounds is still unclear. In this study, the mechanism underlying the interaction of ferritin, ion iron, and chlorogenic acid was investigated, as well as the effect of chlorogenic acid on the physicochemical properties of ferritin. The results showed that chlorogenic acid could interact with Fe(III) to form chlorogenic acid-Fe(III) complexes, which then bonded with ferritin via hydrogen bonds in the ferritin-chlorogenic acid-Fe(III) complexes. The chlorogenic acid showed a high efficiency in Fe(II) chelation and hydroxyl radical (•OH) capture, and could promote iron oxidation and iron release induced by ferritin. Chlorogenic acid could also effectively reduce the polymerization extent of ferritin induced by Fe(III) and Fe(II). This study elucidates the interactions of multiple components in foodstuffs by using a protein-metal-polyphenol model.

Opportunities for plant-derived enhancers for iron, zinc, and calcium bioavailability: A review

Understanding of the mechanism of interactions between dietary elements, their salts, and complexing/binding ligands is vital to manage both deficiency and toxicity associated with essential element bioavailability. Numerous mineral ligands are found in both animal and plant foods and are known to exert bioactivity via element chelation resulting in modulation of antioxidant capacity or micobiome metabolism among other physiological outcomes. However, little is explored in the context of dietary mineral ligands and element bioavailability enhancement, particularly with respect to ligands from plant-derived food sources. This review highlights a novel perspective to consider various plant macro/micronutrients as prospective bioavailability enhancing ligands of three essential elements (Fe, Zn, and Ca). We also delineate the molecular mechanisms of the ligand-binding interactions underlying mineral bioaccessibility at the luminal level. We conclude that despite current understandings of some of the structure-activity relationships associated with strong mineral-ligand binding, the physiological links between ligands as element carriers and uptake at targeted sites throughout the gastrointestinal (GI) tract still require more research. The binding behavior of potential ligands in the human diet should be further elucidated and validated using pharmacokinetic approaches and GI models.

Ferritin Nanocage: A Versatile Nanocarrier Utilized in the Field of Food, Nutrition, and Medicine

Compared with other nanocarriers such as liposomes, mesoporous silica, and cyclodextrin, ferritin as a typical protein nanocage has received considerable attention in the field of food, nutrition, and medicine owing to its inherent cavity size, excellent water solubility, and biocompatibility. Additionally, ferritin nanocage also serves as a versatile bio-template for the synthesis of a variety of nanoparticles. Recently, scientists have explored the ferritin nanocage structure for encapsulation and delivery of guest molecules such as nutrients, bioactive molecules, anticancer drugs, and mineral metal ions by taking advantage of its unique reversible disassembly and reassembly property and biomineralization. In this review, we mainly focus on the preparation and structure of ferritin-based nanocarriers, and regulation of their self-assembly. Moreover, the recent advances of their applications in food nutrient delivery and medical diagnostics are highlighted. Finally, the main challenges and future development in ferritin-directed nanoparticles’ synthesis and multifunctional applications are discussed.

Re-designing ferritin nanocages for mercuric ion detection

Protein nanocages have recently received considerable attention in the fields of nanoscience and nanomedicine and have been used as either biotemplates for the preparation of a variety of nanomaterials or vehicles for drugs or imaging agents. However, their utilization for detection of heavy metal ions has yet to be explored. In this study, by grafting a mercury binding peptide (MBP) on the exterior surface of a recombinant human H-chain ferritin (rHuHF) nanocage, we successfully prepared a new protein nanocage (HuHF-MBP) which exhibits high binding capacity and affinity for Hg2+. The fluorescence of HuHF-MBP labeled with a green fluorescent dye fluorescein isothiocyanate (FITC) can be quenched by graphene oxide (GO), while addition of Hg2+ to the above solution recovered the quenched fluorescence in a dose-dependent manner. Thus, this system consisting of FITC-labeled HuHF-MBP and GO, where FITC and graphene oxide were used as fluorescent reporter probes, has great potential to be explored as a sensor for Hg2+ detection. Indeed, this newly constructed protein sensor exhibited high sensitivity and selectivity for Hg2+, and the limit of detection was 1.0 nM. The construction of this system provides an alternative strategy for the preparation of heavy metal ion sensors by using protein nanocages as biotemplates.

Influence of Manothermosonication on the Physicochemical and Functional Properties of Ferritin as a Nanocarrier of Iron or Bioactive Compounds

Ferritin is a multisubunit protein with a hollow interior interface and modifiable surfaces. In this study, the manothermosonication (MTS) technology was applied to apo-red bean seed ferritin (apoRBF) to produce the MTS-treated apoRBF (MTFS). MTS treatment (200 kPa, 50 °C, and 40 s) maintained the spherical morphology of apoRBF (12 nm), but reduced the content of α-helix structure and increased the content of random coil structure, and correspondingly decreased the ferritin stability. The MTS treatment also affected the ferritin's iron storage function by decreasing its iron oxidative deposition activity and increasing the iron release activity. Importantly, the disassembly and reassembly properties of the MTFS induced by pH changes were retained, which facilitated its usage in encapsulation of tea polyphenol-epigallocatechin gallate (EGCG) into the ferritin by a relatively benign pH conversion routine (pH 3.0/6.8). In addition, the water solubility of the MTFS was increased, leading to the improved encapsulation efficiency of the EGCG molecules. This study will facilitate the ferritin modification and functionalization by MTS to design a protein variant to be used as new scaffold for iron and bioactive compounds.

Thermostability of protein nanocages: the effect of natural extra peptide on the exterior surface

Protein nanocages have been used as functional bio-templates for the synthesis or organization of nanomaterials. However, the stability of these protein nanocages is nonideal, which limits their applications. Herein, we characterized the high thermal stability of plant ferritin, soybean seed H-2 ferritin (SSFH-2), the melting point (T_m) of which is 106 °C. We demonstrated that the hyperthermostability of SSFH-2 is derived from extra peptides (EP) located on its outer surface. Indeed, removal of the EP domains resulted in a dramatic decrease in T_m to 88 °C. Similar to EP-deleted plant ferritin, human H-chain ferritin (HuHF) has a T_m of 82 °C. Excitingly, the graft of the EP domain on the exterior surface of HuHF pronouncedly improved its T_m to 103 °C, which represents a simple, efficient approach to the construction of protein architectures with high stability. The remarkable stability of protein nanocages will greatly facilitate their application as robust biotemplates in the field of nanoscience.

Ferritin nanocage exhibits hyperthermostability with EP domain located on its outer surface.

Interaction between rice bran albumin and epigallocatechin gallate and their physicochemical analysis

Epigallocatechin gallate (EGCG) is sensitive to heat thus its application in food industry is limited. In this work, rice bran albumin protein (RAP) was used as a carrier for EGCG. RAP-EGCG complexes (RAPE) were prepared with the binding number n of 0.0505:1 (EGCG: RAP, w/w) and binding constant K of (0.74 ± 0.002) × 10⁴ M^-1, which suggests that hydrogen bond/van der Waals forces played important roles in such binding. FTIR analysis demonstrated that EGCG could induce the secondary structure changes of RAP above the ratio of 1.92:1 (EGCG:RAP, w/w). Dynamic light scattering and scanning electron microscope results showed that EGCG could trigger RAP association. Furthermore, the EGCG stability in RAPE was significantly improved than that of free EGCG in 10-60 °C. The antioxidant ability of EGCG in RAPE was partially retained. These findings prove that RAP is a potential carrier for polyphenols and is beneficial for mechanism investigation between protein and polyphenols.

On-Axis Alignment of Protein Nanocage Assemblies from 2D to 3D through the Aromatic Stacking Interactions of Amino Acid Residues

Aromatic-aromatic interactions between natural aromatic amino acids Phe, Tyr, and Trp play crucial roles in protein-protein recognition and protein folding. However, the function of such interactions in the preparation of different dimensional, ordered protein superstructures has not been recognized. Herein, by a combination of the directionality of the symmetry axes of protein building blocks and the strength of the aromatic-aromatic interactions coming from a group of aromatic amino acid residues, we built an engineering strategy to construct protein superlattices. Based on this strategy, substitution of single amino acid residue Glu162 around the C₄ rotation axes near the outer surface of 24-mer ferritin nanocage with Phe, Tyr, and Trp, respectively, resulted in 2D and 3D protein superlattices where protein cages are aligned along the C₄ axes, imposing a fixed disposition of neighboring ferritins. The self-assembly of these superlattices is reversible, which can be tuned by external stimuli (salt concentration or pH). Moreover, these superlattices can serve as biotemplates for the fabrication of 2D and 3D inorganic nanoparticle arrays.

Pulsed Electric Fields-Modified Ferritin Realizes Loading of Rutin by a Moderate pH Transition

Ferritin shares a conserved 24-subunit spherical structure and a unique reversible self-assembly characteristic. In the present work, pulsed electric fields (PEF) technology was used to treat red bean seed ferritin deprived of iron (apoRBF) to fabricate a PEF-modified apoRBF (PEFF). Results indicated that PEF treatment at 20 kV/cm for 7.05 ms retained the spherical structure but decreased the α-helix/β-sheet contents of ferritin. Differential scanning calorimetry (DSC) and UV-vis analyses proved that the thermal stability of the PEFF was decreased. Consequently, PEFF disassembled at pH 3.6 and reassembled when the pH was restored to 7.0, exhibiting a more moderate condition relevant to the traditional approach. Using the pH 3.6/7.0 transition routine, rutin molecules were successfully loaded within PEFF nanoparticle. The rutin-loaded PEFF showed a diameter of 12 nm with an encapsulation ratio of 13.7% (w/w). Moreover, PEFF played a role in protecting the encapsulated rutin molecules upon thermal treatment (20-70 °C). This work will be beneficial for extension of PEF application in protein modification and will improve ferritin functionalization as a carrier for food bioactive molecules by a moderate pH transition method.

Pea Ferritin Stability under Gastric pH Conditions Determines the Mechanism of Iron Uptake in Caco-2 Cells

Background

Iron deficiency is an enduring global health problem that requires new remedial approaches. Iron absorption from soybean-derived ferritin, an ∼550-kDa iron storage protein, is comparable to bioavailable ferrous sulfate (FeSO4). However, the absorption of ferritin is reported to involve an endocytic mechanism, independent of divalent metal ion transporter 1 (DMT-1), the transporter for nonheme iron.

Objective

Our overall aim was to examine the potential of purified ferritin from peas (Pisum sativum) as a food supplement by measuring its stability under gastric pH treatment and the mechanisms of iron uptake into Caco-2 cells.

Methods

Caco-2 cells were treated with native or gastric pH-treated pea ferritin in combination with dietary modulators of nonheme iron uptake, small interfering RNA targeting DMT-1, or chemical inhibitors of endocytosis. Cellular ferritin formation, a surrogate measure of iron uptake, and internalization of pea ferritin with the use of specific antibodies were measured. The production of reactive oxygen species (ROS) in response to equimolar concentrations of native pea ferritin and FeSO4 was also compared.

Results

Pea ferritin exposed to gastric pH treatment was degraded, and the released iron was transported into Caco-2 cells by DMT-1. Inhibitors of DMT-1 and nonheme iron absorption reduced iron uptake by 26-40%. Conversely, in the absence of gastric pH treatment, the iron uptake of native pea ferritin was unaffected by inhibitors of nonheme iron absorption, and the protein was observed to be internalized in Caco-2 cells. Chlorpromazine (clathrin-mediated endocytosis inhibitor) reduced the native pea ferritin content within cells by ∼30%, which confirmed that the native pea ferritin was transported into cells via a clathrin-mediated endocytic pathway. In addition, 60% less ROS production resulted from native pea ferritin in comparison to FeSO4.

Conclusion

With consideration that nonheme dietary inhibitors display no effect on iron uptake and the low oxidative potential relative to FeSO4, intact pea ferritin appears to be a promising iron supplement.

Alcalase Enzymolysis of Red Bean (adzuki) Ferritin Achieves Nanoencapsulation of Food Nutrients in a Mild Condition

Classical methods to fabricate ferritin-nutrients shell-core nanoparticles usually apply extremely acid/alkaline pH transition, which may cause the activity loss of nutrients or the formation of insoluble aggregates. In this work, we prepared an extension peptide (EP) deleted red bean (adzuki) ferritin (apoRBFΔEP) by Alcalase 3.0T enzymolysis. Such enzymolysis could delete the EP domain and remain the typical shell-like structure of the ferritin. Meanwhile, the α-helix content of apoRBFΔEP was decreased by 5.5%, and the transition temperature (T_m) was decreased by 4.1 °C. Interestingly, the apoRBFΔEP can be disassembled into subunits under a benign condition at pH 4.0 and is assembled to form an intact cage protein when the pH was increased to 6.7. By using this novel route, the epigallocatechin gallate (EGCG) molecules were successfully encapsulated into the apoRBFΔEP cage with an encapsulation ratio of 11.6% (w/w), which was comparable with that by the traditional pH 2.0 transition. The newly prepared EGCG-loaded apoRBFΔEP exhibited a similarly protective effect on the EGCG upon simulated gastrointestinal tract and thermal treatment as compared with the control. In addition, the EGCG-loaded apoRBFΔEP could significantly relieve the ferritin association induced by pH transition, which was superior to traditional method. The thinking of this work will be especially suitable for encapsulating pH-sensitive molecules based on ferritin in a benign condition.

Ferritin cage for encapsulation and delivery of bioactive nutrients: From structure, property to applications

Ferritin is a class of naturally occurring iron storage proteins, which is distributed widely in animal, plant, and bacteria. It usually consists of 24 subunits that form a hollow protein shell with high symmetry. One holoferritin molecule can store up to 4500 iron atom within its inner cavity, and it becomes apoferritin upon removal of iron from the cavity. Recently, scientists have subverted these nature functions and used reversibly self-assembled property of apoferritin cage controlled by pH for the encapsulation and delivery of bioactive nutrients or anticancer drug. In all these cases, the ferritin cages shield their cargo from the influence of external conditions and provide a controlled microenvironment. More importantly, upon encapsulation, ferritin shell greatly improved the water solubility, thermal stability, photostability, and cellular uptake activity of these small bioactive compounds. This review aims to highlight recent advances in applications of ferritin cage as a novel vehicle in the field of food science and nutrition. Future outlooks are highlighted with the aim to suggest a research line to follow for further studies.

The Size Flexibility of Ferritin Nanocage Opens a New Way to Prepare Nanomaterials

Ferritins are ubiquitous iron storage proteins where Fe(II) sequestration prevents not only its spontaneous oxidation to Fe(III) but also production of toxic free radicals. Recently, scientists have subverted these nature functions and used ferritin cage structures of nanometer dimensions for encapsulation of guest molecules such as anti-cancer drugs or bioactive nutrients based on pH induced ferritin disassembly and reassembly property. However, prior to this study, ferritin nanocage was required to disassemble only under harsh pH conditions (≤2.0 or ≥11.0), followed by reassembly at near neutral pH. Such harsh conditions can cause protein or guest molecules damage to a great extent during this pH-induced unfolding-refolding process. Here, we provide evidence demonstrating that the apoferritin shell is flexible rather than rigid. Indeed, we found that two large complex molecules, uranyl acetate dihydrate and phosphotungstic acid, can reach the cavity of both plant and animal apoferritin followed by mineralization. Moreover, large organic compound such as curcumin and doxorubicin can also be encapsulated within ferritin cavity by its mixing with protein. This strategy will increase the use of ferritin in nanotechnology, and could be also applicable to other shell-like proteins as templates to prepare nanomaterials.

Urea-Driven Epigallocatechin Gallate (EGCG) Permeation into the Ferritin Cage, an Innovative Method for Fabrication of Protein-Polyphenol Co-assemblies

The 8 nm diameter cavity endows the ferritin cage with a natural space to encapsulate food components. In this work, urea was explored as a novel medium to facilitate the formation of ferritin-polyphenol co-assemblies. Results indicated that urea (20 mM) could expand the 4-fold channel size of apo-red bean ferritin (apoRBF) with an increased initial iron release rate υ₀ (0.22 ± 0.02 μM min^-1) and decreased α-helix content (5.6%). Moreover, urea (20 mM) could facilitate the permeation of EGCG into the apoRBF without destroying the ferritin structure and thus form ferritin-EGCG co-assemblies (FECs) with an encapsulation ratio and loading capacity of 17.6 and 2.1% (w/w), respectively. TEM exhibited that FECs maintained a spherical morphology with a 12 nm diameter in size. Fluorescence analysis showed that urea intervention could improve the binding constant K [(1.22 ± 0.8) × 10⁴ M^-1] of EGCG to apoRBF. Furthermore, the EGCG thermal stability was significantly improved (20-60 °C) compared with free EGCG. Additionally, this urea-involved method was applicable for chlorogenic acid and anthocyanin encapsulation by the apoRBF cage. Thus, urea shows potential as a novel potential medium to encapsulate and stabilize bioactive polyphenols for food usage based on the ferritin protein cage structure.

Interactions of β-Conglycinin (7S) with Different Phenolic Acids-Impact on Structural Characteristics and Proteolytic Degradation of Proteins

p-Coumalic acid (PCA), caffeic acid (CA), gallic acid (GA) and chlorogenic acid (CGA) are the major phenolic acids that co-exist with soy protein components in foodstuffs. Surprisingly, there are only a handful of reports that describe their interaction with β-Conglycinin (7S), a major soy protein. In this report, we investigated the interaction between phenolic acids and soy protein 7S and observed an interaction between each of these phenolic acids and soy protein 7S, which was carried out by binding. Further analysis revealed that the binding activity of the phenolic acids was structure dependent. Here, the binding affinity of CA and GA towards 7S was found to be stronger than that of PCA, because CA and GA have one more hydroxyl group. Interestingly, the binding of phenolic acids with soy protein 7S did not affect protein digestion by pepsin and trypsin. These findings aid our understanding of the relationship between different phenolic acids and proteins in complex food systems.

Soybean Ferritin Expression in Saccharomyces cerevisiae Modulates Iron Accumulation and Resistance to Elevated Iron Concentrations

Fungi, including the yeast Saccharomyces cerevisiae, lack ferritin and use vacuoles as iron storage organelles. This work explored how plant ferritin expression influenced baker's yeast iron metabolism. Soybean seed ferritin H1 (SFerH1) and SFerH2 genes were cloned and expressed in yeast cells. Both soybean ferritins assembled as multimeric complexes, which bound yeast intracellular iron in vivo and, consequently, induced the activation of the genes expressed during iron scarcity. Soybean ferritin protected yeast cells that lacked the Ccc1 vacuolar iron detoxification transporter from toxic iron levels by reducing cellular oxidation, thus allowing growth at high iron concentrations. Interestingly, when simultaneously expressed in ccc1Δ cells, SFerH1 and SFerH2 assembled as heteropolymers, which further increased iron resistance and reduced the oxidative stress produced by excess iron compared to ferritin homopolymer complexes. Finally, soybean ferritin expression led to increased iron accumulation in both wild-type and ccc1Δ yeast cells at certain environmental iron concentrations.

IMPORTANCE

Iron deficiency is a worldwide nutritional disorder to which women and children are especially vulnerable. A common strategy to combat iron deficiency consists of dietary supplementation with inorganic iron salts, whose bioavailability is very low. Iron-enriched yeasts and cereals are alternative strategies to diminish iron deficiency. Animals and plants possess large ferritin complexes that accumulate, detoxify, or buffer excess cellular iron. However, the yeast Saccharomyces cerevisiae lacks ferritin and uses vacuoles as iron storage organelles. Here, we explored how soybean ferritin expression influenced yeast iron metabolism, confirming that yeasts that express soybean seed ferritin could be explored as a novel strategy to increase dietary iron absorption.

Synthesis of homogeneous protein-stabilized rutin nanodispersions by reversible assembly of soybean (Glycine max) seed ferritin

We have studied the soybean seed ferritin stabilized rutin nanodispersions with improved water-solubility, thermal stability, and UV radiation stability.

Self-assembly of the sodium salts of fatty acids into limpid hydrogels through non-covalent interactions with peptides

Sodium salts of fatty acids (SFA) self-assemble into a limpid hydrogel in the presence of poly(α,l-lysine) with a high selectivity for the size of SFA and poly(α,l-lysine).

Four-fold channels are involved in iron diffusion into the inner cavity of plant ferritin

From an evolutionary point of view, plant and animal ferritins arose from a common ancestor, but plant ferritin exhibits different features as compared with the animal analogue. One major difference is that the 4-fold channels naturally occurring in plant ferritin are hydrophilic, whereas the 4-fold channels in animal ferritin are hydrophobic. Prior to this study, however, the function of the 4-fold channels in oxidative deposition of iron in phytoferritin remained unknown. To elucidate the role of the 4-fold channels in iron oxidative deposition in ferritin, three mutants of recombinant soybean seed H-2 ferritin (rH-2) were prepared by site-directed mutagenesis, which contained H193A/H197A, a 4-fold channel mutant, E165I/E167A/E171A, a 3-fold channel mutant, and E165I/E167A/E171A/H193A/H197A, where both 3- and 4-channels were mutated. Stopped-flow, electrode oximetry, and transmission electron microscopy (TEM) results showed that H193A/H197A and E165I/E167A/E171A exhibited a similar catalyzing activity of iron oxidation with each other, but a pronounced low activity compared to rH-2, demonstrating that both the 4-fold and 3-fold hydrophilic channels are necessary for iron diffusion in ferritin, followed by oxidation. Indeed, among all tested ferritin, the catalyzing activity of E165I/E167A/E171A/H193A/H197A was weakest because its 3- and 4- fold channels were blocked. These findings advance our understanding of the function of 4-fold channels of plant ferritin and the relationship of the structure and function of ferritin.

Dissecting plant iron homeostasis under short and long-term iron fluctuations

A wealth of information on the different aspects of iron homeostasis in plants has been obtained during the last decade. However, there is no clear road-map integrating the relationships between the various components. The principal aim of the current review is to fill this gap. In this context we discuss the lack of low affinity iron uptake mechanisms in plants, the utilization of a different uptake mechanism by graminaceous plants compared to the others, as well as the roles of riboflavin, ferritin isoforms, nitric oxide, nitrosylation, heme, aconitase, and vacuolar pH. Cross-homeostasis between elements is also considered, with a specific emphasis on the relationship between iron homeostasis and phosphorus and copper deficiencies. As the environment is a crucial parameter for modulating plant responses, we also highlight how diurnal fluctuations govern iron metabolism. Evolutionary aspects of iron homeostasis have so far attracted little attention. Looking into the past can inform us on how long-term oxygen and iron-availability fluctuations have influenced the evolution of iron uptake mechanisms. Finally, we evaluate to what extent this homeostastic road map can be used for the development of novel biofortification strategies in order to alleviate iron deficiency in human.

Stability and iron oxidation properties of a novel homopolymeric plant ferritin from adzuki bean seeds: a comparative analysis with recombinant soybean seed H-1 chain ferritin

BACKGROUND

All reported plant ferritins are heteropolymers comprising two different H-type subunits. Whether or not homopolymeric plant ferritin occurs in nature is an open question.

METHODS

A homopolymeric phytoferritin from adzuki bean seeds (ASF) was obtained by various protein purification techniques for the first time, which shares the highest identity (89.6%) with soybean seed H-1 ferritin (rH-1). Therefore, we compared iron oxidation activity and protein stability of ASF with those of rH-1 by stopped-flow combined with light scattering or UV/Vis spectrophotography, SDS- and native- PAGE analyses. Additionally, a new rH-1 variant (S68E) was prepared by site-directed mutagenesis approach to elucidate their difference in protein stability.

RESULTS

At high iron loading of protein, the extension peptide (EP) of plant ferritin was involved in iron oxidation, and the EP of ASF exhibited a much stronger iron oxidative activity than that of rH-1. Besides, ASF is more stable than rH-1 during storage, which is ascribed to one amino acid residue, Ser68.

CONCLUSIONS

ASF exhibits a different mechanism in iron oxidation from rH-1 at high iron loading of protein, and a higher stability than rH-1. These differences are mainly stemmed from their different EP sequences.

GENERAL SIGNIFICANCE

This work demonstrates that plant cells have evolved the EP of phytoferritin to control iron chemistry and protein stability by exerting a fine tuning of its amino acid sequence.

A novel strategy of natural plant ferritin to protect DNA from oxidative damage during iron oxidation

Plant ferritin is a naturally occurring heteropolymer in plastids, where Fe(2+) is oxidatively deposited into the protein. However, the effect of this process on the coexistence of DNA and plant ferritin in the plastids is unknown. To investigate this effect, we built a system in which various plant ferritins and DNA coexist, followed by treatment with ferrous ions under aerobic conditions. Interestingly, naturally occurring soybean seed ferritin (SSF), a heteropolymer with an H-1/H-2 ratio of 1 to 1 in the apo form, completely protected DNA from oxidative damage during iron oxidative deposition into protein, and a similar result was obtained with its recombinant form, but not with its homopolymeric counterparts, apo rH-1 and apo rH-2. We demonstrate that the difference in DNA protection between heteropolymeric and homopolymeric plant ferritins stems from their different strategies to control iron chemistry during the above oxidative process. For example, the detoxification reaction occurs only in the presence of apo heteropolymeric SSF (hSSF), thereby preventing the production of hydroxyl radicals. In contrast, hydroxyl radicals are apparently generated via the Fenton reaction when apo rH-1 or rH-2 is used instead of apo hSSF. Thus, a combination of H-1 and H-2 subunits in hSSF seems to impart a unique DNA-protective function to the protein, which was previously unrecognized. This new finding advances our understanding of the structure and function of ferritin and of the widespread occurrence of heteropolymeric plant ferritin in nature.

Effect of tannic acid on properties of soybean (Glycine max) seed ferritin: a model for interaction between naturally-occurring components in foodstuffs

There are many components with different properties co-existing in food, so interactions among these components are likely to occur, thereby affecting food quality. However, relatively little information is available on such interactions. In this study, we focus on the interaction between tannic acid (TA) and soybean seed ferritin (SSF), since they co-exist in many foodstuffs, and the consequence of this interaction. As expected, TA interacts with SSF, resulting in changes in the tertiary/quaternary structure of the protein, while having no effect on its primary and secondary structure. On one hand, such interaction leads to protein association, which markedly inhibited ferritin degradation by pepsin at pH 4.0 and trypsin at pH 7.5. On the other hand, iron release was faster with TA than with ascorbic acid, and such release has a negative effect on iron supplementation. These results help to understand the interactions of food components.

The extension peptide of plant ferritin from sea lettuce contributes to shell stability and surface hydrophobicity

Plant ferritins have some unique structural and functional features. Most of these features can be related to the plant-specific "extension peptide" (EP), which exists in the N-terminus of the mature region of a plant ferritin. Recent crystallographic analysis of a plant ferritin revealed the structure of the EP, however, two points remain unclear: (i) whether the structures of well-conserved EP of plant ferritins are common in all plants, and (ii) whether the EP truly contributes to the shell stability of the plant ferritin oligomer. To clarify these matters, we have cloned a green-plant-type ferritin cDNA from a green alga, Ulva pertusa, and investigated its crystal structure. Ulva pertusa ferritin (UpFER) has a plant-ferritin-specific extension peptide composed of 28 amino acid residues. In the crystal structure of UpFER, the EP lay on and interacted with the neighboring threefold symmetry-related subunit. The amino acid residues involved in the interaction were very highly conserved among plant ferritins. The EPs masked the hydrophobic pockets on the ferritin shell surface by lying on them, and this made the ferritin oligomer more hydrophilic. Furthermore, differential scanning calorimetric analysis of the native and its EP-deletion mutant suggested that the EP contributed to the thermal stability of the plant ferritin shell. Thus, the shell stability and surface hydrophobicity of plant ferritin were controlled by the presence or absence of the plant-ferritin-specific EP. This regulation can account for those processes such as shell stability, degradation, and association of plant ferritin, which are significantly related to iron utilization in plants.

Soybean ferritin: isolation, characterization, and free radical generation

The main aim of this work was to assess the multi-task role of ferritin (Ft) in the oxidative metabolism of soybean (Glycine max). Soybean seeds incubated for 24 h yielded 41 ± 5 μg Ft/g fresh weight. The rate of in vitro incorporation of iron (Fe) into Ft was tested by supplementing the reaction medium with physiological Fe chelators. The control rate, observed in the presence of 100 μM Fe, was not significantly different from the values observed in the presence of 100 μM Fe-his. However, it was significantly higher in the presence of 100 μM Fe-citrate (approximately 4.5-fold) or of 100 μM Fe-ATP (approximately 14-fold). Moreover, a substantial decrease in the Trp-dependent fluorescence of the Ft protein was determined during Fe uptake from Fe-citrate, as compared with the control. On the other hand, Ft addition to homogenates from soybean embryonic axes reduced endogenously generated ascorbyl radical, according to its capacity for Fe uptake. The data presented here suggest that Ft could be involved in the generation of free radicals, such as hydroxyl radical, by Fe-catalyzed reactions. Moreover, the scavenging of these radicals by Ft itself could then lead to protein damage. However, Ft could also prevent cellular damage by the uptake of catalytically active Fe.

Moving Iron through ferritin protein nanocages depends on residues throughout each four α-helix bundle subunit

Eukaryotic H ferritins move iron through protein cages to form biologically required, iron mineral concentrates. The biominerals are synthesized during protein-based Fe²⁺/O₂ oxidoreduction and formation of [Fe³⁺O](n) multimers within the protein cage, en route to the cavity, at sites distributed over ~50 Å. Recent NMR and Co²⁺-protein x-ray diffraction (XRD) studies identified the entire iron path and new metal-protein interactions: (i) lines of metal ions in 8 Fe²⁺ ion entry channels with three-way metal distribution points at channel exits and (ii) interior Fe³⁺O nucleation channels. To obtain functional information on the newly identified metal-protein interactions, we analyzed effects of amino acid substitution on formation of the earliest catalytic intermediate (diferric peroxo-A(650 nm)) and on mineral growth (Fe³⁺O-A(350 nm)), in A26S, V42G, D127A, E130A, and T149C. The results show that all of the residues influenced catalysis significantly (p < 0.01), with effects on four functions: (i) Fe²⁺ access/selectivity to the active sites (Glu¹³⁰), (ii) distribution of Fe²⁺ to each of the three active sites near each ion channel (Asp¹²⁷), (iii) product (diferric oxo) release into the Fe³⁺O nucleation channels (Ala²⁶), and (iv) [Fe³⁺O](n) transit through subunits (Val⁴², Thr¹⁴⁹). Synthesis of ferritin biominerals depends on residues along the entire length of H subunits from Fe²⁺ substrate entry at 3-fold cage axes at one subunit end through active sites and nucleation channels, at the other subunit end, inside the cage at 4-fold cage axes. Ferritin subunit-subunit geometry contributes to mineral order and explains the physiological impact of ferritin H and L subunits.

Protein association and dissociation regulated by extension peptide: a mode for iron control by phytoferritin in seeds

Most of the iron in legume seeds is stored in ferritin located in the amyloplast, which is used during seed germination. However, there is a lack of information on the regulation of iron by phytoferritin. In this study, soluble and insoluble forms of pea (Pisum sativum) seed ferritin (PSF) isolated from dried seeds were found to be identical 24-mer ferritins comprising H-1 and H-2 subunits. The insoluble form is favored at low pH, whereas the two forms reversibly interconvert in the pH range of 6.0 to 7.8, with an apparent pK(a) of 6.7. This phenomenon was not observed in animal ferritins, indicating that PSF is unique. The pH of the amyloplast was found to be approximately 6.0, thus facilitating PSF association, which is consistent with the role of PSF in long-term iron storage. Similar to previous studies, the results of this work showed that protein degradation occurs in purified PSF during storage, thus proving that phytoferritin also undergoes degradation during seedling germination. In contrast, no degradation was observed in animal ferritins, suggesting that this degradation of phytoferritin may be due to the extension peptide (EP), a specific domain found only in phytoferritin. Indeed, removal of EP from PSF significantly increased protein stability and prevented degradation under identical conditions while promoting protein dissociation. Correlated with such dissociation was a considerable increase in the rate of ascorbate-induced iron release from PSF at pH 6.0. Thus, phytoferritin may have facilitated the evolution of EP to enable it to regulate iron for storage or complement in seeds.

Role of H-1 and H-2 subunits of soybean seed ferritin in oxidative deposition of iron in protein

Naturally occurring phytoferritin is a heteropolymer consisting of two different H-type subunits, H-1 and H-2. Prior to this study, however, the function of the two subunits in oxidative deposition of iron in ferritin was unknown. The data show that, upon aerobic addition of 48-200 Fe(2+)/shell to apoferritin, iron oxidation occurs only at the diiron ferroxidase center of recombinant H1 (rH-1). In addition to the diiron ferroxidase mechanism, such oxidation is catalyzed by the extension peptide (a specific domain found in phytoferritin) of rH-2, because the H-1 subunit is able to remove Fe(3+) from the center to the inner cavity better than the H-2 subunit. These findings support the idea that the H-1 and H-2 subunits play different roles in iron mineralization in protein. Interestingly, at medium iron loading (200 irons/shell), wild-type (WT) soybean seed ferritin (SSF) exhibits a stronger activity in catalyzing iron oxidation (1.10 ± 0.13 μm iron/subunit/s) than rH-1 (0.59 ± 0.07 μm iron/subunit/s) and rH-2 (0.48 ± 0.04 μm iron/subunit/s), demonstrating that a synergistic interaction exists between the H-1 and H-2 subunits in SSF during iron mineralization. Such synergistic interaction becomes considerably stronger at high iron loading (400 irons/shell) as indicated by the observation that the iron oxidation activity of WT SSF is ∼10 times larger than those of rH-1 and rH-2. This helps elucidate the widespread occurrence of heteropolymeric ferritins in plants.

A novel EP-involved pathway for iron release from soya bean seed ferritin

Iron in phytoferritin from legume seeds is required for seedling germination and early growth. However, the mechanism by which phytoferritin regulates its iron complement to these physiological processes remains unknown. In the present study, protein degradation is found to occur in purified SSF (soya bean seed ferritin) (consisting of H-1 and H-2 subunits) during storage, consistent with previous results that such degradation also occurs during seedling germination. In contrast, no degradation is observed with animal ferritin under identical conditions, suggesting that SSF autodegradation might be due to the EP (extension peptide) on the exterior surface of the protein, a specific domain found only in phytoferritin. Indeed, EP-deleted SSF becomes stable, confirming the above hypothesis. Further support comes from a protease activity assay showing that EP-1 (corresponding to the EP of the H-1 subunit) exhibits significant serine protease-like activity, whereas the activity of EP-2 (corresponding to the EP of the H-2 subunit) is much weaker. Consistent with the observation above, rH-1 (recombinant H-1 ferritin) is prone to degradation, whereas its analogue, rH-2, becomes very stable under identical conditions. This demonstrates that SSF degradation mainly originates from the serine protease-like activity of EP-1. Associated with EP degradation is a considerable increase in the rate of iron release from SSF induced by ascorbate in the amyloplast (pH range, 5.8-6.1). Thus phytoferritin may have facilitated the evolution of the specific domain to control its iron complement in response to cell iron need in the seedling stage.

Crystal structure of plant ferritin reveals a novel metal binding site that functions as a transit site for metal transfer in ferritin

Ferritins are important iron storage and detoxification proteins that are widely distributed in living kingdoms. Because plant ferritin possesses both a ferroxidase site and a ferrihydrite nucleation site, it is a suitable model for studying the mechanism of iron storage in ferritin. This article presents for the first time the crystal structure of a plant ferritin from soybean at 1.8-A resolution. The soybean ferritin 4 (SFER4) had a high structural similarity to vertebrate ferritin, except for the N-terminal extension region, the C-terminal short helix E, and the end of the BC-loop. Similar to the crystal structures of other ferritins, metal binding sites were observed in the iron entry channel, ferroxidase center, and nucleation site of SFER4. In addition to these conventional sites, a novel metal binding site was discovered intermediate between the iron entry channel and the ferroxidase site. This site was coordinated by the acidic side chain of Glu(173) and carbonyl oxygen of Thr(168), which correspond, respectively, to Glu(140) and Thr(135) of human H chain ferritin according to their sequences. A comparison of the ferroxidase activities of the native and the E173A mutant of SFER4 clearly showed a delay in the iron oxidation rate of the mutant. This indicated that the glutamate residue functions as a transit site of iron from the 3-fold entry channel to the ferroxidase site, which may be universal among ferritins.