The iron redox and hydrolysis chemistry of the ferritins

The role of iron transporters and regulators in Alzheimer's disease and Parkinson's disease: Pathophysiological insights and therapeutic prospects

Brain iron homeostasis plays a vital role in maintaining brain development and controlling neuronal function under physiological conditions. Many studies have shown that the imbalance of brain iron homeostasis is closely related to the pathogenesis of neurodegenerative diseases (NDs), such as Alzheimer's disease (AD) and Parkinson's disease (PD). Recent advances have revealed the importance of iron transporters and regulatory molecules in the pathogenesis and treatment of NDs. This review summarizes the research progress on brain iron overload and the aberrant expression of several key iron transporters and regulators in AD and PD, emphasizes the pathological roles of these molecules in the pathogenesis of AD and PD, and highlights the therapeutic prospects of targeting these iron transporters and regulators to restore brain iron homeostasis in the treatment of AD and PD. A comprehensive understanding of the pathophysiological roles of iron, iron transporters and regulators, and their regulations in NDs may provide new therapeutic avenues for more targeted neurotherapeutic strategies for treating these diseases.

Effects of ferritin iron loading, subunit composition, and the NCOA4-iron sulfur cluster on ferritin-NCOA4 interactions: An isothermal titration calorimetry study

Ferritin is a 24-mer protein nanocage that stores iron and regulates intracellular iron homeostasis. The nuclear receptor coactivator-4 (NCOA4) binds specifically to ferritin H subunits and facilitates the autophagic trafficking of ferritin to the lysosome for degradation and iron release. Using isothermal titration calorimetry (ITC), we studied the thermodynamics of the interactions between ferritin and the soluble fragment of NCOA4 (residues 383-522), focusing on the effects of the recently identified FeS cluster bound to NCOA4, ferritin subunit composition, and ferritin-iron loading. Our findings show that in the presence of the FeS cluster, the binding is driven by a more favorable enthalpy change and a decrease in entropy change, indicating a key role for the FeS cluster in the structural organization and stability of the complex. The ferritin iron core further enhances this association, increasing binding enthalpy and stabilizing the NCOA4-ferritin complex. The ferritin subunit composition primarily affects binding stoichiometry of the reaction based on the number of H subunits in the ferritin H/L oligomer. Our results demonstrate that both the FeS cluster and the ferritin iron core significantly affect the binding thermodynamics of the NCOA4-ferritin interactions, suggesting regulatory roles for the FeS cluster and ferritin iron content in ferritinophagy.

Gastric stability of bare and chitosan-fabricated ferritin and its bio-mineral: implication for potential dietary iron supplements

Iron deficiency anaemia (IDA), the most widespread nutritional disorder, is a persistent global health issue affecting millions, especially in resource-limited geographies. Oral iron supplementation is usually the first choice for exogenous iron administration owing to its convenience, effectiveness and low cost. However, commercially available iron supplementations are often associated with oxidative stress, gastrointestinal side effects, infections and solubility issues. Herein, we aim to address these limitations by employing ferritin proteins-self-assembled nanocaged architectures functioning as a soluble cellular iron repository-as a non-toxic and biocompatible alternative. Our in vitro studies based on PAGE and TEM indicate that bare ferritin proteins are resistant to gastric conditions but their cage integrity is compromised under longer incubation periods and at higher concentrations of pepsin, which is a critical component of gastric juice. To ensure the safe delivery of encapsulated iron cargo, with minimal cage disintegration/degradation and iron leakage along the gastrointestinal tract, we fabricated the surface of ferritin with chitosan. Further, the stoichiometry and absorptivity of iron-chelator complexes at both gastric and circumneutral pH were estimated using Job's plot. Unlike bipyridyl, deferiprone exhibited pH dependency. In vitro kinetics was studied to evaluate iron release from bare and chitosan-fabricated ferritins employing both reductive (in the presence of ascorbate and bipyridyl) and non-reductive (direct chelation by deferiprone) pathways to determine their bio-mineral stabilities. Chitosan-decorated ferritin displayed superior cage integrity and iron retention capability over bare ferritin in simulated gastric fluid. The ability of ferritins to naturally facilitate controlled iron release in conjugation with enteric coating provided by chitosan may mitigate the aforementioned side effects and enhance iron absorption in the intestine. The results of the current study could pave the way for the development of an oral formulation based on ferritin-caged iron bio-mineral that can be a promising alternative for the treatment of IDA, offering better therapeutic outcomes.

Unveiling the stochastic nature of human heteropolymer ferritin self-assembly mechanism

Despite ferritin's critical role in regulating cellular and systemic iron levels, our understanding of the structure and assembly mechanism of isoferritins, discovered over eight decades ago, remains limited. Unveiling how the composition and molecular architecture of hetero-oligomeric ferritins confer distinct functionality to isoferritins is essential to understanding how the structural intricacies of H and L subunits influence their interactions with cellular machinery. In this study, ferritin heteropolymers with specific H to L subunit ratios were synthesized using a uniquely engineered plasmid design, followed by high-resolution cryo-electron microscopy analysis and deep learning-based amino acid modeling. Our structural examination revealed unique architectural features during the self-assembly mechanism of heteropolymer ferritins and demonstrated a significant preference for H-L heterodimer formation over H-H or L-L homodimers. Unexpectedly, while dimers seem essential building blocks in the protein self-assembly process, the overall mechanism of ferritin self-assembly is observed to proceed randomly through diverse pathways. The physiological significance of these findings is discussed including how ferritin microheterogeneity could represent a tissue-specific adaptation process that imparts distinctive tissue-specific functions to isoferritins.

Ferritin Single-Electron Transistor

We report on the fabrication of a single-electron transistor based on ferritin using wide self-aligned nanogap devices. A local gate below the gap area enables three-terminal electrical measurements, showing the Coulomb blockade in good agreement with the single-electron tunneling theory. Comparison with this theory allows extraction of the tunnel resistances, capacitances, and gate coupling. Additionally, the data suggest the presence of two separate islands coupled in series or in parallel: information that was not possible to distinguish by using only two-terminal measurements. To interpret the charge transport features, we propose a scenario based on the established configuration structures of ferritin involving either iron sites in the organic shell or two dissimilar clusters within the core.

Effects of ferritin heavy chain on oxidative stress, cell proliferation and apoptosis in geese follicular granulosa cells

1. The ferritin heavy chain (FHC) has a vital impact on follicular development in geese, due to its ability to regulate apoptosis of granulosa cells (GCs) and follicular atresia. However, its specific regulatory mechanisms remain unclear. The present study characterised how FHC regulates oxidative stress, cell proliferation and apoptosis in goose GCs by interfering with and overexpressing the FHC gene.2. After 72 h of interference with FHC expression, the activity of GCs decreased remarkably (p < 0.05), reactive oxygen species (ROS) levels and the expression levels of antioxidant enzyme genes catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) increased significantly (p < 0.05). The overexpression of FHC for 72 h was found to significantly reduce the expression of CAT and SOD genes (p < 0.05).3. Interfering with FHC expression revealed that the expression levels of the cell proliferation gene Aurora kinase A (AURORA-A) were significantly decreased (p < 0.05), while the expression levels of the apoptosis genes B-cell lymphoma-2 (BCL-2) and cysteine aspartate-specific protease 8 (CASPASE 8) increased (p < 0.05). Further research has shown that, when interfering with FHC expression for 72 h, apoptosis rate increased by 1.19-fold (p < 0.05), but the current data showed a lower apoptosis rate after FHC overexpression by 59.41%, 63.39%, and 52.31% at three different treatment times (p < 0.05).4. In conclusion, FHC improved the antioxidant capacity of GCs, promotes GCs proliferation, and inhibits GCs apoptosis of ovarian follicles in Sichuan white geese.

Iron mobilization from intact ferritin: effect of differential redox activity of quinone derivatives with NADH/O2 and in situ-generated ROS

Ferritins are multimeric nanocage proteins that sequester/concentrate excess of free iron and catalytically synthesize a hydrated ferric oxyhydroxide bio-mineral. Besides functioning as the primary intracellular iron storehouses, these supramolecular assemblies also oversee the controlled release of iron to meet physiologic demands. By virtue of the reducing nature of the cytosol, reductive dissolution of ferritin-iron bio-mineral by physiologic reducing agents might be a probable pathway operating in vivo. Herein, to explore this reductive iron-release pathway, a series of quinone analogs differing in size, position/nature of substituents and redox potentials were employed to relay electrons from physiologic reducing agent, NADH, to the ferritin core. Quinones are well known natural electron/proton mediators capable of facilitating both 1/2 electron transfer processes and have been implicated in iron/nutrient acquisition in plants and energy transduction. Our findings on the structure-reactivity of quinone mediators highlight that iron release from ferritin is dictated by electron-relay capability (dependent on E1/2 values) of quinones, their molecular structure (i.e., the presence of iron-chelation sites and the propensity for H-bonding) and the type/amount of reactive oxygen species (ROS) they generate in situ. Juglone/Plumbagin released maximum iron due to their intermediate E1/2 values, presence of iron chelation sites, the ability to inhibit in situ generation of H2O2 and form intramolecular H-bonding (possibly promotes semiquinone formation). This study may strengthen our understanding of the ferritin-iron-release process and their significance in bioenergetics/O2-based cellular metabolism/toxicity while providing insights on microbial/plant iron acquisition and the dynamic host-pathogen interactions.

Encapsulated Ferritin-like Proteins: A Structural Perspective

Encapsulins are self-assembling nano-compartments that naturally occur in bacteria and archaea. These nano-compartments encapsulate cargo proteins that bind to the shell's interior through specific recognition sequences and perform various metabolic processes. Encapsulation enables organisms to perform chemical reactions without exposing the rest of the cell to potentially harmful substances while shielding cargo molecules from degradation and other adverse effects of the surrounding environment. One particular type of cargo protein, the ferritin-like protein (FLP), is the focus of this review. Encapsulated FLPs are members of the ferritin-like protein superfamily, and they play a crucial role in converting ferrous iron (Fe+2) to ferric iron (Fe+3), which is then stored inside the encapsulin in mineralized form. As such, FLPs regulate iron homeostasis and protect organisms against oxidative stress. Recent studies have demonstrated that FLPs have tremendous potential as biosensors and bioreactors because of their ability to catalyze the oxidation of ferrous iron with high specificity and efficiency. Moreover, they have been investigated as potential targets for therapeutic intervention in cancer drug development and bacterial pathogenesis. Further research will likely lead to new insights and applications for these remarkable proteins in biomedicine and biotechnology.

Structural basis for the intracellular regulation of ferritin degradation

The interaction between nuclear receptor coactivator 4 (NCOA4) and the iron storage protein ferritin is a crucial component of cellular iron homeostasis. The binding of NCOA4 to the FTH1 subunits of ferritin initiates ferritinophagy-a ferritin-specific autophagic pathway leading to the release of the iron stored inside ferritin. The dysregulation of NCOA4 is associated with several diseases, including neurodegenerative disorders and cancer, highlighting the NCOA4-ferritin interface as a prime target for drug development. Here, we present the cryo-EM structure of the NCOA4-FTH1 interface, resolving 16 amino acids of NCOA4 that are crucial for the interaction. The characterization of mutants, designed to modulate the NCOA4-FTH1 interaction, is used to validate the significance of the different features of the binding site. Our results explain the role of the large solvent-exposed hydrophobic patch found on the surface of FTH1 and pave the way for the rational development of ferritinophagy modulators.

The application of ferritin in transporting and binding diverse metal ions

The ferritin cage can not only load iron ions in its inner cavity, but also has the capacity to carry other metal ions, thus constructing a new biological nano-transport system. The nanoparticles formed by ferritin and minerals can be used as ingredients of mineral supplements, which overcome the shortcomings of traditional mineral ingredients such as low bioavailability. Moreover, ferritin can be used to remove heavy metal ions from contaminated food. Silver and palladium nanoparticles formed by ferritin are also applied as anticancer agents. Ferritin combined with metal ions can be also used to detect harmful substances. This review aims to provide a comprehensive overview of ferritin's function in transporting and binding metal ions, and discusses the limitations and future prospects, which offers valuable insights for the application of ferritin in mineral supplements, food detoxifiers, anticancer agents, and food detections.

Structural Insights into the Reaction between Hydrogen Peroxide and Di-iron Complexes at the Ferroxidase Center of Ferritin

The Fe(II) oxidation mechanism in the ferroxidase center of heavy chain ferritin has been studied extensively. However, the actual production of H2O2 was found to be substantially lower than expected at low flux of Fe(II) to ferritin subunits. Here, we demonstrated that H2O2 could interact with the di-iron nuclear center, leading to the production of hydroxyl radicals and oxygen. Two reaction intermediates were captured in the ferroxidase center by using the time-lapse crystallographic techniques in a shellfish ferritin. The crystal structures revealed the binding of H2O2 as a μ -1,2-peroxo-diferric species and the binding of O2 to the diferric structure. This investigation sheds light on the reaction between the di-iron nuclear center and H2O2 and provides insights for the exploitation of metalloenzymes.

Two-dimensional oxygen-deficient ZnO1-x nanosheet as a highly selective and sensitive fluorescence probe for ferritin detection: the electron transfer biosensor (ETBS)

Iron proteins are of great scientific interest due to their importance as an excellent biomarker for human diseases. Ferritin (Fe3+), being an iron-rich blood protein, is related to various diseases like anemia and cancer. For the first time, we have developed a highly sensitive and selective ferritin biosensor based on fluorescent oxygen-deficient zinc oxide nanosheets through hydrothermal and probe-ultrasonication combined methods. The fluorescence study showed an intense bluish-green fluorescence at λex = 370 nm, after optimization at different excitation wavelengths. In addition, the fluorescence of ZnO1-x nanosheets can be efficiently quenched due to electron transfer reactions in order to achieve quantification analysis. The limit of detection (LOD) was calculated to be 0.015 nM (7.2 ng mL-1) with high linearity (R2 = 0.9930). In addition, the real-world application of the proposed biosensor has been performed on human blood serum samples in the presence of various interfering analytes showing high selectivity and sensitivity with a regression value R2 = 0.9980 indicating the current approach is an excellent biosensor platform.

Iron homeostasis and post-hemorrhagic hydrocephalus: a review

Iron physiology is regulated by a complex interplay of extracellular transport systems, coordinated transcriptional responses, and iron efflux mechanisms. Dysregulation of iron metabolism can result in defects in myelination, neurotransmitter synthesis, and neuronal maturation. In neonates, germinal matrix-intraventricular hemorrhage (GMH-IVH) causes iron overload as a result of blood breakdown in the ventricles and brain parenchyma which can lead to post-hemorrhagic hydrocephalus (PHH). However, the precise mechanisms by which GMH-IVH results in PHH remain elusive. Understanding the molecular determinants of iron homeostasis in the developing brain may lead to improved therapies. This manuscript reviews the various roles iron has in brain development, characterizes our understanding of iron transport in the developing brain, and describes potential mechanisms by which iron overload may cause PHH and brain injury. We also review novel preclinical treatments for IVH that specifically target iron. Understanding iron handling within the brain and central nervous system may provide a basis for preventative, targeted treatments for iron-mediated pathogenesis of GMH-IVH and PHH.

Fenton Reaction in vivo and in vitro. Possibilities and Limitations

The review considers the problem of hydrogen peroxide decomposition and hydroxyl radical formation in the presence of iron in vivo and in vitro. Analysis of the literature data allows us to conclude that, under physiological conditions, transport of iron, carried out with the help of carrier proteins, minimizes the possibility of appearance of free iron ions in cytoplasm of the cell. Under pathological conditions, when the process of transferring an iron ion from a donor protein to an acceptor protein can be disrupted due to modifications of the carrier proteins, iron ions can enter cytosol. However, at pH values close to neutral, which is typical for cytosol, iron ions are converted into water-insoluble hydroxides. This makes it impossible to decompose hydrogen peroxide according to the mechanism of the classical Fenton reaction. A similar situation is observed in vitro, since buffers with pH close to neutral are used to simulate free radical oxidation. At the same time, iron hydroxides are able to catalyze decomposition of hydrogen peroxide with formation of a hydroxyl radical. Decomposition of hydrogen peroxide with iron hydroxides is called Fenton-like reaction. Studying the features of Fenton-like reaction in biological systems is the subject of future research.

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.

New Perspectives on Circulating Ferritin: Its Role in Health and Disease

The diagnosis of iron disturbances usually includes the evaluation of serum parameters. Serum iron is assumed to be entirely bound to transferrin, and transferrin saturation-the ratio between the serum iron concentration and serum transferrin-usually reflects iron availability. Additionally, serum ferritin is commonly used as a surrogate of tissue iron levels. Low serum ferritin values are interpreted as a sign of iron deficiency, and high values are the main indicator of pathological iron overload. However, in situations of inflammation, serum ferritin levels may be very high, independently of tissue iron levels. This presents a particularly puzzling challenge for the clinician evaluating the overall iron status of the patient in the presence of an inflammatory condition. The increase in serum ferritin during inflammation is one of the enigmas regarding iron metabolism. Neither the origin, the mechanism of release, nor the effects of serum ferritin are known. The use of serum ferritin as a biomarker of disease has been rising, and it has become increasingly diverse, but whether or not it contributes to controlling the disease or host pathology, and how it would do it, are important, open questions. These will be discussed here, where we spotlight circulating ferritin and revise the recent clinical and preclinical data regarding its role in health and disease.

Ferritin microheterogeneity, subunit composition, functional, and physiological implications

Ferritin is a ubiquitous intracellular iron storage protein that plays a crucial role in iron homeostasis. Animal tissue ferritins consist of multiple isoforms (or isoferritins) with different proportions of H and L subunits that contribute to their structural and compositional heterogeneity, and thus physiological functions. Using size exclusion and anion exchange chromatography, capillary isoelectric focusing (cIEF), and SDS-capillary gel electrophoresis (SDS-CGE), we reveal for the first time a significant variation in ferritin subunit composition and isoelectric points, in both recombinant and native ferritins extracted from animal organs. Our results indicate that subunits composition is the main determinant of the mean pI of recombinant ferritin heteropolymers, and that ferritin microheterogeneity is a common property of both natural and recombinant proteins and appears to be an intrinsic feature of the cellular machinery during ferritin expression, regulation, post-translational modifications, and post-subunits assembly. The functional significance and physiological implications of ferritin heterogeneity in terms of iron metabolism, response to oxidative stress, tissue-specific functions, and pathological processes are discussed.

Clostridioides difficile ferrosome organelles combat nutritional immunity

Iron is indispensable for almost all forms of life but toxic at elevated levels1-4. To survive within their hosts, bacterial pathogens have evolved iron uptake, storage and detoxification strategies to maintain iron homeostasis1,5,6. Recent studies showed that three Gram-negative environmental anaerobes produce iron-containing ferrosome granules7,8. However, it remains unclear whether ferrosomes are generated exclusively by Gram-negative bacteria. The Gram-positive bacterium Clostridioides difficile is the leading cause of nosocomial and antibiotic-associated infections in the USA9. Here we report that C. difficile undergoes an intracellular iron biomineralization process and stores iron in membrane-bound ferrosome organelles containing non-crystalline iron phosphate biominerals. We found that a membrane protein (FezA) and a P1B6-ATPase transporter (FezB), repressed by both iron and the ferric uptake regulator Fur, are required for ferrosome formation and play an important role in iron homeostasis during transition from iron deficiency to excess. Additionally, ferrosomes are often localized adjacent to cellular membranes as shown by cryo-electron tomography. Furthermore, using two mouse models of C. difficile infection, we demonstrated that the ferrosome system is activated in the inflamed gut to combat calprotectin-mediated iron sequestration and is important for bacterial colonization and survival during C. difficile infection.

Ferritin Is Secreted from Primary Cultured Astrocyte in Response to Iron Treatment via TRPML1-Mediated Exocytosis

Impaired iron homeostasis has been proven to be one of the critical contributors to the pathology of Parkinson's disease (PD). Ferritin is considered an intracellular protein responsible for storing cytosolic iron. Recent studies have found that ferritin can be secreted from cells independent of the classical endoplasmic reticulum-Golgi system. However, the precise mechanisms underlying the secretion of ferritin in the brain were not elucidated. In the present study, we demonstrated that the primary cultured astrocytes do have the ability to secrete ferritin, which is enhanced by iron treatment. Increased ferritin secretion was accompanied by increased protein expression of ferritin response to iron stimulation. Further study showed that iron-induced expression and secretion of ferritin could be inhibited by CQ or 3-MA pretreatment. In addition, the knockdown of transient receptor potential mucolipin 1 (TRPML1) antagonized iron-induced ferritin secretion, accompanied by further increased intracellular protein levels of ferritin. Further study demonstrated that ferritin colocalized with LAMP1 in iron-treated astrocytes. On the contrary, ras-associated protein 27a (Rab27a) knockdown further enhanced iron-induced ferritin secretion and decreased intracellular protein levels of ferritin. Furthermore, we also showed that the secretory autophagy protein tripartite motif containing 16 (TRIM16) and sec22b decreased in iron-treated astrocytes. These results suggested that astrocytes might secrete ferritin via TRPML1-mediated exocytosis. This provides new evidence for the mechanisms underlying the secretion of ferritin in primary cultured astrocytes under a high iron environment.

Engineered Ferritin with Eu3+ as a Bright Nanovector: A Photoluminescence Study

Ferritin nanoparticles play many important roles in theranostic and bioengineering applications and have been successfully used as nanovectors for the targeted delivery of drugs due to their ability to specifically bind the transferrin receptor (TfR1, or CD71). They can be either genetically or chemically modified for encapsulating therapeutics or probes in their inner cavity. Here, we analyzed a new engineered ferritin nanoparticle, made of the H chain mouse ferritin (HFt) fused with a specific lanthanide binding tag (LBT). The HFt-LBT has one high affinity lanthanide binding site per each of the 24 subunits and a tryptophane residue within the tag that acts as an antenna able to transfer the energy to the lanthanide ions via a LRET process. In this study, among lanthanides, we selected europium for its red emission that allows to reduce overlap with tissue auto-fluorescence. Steady state emission measurements and time-resolved emission spectroscopy have been employed to investigate the interaction between the HFt-LBT and the Eu3+ ions. This allowed us to identify the Eu3+ energy states involved in the process and to pave the way for the future use of HFt-LBT Eu3+ complex in theranostics.

Ferroptosis regulates key signaling pathways in gastrointestinal tumors: Underlying mechanisms and therapeutic strategies

Ferroptosis is an emerging novel form of non-apoptotic, regulated cell death that is heavily dependent on iron and characterized by rupture in plasma membrane. Ferroptosis is distinct from other regulated cell death modalities at the biochemical, morphological, and molecular levels. The ferroptotic signature includes high membrane density, cytoplasmic swelling, condensed mitochondrial membrane, and outer mitochondrial rupture with associated features of accumulation of reactive oxygen species and lipid peroxidation. The selenoenzyme glutathione peroxidase 4, a key regulator of ferroptosis, greatly reduces the lipid overload and protects the cell membrane against oxidative damage. Ferroptosis exerts a momentous role in regulating cancer signaling pathways and serves as a therapeutic target in cancers. Dysregulated ferroptosis orchestrates gastrointestinal (GI) cancer signaling pathways leading to GI tumors such as colonic cancer, pancreatic cancer, and hepatocellular carcinoma. Crosstalk exists between ferroptosis and other cell death modalities. While apoptosis and autophagy play a detrimental role in tumor progression, depending upon the factors associated with tumor microenvironment, ferroptosis plays a decisive role in either promoting tumor growth or suppressing it. Several transcription factors, such as TP53, activating transcription factors 3 and 4, are involved in influencing ferroptosis. Importantly, several molecular mediators of ferroptosis, such as p53, nuclear factor erythroid 2-related factor 2/heme oxygenase-1, hypoxia inducible factor 1, and sirtuins, coordinate with ferroptosis in GI cancers. In this review, we elaborated on key molecular mechanisms of ferroptosis and the signaling pathways that connect ferroptosis to GI tumors.

Structural and Biochemical Characterization of Silver/Copper Binding by Dendrorhynchus zhejiangensis Ferritin

Ferritin with a highly symmetrical cage-like structure is not only key in the reversible storage of iron in efficient ferroxidase activity; it also provides unique coordination environments for the conjugation of heavy metal ions other than those associated with iron. However, research regarding the effect of these bound heavy metal ions on ferritin is scarce. In the present study, we prepared a marine invertebrate ferritin from Dendrorhynchus zhejiangensis (DzFer) and found that it could withstand extreme pH fluctuation. We then demonstrated its capacity to interact with Ag⁺ or Cu²⁺ ions using various biochemical and spectroscopic methods and X-ray crystallography. Structural and biochemical analyses revealed that both Ag⁺ and Cu²⁺ were able to bind to the DzFer cage via metal-coordination bonds and that their binding sites were mainly located inside the three-fold channel of DzFer. Furthermore, Ag⁺ was shown to have a higher selectivity for sulfur-containing amino acid residues and appeared to bind preferentially at the ferroxidase site of DzFer as compared with Cu²⁺. Thus, it is far more likely to inhibit the ferroxidase activity of DzFer. The results provide new insights into the effect of heavy metal ions on the iron-binding capacity of a marine invertebrate ferritin.

Modeling the iron storage protein ferritin reveals how residual ferrihydrite iron determines initial ferritin iron sequestration kinetics

Computational models can be created more efficiently by composing them from smaller, well-defined sub-models that represent specific cellular structures that appear often in different contexts. Cellular iron metabolism is a prime example of this as multiple cell types tend to rely on a similar set of components (proteins and regulatory mechanisms) to ensure iron balance. One recurrent component, ferritin, is the primary iron storage protein in mammalian cells and is necessary for cellular iron homeostasis. Its ability to sequester iron protects cells from rising concentrations of ferrous iron limiting oxidative cell damage. The focus of the present work is establishing a model that tractably represents the ferritin iron sequestration kinetics such that it can be incorporated into larger cell models, in addition to contributing to the understanding of general ferritin iron sequestration dynamics within cells. The model's parameter values were determined from published kinetic and binding experiments and the model was validated against independent data not used in its construction. Simulation results indicate that FT concentration is the most impactful on overall sequestration dynamics, while the FT iron saturation (number of iron atoms sequestered per FT cage) fine tunes the initial rates. Finally, because this model has a small number of reactions and species, was built to represent important details of FT kinetics, and has flexibility to include subtle changes in subunit composition, we propose it to be used as a building block in a variety of specific cell type models of iron metabolism.

Iron Homeostasis in Insects

Iron is an essential micronutrient for all types of organisms; however, iron has chemical properties that can be harmful to cells. Because iron is both necessary and potentially damaging, insects have homeostatic processes that control the redox state, quantity, and location of iron in the body. These processes include uptake of iron from the diet, intracellular and extracellular iron transport, and iron storage. Early studies of iron-binding proteins in insects suggested that insects and mammals have surprisingly different mechanisms of iron homeostasis, including different primary mechanisms for exporting iron from cells and for transporting iron from one cell to another, and subsequent studies have continued to support this view. This review summarizes current knowledge about iron homeostasis in insects, compares insect and mammalian iron homeostasis mechanisms, and calls attention to key remaining knowledge gaps.

Modification of 4-Fold and B-Pores in Bacterioferritin from Mycobacterium tuberculosis Reveals Their Role in Fe2+ Entry and Oxidoreductase Activity

The self-assembled ferritin nanocages, nature's solution to iron toxicity and its low solubility, scavenge free iron to synthesize hydrated ferric oxyhydroxide mineral inside their central cavity by protein-mediated ferroxidase and hydrolytic/nucleation reactions. These complex processes in ferritin commence with the rapid influx of Fe²⁺ ions via the inter-subunit contact points (i.e., pores/channels). Investigation of these pores as Fe²⁺ uptake routes in ferritins remains a subject of intense research, in iron metabolism, toxicity, and bacterial pathogenesis, which are yet to be established in the bacterioferritin (BfrA) from Mycobacterium tuberculosis (Mtb). The electrostatic properties of this protein indicate that the 4-fold and B-pores might serve as potential Fe²⁺ entry routes. Therefore, in the current work, electrostatics at/along these pores was altered by site-directed mutagenesis to establish their role in Fe²⁺ uptake/oxidation (ferroxidase activity) in Mtb BfrA. Despite forming self-assembled protein nanocompartment, these 4-fold and B-pore variants exhibited partial loss of ferroxidase activity and lower accumulation of transient species, which not only indicated their role in Fe²⁺ entry but also suggested the existence of multiple pathways. Although the B-pore variants inhibited the rapid ferroxidase activity to a larger extent, they had minimal impact on their cage stability. The current work revealed the relative contribution of these pores toward rapid Fe²⁺ uptake/oxidation and cage stability, possibly as consequences of their differential symmetry, number of modified residues (at each pore), and heme content. Therefore, these findings may help to understand the role of these pores in iron acquisition and Mtb proliferation under iron-limiting conditions to control its pathogenesis.

Hyperthermostable recombinant human heteropolymer ferritin derived from a novel plasmid design

Mammalian ferritins are predominantly heteropolymeric species consisting of 2 structurally similar, but functionally and genetically distinct subunit types, called H (Heavy) and L (Light). The two subunits co-assemble in different H and L ratios to form 24-mer shell-like protein nanocages where thousands of iron atoms can be mineralized inside a hollow cavity. Here, we use differential scanning calorimetry (DSC) to study ferritin stability and understand how various combinations of H and L subunits confer aspects of protein structure-function relationships. Using a recently engineered plasmid design that enables the synthesis of complex ferritin nanostructures with specific H to L subunit ratios, we show that homopolymer L and heteropolymer L-rich ferritins have a remarkable hyperthermostability (Tm = 115 ± 1°C) compared to their H-ferritin homologues (Tm = 93 ± 1°C). Our data reveal a significant linear correlation between protein thermal stability and the number of L subunits present on the ferritin shell. A strong and unexpected iron-induced protein thermal destabilization effect (ΔTm up to 20°C) is observed. To our knowledge, this is the first report of recombinant human homo- and hetero-polymer ferritins that exhibit surprisingly high dissociation temperatures, the highest among all known ferritin species, including many known hyperthermophilic proteins and enzymes. This extreme thermostability of our L and L-rich ferritins may have great potential for biotechnological applications.

Iron Accumulation in Ferritin

Understanding the iron accumulation process in ferritin protein nanocages has remained a centerpiece in the field of iron biochemistry/biomineralization, which ultimately has implications in health and diseases. Although mechanistic differences of iron acquisition and mineralization exist in the superfamily of ferritins, we describe the techniques that can be used to investigate the accumulation of iron in all the ferritin proteins by in vitro iron mineralization process. In this chapter, we report that the non-denaturing polyacrylamide gel electrophoresis coupled with Prussian blue staining (in-gel assay) can be useful to investigate the iron-loading efficiency in ferritin protein nanocage, by estimating the relative amount of iron incorporated inside it. Similarly, the absolute size of the iron mineral core and the amount of total iron accumulated inside its nanocavity can be determined by using transmission electron microscopy and spectrophotometry, respectively.

Micromagnetic and morphological characterization of heteropolymer human ferritin cores

The physical properties of in vitro iron-reconstituted and genetically engineered human heteropolymer ferritins were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), electron energy-loss spectroscopy (EELS), and 57Fe Mössbauer spectroscopy were employed to ascertain (1) the microstructural, electronic, and micromagnetic properties of the nanosized iron cores, and (2) the effect of the H and L ferritin subunit ratios on these properties. Mössbauer spectroscopic signatures indicate that all iron within the core is in the high spin ferric state. Variable temperature Mössbauer spectroscopy for H-rich (H21/L3) and L-rich (H2/L22) ferritins reconstituted at 1000 57Fe/protein indicates superparamagnetic behavior with blocking temperatures of 19 K and 28 K, while HAADF-STEM measurements give average core diameters of (3.7 ± 0.6) nm and (5.9 ± 1.0) nm, respectively. Most significantly, H-rich proteins reveal elongated, dumbbell, and crescent-shaped cores, while L-rich proteins present spherical cores, pointing to a correlation between core shape and protein shell composition. Assuming an attempt time for spin reversal of τ 0 = 10-11 s, the Néel-Brown formula for spin-relaxation time predicts effective magnetic anisotropy energy densities of 6.83 × 104 J m-3 and 2.75 × 104 J m-3 for H-rich and L-rich proteins, respectively, due to differences in surface and shape contributions to magnetic anisotropy in the two heteropolymers. The observed differences in shape, size, and effective magnetic anisotropies of the derived biomineral cores are discussed in terms of the iron nucleation sites within the interior surface of the heteropolymer shells for H-rich and L-rich proteins. Overall, our results imply that site-directed nucleation and core growth within the protein cavity play a determinant role in the resulting core morphology. Our findings have relevance to iron biomineralization processes in nature and the growth of designer's magnetic nanoparticles within recombinant apoferritin nano-templates for nanotechnology.

Destruction of Lysozyme Amyloid Fibrils Induced by Magnetoferritin and Reconstructed Ferritin

Neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), or systemic amyloidosis, are characterized by the specific protein transformation from the native state to stable insoluble deposits, e.g., amyloid plaques. The design of potential therapeutic agents and drugs focuses on the destabilization of the bonds in their beta-rich structures. Surprisingly, ferritin derivatives have recently been proposed to destabilize fibril structures. Using atomic force microscopy (AFM) and fluorescence spectrophotometry, we confirmed the destructive effect of reconstructed ferritin (RF) and magnetoferritin (MF) on lysosome amyloid fibrils (LAF). The presence of iron was shown to be the main factor responsible for the destruction of LAF. Moreover, we found that the interaction of RF and MF with LAF caused a significant increase in the release of potentially harmful ferrous ions. Zeta potential and UV spectroscopic measurements of LAF and ferritin derivative mixtures revealed a considerable difference in RF compared to MF. Our results contribute to a better understanding of the mechanism of fibril destabilization by ferritin-like proteins. From this point of view, ferritin derivatives seem to have a dual effect: therapeutic (fibril destruction) and adverse (oxidative stress initiated by increased Fe²⁺ release). Thus, ferritins may play a significant role in various future biomedical applications.

Effect of Phosphate and Ferritin Subunit Composition on the Kinetics, Structure, and Reactivity of the Iron Core in Human Homo- and Heteropolymer Ferritins

Ferritins are highly conserved supramolecular protein nanostructures that play a key role in iron homeostasis. Thousands of iron atoms can be stored inside their hollow cavity as a hydrated ferric oxyhydroxide mineral. Although phosphate associates with the ferritin iron nanoparticles, the effect of physiological concentrations on the kinetics, structure, and reactivity of ferritin iron cores has not yet been explored. Here, the iron loading and mobilization kinetics were studied in the presence of 1-10 mM phosphate using homopolymer and heteropolymer ferritins having different H to L subunit ratios. In the absence of ferritin, phosphate enhances the rate of ferrous ion oxidation and forms large and soluble polymeric Fe(III)-phosphate species. In the presence of phosphate, Fe(II) oxidation and core formation in ferritin is significantly accelerated with oxidation rates several-fold higher than with phosphate alone. High-angle annular dark-field scanning transmission electron microscopy measurements revealed a strong phosphate effect on both the size and morphology of the iron mineral in H-rich (but not L-rich) ferritins. While iron nanoparticles in L-rich ferritins have spherical shape in the absence and presence of phosphate, iron nanoparticles in H-rich ferritins change from irregular shapes in the absence of phosphate to spherical particles in the presence of phosphate with larger size distribution and smaller particle size. In the presence of phosphate, the kinetics of iron-reductive mobilization from ferritin releases twice as much iron than in its absence. Altogether, our results demonstrate an important role for phosphate, and the ferritin H and L subunit composition toward the kinetics of iron oxidation and removal from ferritin, as well as the structure and reactivity of the iron mineral, and may have an important implication on ferritin iron management in vivo.

The crosstalk effect between ferrous and other ions metabolism in ferroptosis for therapy of cancer

Ferroptosis is an iron-dependent cell death process characterized by excessive accumulation of reactive oxygen species and lipid peroxidation. The elucidation of ferroptosis pathways may lead to novel cancer therapies. Current evidence suggests that the mechanism of ferroptosis can be summarized as oxidative stress and antioxidant defense mechanisms. During this process, ferrous ions play a crucial role in cellular oxidation, plasma membrane damage, reactive oxygen species removal imbalance and lipid peroxidation. Although, disregulation of intracellular cations (Fe²⁺, Ca²⁺, Zn²⁺, etc.) and anions (Cl^-, etc.) have been widely reported to be involved in ferroptosis, their specific regulatory mechanisms have not been established. To further understand the crosstalk effect between ferrous and other ions in ferroptosis, we reviewed the ferroptosis process from the perspective of ions metabolism. In addition, the role of ferrous and other ions in tumor therapy is briefly summarized.

Ferritin: A Promising Nanoreactor and Nanocarrier for Bionanotechnology

The essence of bionanotechnology lies in the application of nanotechnology/nanomaterials to solve the biological problems. Quantum dots and nanoparticles hold potential biomedical applications, but their inherent problems such as low solubility and associated toxicity due to their interactions at nonspecific target sites is a major concern. The self-assembled, thermostable, ferritin protein nanocages possessing natural iron scavenging ability have emerged as a potential solution to all the above-mentioned problems by acting as nanoreactor and nanocarrier. Ferritins, the cellular iron repositories, are hollow, spherical, symmetric multimeric protein nanocages, which sequester the excess of free Fe(II) and synthesize iron biominerals (Fe₂O₃·H₂O) inside their ∼5-8 nm central cavity. The electrostatics and dynamics of the pore residues not only drives the natural substrate Fe²⁺ inside ferritin nanocages but also uptakes a set of other metals ions/counterions during in vitro synthesis of nanomaterial. The current review aims to report the recent developments/understanding on ferritin structure (self-assembly, surface/pores electrostatics, metal ion binding sites) and chemistry occurring inside these supramolecular protein cages (protein mediated metal ion uptake and mineralization/nanoparticle formation) along with its surface modification to exploit them for various nanobiotechnological applications. Furthermore, a better understanding of ferritin self-assembly would be highly useful for optimizing the incorporation of nanomaterials via the disassembly/reassembly approach. Several studies have reported the successful engineering of these ferritin protein nanocages in order to utilize them as potential nanoreactor for synthesizing/incorporating nanoparticles and as nanocarrier for delivering imaging agents/drugs at cell specific target sites. Therefore, the combination of nanoscience (nanomaterials) and bioscience (ferritin protein) projects several benefits for various applications ranging from electronics to medicine.

Iron Mobilization from Ferritin in Yeast Cell Lysate and Physiological Implications

Most in vitro iron mobilization studies from ferritin have been performed in aqueous buffered solutions using a variety of reducing substances. The kinetics of iron mobilization from ferritin in a medium that resembles the complex milieu of cells could dramatically differ from those in aqueous solutions, and to our knowledge, no such studies have been performed. Here, we have studied the kinetics of iron release from ferritin in fresh yeast cell lysates and examined the effect of cellular metabolites on this process. Our results show that iron release from ferritin in buffer is extremely slow compared to cell lysate under identical experimental conditions, suggesting that certain cellular metabolites present in yeast cell lysate facilitate the reductive release of ferric iron from the ferritin core. Using filtration membranes with different molecular weight cut-offs (3, 10, 30, 50, and 100 kDa), we demonstrate that a cellular component >50 kDa is implicated in the reductive release of iron. When the cell lysate was washed three times with buffer, or when NADPH was omitted from the solution, a dramatic decrease in iron mobilization rates was observed. The addition of physiological concentrations of free flavins, such as FMN, FAD, and riboflavin showed about a two-fold increase in the amount of released iron. Notably, all iron release kinetics occurred while the solution oxygen level was still high. Altogether, our results indicate that in addition to ferritin proteolysis, there exists an auxiliary iron reductive mechanism that involves long-range electron transfer reactions facilitated by the ferritin shell. The physiological implications of such iron reductive mechanisms are discussed.

Potential of Ferritin-Based Platforms for Tumor Immunotherapy

Ferritin is an iron storage protein that plays a key role in iron homeostasis and cellular antioxidant activity. Ferritin has many advantages as a tumor immunotherapy platform, including a small particle size that allows for penetration into tumor-draining lymph nodes or tumor tissue, a unique structure consisting of 24 self-assembled subunits, cavities that can encapsulate drugs, natural targeting functions, and a modifiable outer surface. In this review, we summarize related research applying ferritin as a tumor immune vaccine or a nanocarrier for immunomodulator drugs based on different targeting mechanisms (including dendritic cells, tumor-associated macrophages, tumor-associated fibroblasts, and tumor cells). In addition, a ferritin-based tumor vaccine expected to protect against a wide range of coronaviruses by targeting multiple variants of SARS-CoV-2 has entered phase I clinical trials, and its efficacy is described in this review. Although ferritin is already on the road to transformation, there are still many difficulties to overcome. Therefore, three barriers (drug loading, modification sites, and animal models) are also discussed in this paper. Notwithstanding, the ferritin-based nanoplatform has great potential for tumor immunotherapy, with greater possibility of clinical transformation.

Noncovalent Engineering of Apoferritin with a PEGylated [FeFe] Hydrogenase Mimic for In Situ Polymerization

Apoferritin can act as a scaffold for functionalization in the inner and outer surfaces. However, traditional covalent modification methods have a risk of disrupting the structure and physicochemical properties of apoferritin. Herein, we report a method for designing versatile apoferritin-based nanosystems through noncovalent interaction between a PEGylated [FeFe]-hydrogenase mimic (FeFe-PEG-N₃) and apoferritin. FeFe-PEG-N₃ can be anchored into the threefold channels of apoferritin via program injection, at a number of ∼8 per protein. We also engineered apoferritin with an FeFe-PEG-N₃/ATRP initiator conjugate for in situ and noninvasive atom transfer radical polymerization (ATRP) at the apoferritin surface. This "grafting-from" method for noncovalent apoferritin engineering has the advantages of simple preparation, good controllability, and high efficiency and affords opportunities for the construction of multifunctional apoferritin-based nanosystems for broad applications such as drug delivery and catalysis.

Crystallographic characterization of a marine invertebrate ferritin from the sea cucumber Apostichopus japonicus

Ferritin is considered to be an ubiquitous and conserved iron-binding protein that plays a crucial role in iron storage, detoxification and immune response. Although ferritin is of critical importance for almost all kingdoms of life, there is a lack of knowledge about its role in the marine invertebrate sea cucumber (Apostichopus japonicus). In this study, we characterized the first crystal structure of Apostichopus japonicas ferritin (AjFER) at 2.75 Å resolution. The structure of AjFER shows a 4-3-2 symmetry cage-like hollow shell composed of 24 subunits, mostly similar to the structural characteristics of other known ferritin species, including the conserved ferroxidase center and 3-fold channel. The 3-fold channel consisting of three 3-fold negative amino acid rings suggests a potential pathway in which metal ions can be first captured by Asp120 from the outside environment, attracted by His116 and Cys128 when entering the channel, and then transferred by Glu138 from the 3-fold channel to the ferroxidase site. Overall, the presented crystal structure of AjFER may provide insights into the potential mechanism of the metal transport pathway for related marine invertebrate ferritins.

Ferritin-Like Proteins: A Conserved Core for a Myriad of Enzyme Complexes

Ferritin-like proteins share a common fold, a four α-helix bundle core, often coordinating a pair of metal ions. Although conserved, the ferritin fold permits a diverse set of reactions, and is central in a multitude of macromolecular enzyme complexes. Here, we emphasize this diversity through three members of the ferritin-like superfamily: the soluble methane monooxygenase, the class I ribonucleotide reductase and the aldehyde deformylating oxygenase. They all rely on dinuclear metal cofactors to catalyze different challenging oxygen-dependent reactions through the formation of multi-protein complexes. Recent studies using cryo-electron microscopy, serial femtosecond crystallography at an X-ray free electron laser source, or single-crystal X-ray diffraction, have reported the structures of the active protein complexes, and revealed unprecedented insights into the molecular mechanisms of these three enzymes.

Ferroptosis in Intracerebral Hemorrhage: A Panoramic Perspective of the Metabolism, Mechanism and Theranostics

Iron is one of the most crucial elements in the human body. In recent years, a kind of programmed, non-apoptotic cell death closely related to iron metabolism-called ferroptosis- has aroused much interest among many scientists. Ferroptosis also interacts with other pathways involved in cell death including iron abnormality, the cystine/glutamate antiporter and lipid peroxidation. Together these pathological pathways exert great impacts on intracerebral hemorrhage (ICH), a lethal cerebrovascular disease with a high incidence rate and mortality rate. Furthermore, the ferroptosis also affects different brain cells (neurons and neuroglial cells) and different organelles (mitochondria and endoplasmic reticulum). Clinical treatments for ferroptosis in ICH have been closely investigated recently. This perspective provides a comprehensive summary of ferroptosis mechanisms after ICH and its interaction with other cell death patterns. Understanding the role of ferroptosis in ICH will open new windows for the future treatments and preventions for ICH and other intracerebral diseases.

Alteration of Coaxial Heme Ligands Reveals the Role of Heme in Bacterioferritin from Mycobacterium tuberculosis

The uptake and utilization of iron remains critical for the survival/virulence of the host/pathogens in spite of the limitations (low bioavailability/high toxicity) associated with this nutrient. Both the host and pathogens manage to overcome these problems by utilizing the iron repository protein nanocages, ferritins, which not only sequester and detoxify the free Fe(II) ions but also decrease the iron solubility gap by synthesizing/encapsulating the Fe(III)-oxyhydroxide biomineral in its central hollow nanocavity. Bacterial pathogens including Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, encode a distinct subclass of ferritins called bacterioferritin (BfrA), which binds heme, the versatile redox cofactor, via coaxial, conserved methionine (M52) residues at its subunit-dimer interfaces. However, the exact role of heme in Mtb BfrA remains yet to be established. Therefore, its coaxial ligands were altered via site-directed mutagenesis, which resulted in both heme-bound (M52C; ∼1 heme per cage) and heme-free (M52H and M52L) variants, indicating the importance of M52 residues as preferential heme binding axial ligands in Mtb BfrA. All these variants formed intact nanocages of similar size and iron-loading ability as that of wild-type (WT) Mtb BfrA. However, the as-isolated heme-bound variants (WT and M52C) exhibited enhanced protein stability and reductive iron mobilization as compared to their heme-free analogues (M52H and M52L). Further, increasing the heme content in BfrA variants by reconstitution not only enhanced the cage stability but also facilitated the iron mobilization, suggesting the role of heme. In contrary, heme altered the ferroxidase activity to a lesser extent despite facilitating the accumulation of the reactive intermediates formed during the course of the reaction. The current study suggests that heme in Mtb BfrA enhances the overall stability of the protein and possibly acts as an intrinsic electron relay station to influence the iron mineral dissolution and thus may be associated with Mtb's pathogenicity.

Shrimp ferritin greatly improves the physical and chemical stability of astaxanthin

The low stability of trans-astaxanthin (AX) not only limits its applications as a functional factor in food systems, but also affects the sensor quality of most shrimp products. Therefore, it is important to find an easy, effective way to improve the physical and chemical stability of AX. In this study, by taking advantage of the co-existence of AX and shrimp ferritin (Marsupenaeus japonicus ferritin, MjF), we investigated the interaction of AX with MjF. Results showed that AX molecules are able to bind on the outer surface of MjF to form complexes, and quantitative analyses demonstrated that one ferritin molecule is bound to ∼48 AX molecules. Consequently, such binding not only greatly enhances the water solubility, thermal stability, and photo stability of AX, but also protects AX from Fe²⁺ -induced oxidative damage, as compared to free AX. Thus, MjF could be used as a protective molecule to improve the physical and chemical stability of AX. PRACTICAL APPLICATION: Our study opens up a new avenue for improving the physicochemical properties of bioactive molecules by interacting with protein, and shrimp ferritin could be explored as a protective system for the bioactive molecules.

A Novel Approach for the Synthesis of Human Heteropolymer Ferritins of Different H to L Subunit Ratios

Mammalian ferritins are predominantly heteropolymeric species consisting of 24 structurally similar, but functionally different subunit types, named H and L, that co-assemble in different proportions. Despite their discovery more than 8 decades ago, recombinant human heteropolymer ferritins have never been synthesized, owing to the lack of a good expression system. Here, we describe for the first time a unique approach that uses a novel plasmid design that enables the synthesis of these complex ferritin nanostructures. Our study reveals an original system that can be easily tuned by altering the concentrations of two inducers, allowing the synthesis of a full spectrum of heteropolymer ferritins, from H-rich to L-rich ferritins and any combinations in-between (isoferritins). The H to L subunit composition of purified ferritin heteropolymers was analyzed by SDS-PAGE and capillary gel electrophoresis, and their iron handling properties characterized by light absorption spectroscopy. Our novel approach allows future investigations of the structural and functional differences of isoferritin populations, which remain largely obscure. This is particularly exciting since a change in the ferritin H- to L-subunit ratio could potentially lead to new iron core morphologies for various applications in bio-nanotechnologies.

Imidacloprid activates ROS and causes mortality in honey bees (Apis mellifera) by inducing iron overload

Imidacloprid, a neonicotinoid pesticide widely used for insect pest control, has become a potential pollutant to pollinators. Previous reports have demonstrated the toxicity of this drug in activating oxidative stress resulting in high mortality in the honey bee Apis mellifera. However, the mechanisms underlying the toxicity of imidacloprid have not been fully elucidated. In this study, sublethal (36 ng/bee) and median lethal (132 ng/bee) doses of imidacloprid were administered to bees. The results showed dose-dependent increases in reactive oxygen species (ROS), Fe2+, and mortality in bees. Notably, imidacloprid also induced upregulation of the gene encoding ferritin (AmFth), which plays a pivotal role in reducing Fe2+ overload. Upregulation of AmFth has been suggested to be closely related to ROS accumulation and high mortality in bees. To confirm the role played by AmFth in imidacloprid-activated ROS, dsAmFth double-strand was orally administered to bees after exposure to imidacloprid. The results revealed aggravated Fe2+ overload, higher ROS activation, and elevated mortality in the bees, indicating that imidacloprid activated ROS and caused mortality in the bees, probably by inducing iron overload. This study helps to elucidate the molecular mechanisms underlying the toxicity of imidacloprid from the perspective of iron metabolism.

Bacterial Siderophores and Their Potential Applications: A Review

The bacterial infection is one of the major health issues throughout the world. To protect humans from the infection and infectious agents, it is important to understand the mechanism of interaction of pathogens along with their susceptible hosts. This will help us to develop a novel strategy for designing effective new drugs or vaccines. As iron is an essential metal ion required for all the living systems for their growth, as well, it is needed by pathogenic bacterial cells for their growth and development inside host tissues. To get iron from the host tissues, microbes developed an iron-chelating system called siderophore and also corresponding receptors. Siderophores are low molecular weight organic complex produced by different strains of bacteria for the procurement of iron from the environment or host body under the iron deficient-conditions. Mostly in the environment at physiological pH, the iron is present in the ferric ionic form (Fe3+), which is water- insoluble and thus inaccessible for them. Such a condition promotes the generation of siderophores. These siderophores have been used in different areas such as agriculture, treatment of diseases, culture the unculturable strains of bacteria, promotion of plant growth, controlling phytopathogens, detoxification of heavy metal contamination, etc. In the medical field, siderophores can be used as "Trojan Horse Strategy", which forms a complex with antibiotics and also delivers these antibiotics to the desired locations, especially in antibiotic-resistant bacteria. The promising application of siderophore-based use of antibiotics for the management of bacterial resistance can be strategies to be used.

Diversity of structures and functions of oxo-bridged non-heme diiron proteins

Oxo-bridged diiron proteins are a distinct class of non-heme iron proteins. Their active sites are composed of two irons that are coordinated by amino acid side chains, and a bridging oxygen that interacts with each iron. These proteins are members of the ferritin superfamily and share the structural feature of a four α-helix bundle that provides the residues that coordinate the irons. The different proteins also display a wide range of structures and functions. A prototype of this family is hemerythrin, which functions as an oxygen transporter. Several other hemerythrin-like proteins have been described with a diversity of functions including oxygen and iron sensing, and catalytic activities. Rubrerythrins react with hydrogen peroxide and rubrerythrin-like proteins possess a rubredoxin domain, in addition to the oxo-bridged diiron center. Other redox enzymes with oxo-bridged irons include flavodiiron proteins that act as O2 or NO reductases, ribonucleotide reductase and methane monooxygenase. Ferritins have an oxo-bridged diiron in the ferroxidase center of the protein, which plays a role in the iron storage function of these proteins. There are also bacterial ferritins that exhibit catalytic activities. The structures and functions of this broad class of oxo-bridged diiron proteins are described and compared in this review.

Flavin-mediated reductive iron mobilization from frog M and Mycobacterial ferritins: impact of their size, charge and reactivities with NADH/O2

In vitro, reductive mobilization of ferritin iron using suitable electron transfer mediators has emerged as a possible mechanism to mimic the iron release process, in vivo. Nature uses flavins as electron relay molecules for important biological oxidation and oxygenation reactions. Therefore, the current work utilizes three flavin analogues: riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which differ in size and charge but have similar redox potentials, to relay electron from nicotinamide adenine dinucleotide (NADH) to ferritin mineral core. Of these, the smallest/neutral analogue, RF, released more iron (~ three fold) in comparison to the larger and negatively charged FMN and FAD. Although iron mobilization got marred during the initial stages under aerobic conditions, but increased with a greater slope at the later stages of the reaction kinetics, which gets inhibited by superoxide dismutase, consistent with the generation of O2∙- in situ. The initial step, i.e., interaction of flavins with NADH played critical role in the iron release process. Overall, the flavin-mediated reductive iron mobilization from ferritins occurred via two competitive pathways, involving the reduced form of flavins either alone (anaerobic condition) or in combination with O2∙- intermediate (aerobic condition). Moreover, faster iron release was observed for ferritins from Mycobacterium tuberculosis than from bullfrog, indicating the importance of protein nanocage and the advantages they provide to the respective organisms. Therefore, these structure-reactivity studies of flavins with NADH/O2 holds significance in ferritin iron release, bioenergetics, O2-based cellular toxicity and may be potentially exploited in the treatment of methemoglobinemia. Smaller sized/neutral flavin analogue, riboflavin (RF) exhibits faster reactivity towards both NADH and O2 generating more amount of O2∙- and releases higher amount of iron from different ferritins, compared to its larger sized/negatively charged derivatives such as FMN and FAD.

Complexation of ferrous ions by ferrozine, 2,2'-bipyridine and 1,10-phenanthroline: Implication for the quantification of iron in biological systems

Iron is an essential nutrient for virtually all forms of life. Because of its redox properties and involvement in a wide range of biological processes, a number of qualitative and quantitative chemical tools have been developed to detect reduced (Fe²⁺) and oxidized (Fe³⁺) forms of iron in biomolecules. These types of measurements are not only important in detecting iron species in solution, but also in understanding iron distribution, accumulation, and role in physiological and pathological processes. Here, we use UV-vis spectrophotometry and three common chromogenic reagents, ferrozine, 2,2'-bipyridine, and 1,10-phenanthroline to detect and quantify the concentration of ferrous ions in aqueous solutions, owing to the unique absorption spectra, specific molar absorptivity, and characteristic colors of these Fe²⁺-chelator complexes. Our results show that the kinetics of the formation of the {Fe²⁺-(ferrozine)₃} complex, but not the{Fe²⁺-(bipyridine)₃} or the {Fe(II)-(phenanthroline)₃} complexes depend on the concentration of the iron chelator, requiring up to 20 min to complete when close to stoichiometric ratios are employed. The molar absorptivity values of these complexes under excess chelator concentrations were ~ 10% to 15% higher than reported literature values (i.e. 31,500 ± 1500 M^-1 cm^-1 for ferrozine at 562 nm, 9950 ± 100 M^-1 cm^-1 for 2,2'-bipyridine at 522 nm, and 12,450 ± 370 M^-1 cm^-1 for 1,10-phenanthroline at 510 nm). Our results have important implications when quantifying iron in biological systems and reveal optimal experimental conditions that must be employed for the accurate measurements of ferrous ions, whether free in solution, or after reduction of protein-bound ferric ions.