Ah, ferritin. The humble protein that hoards iron like a miser hoards gold. Fascinating, in a tedious, biological sort of way. You want me to elaborate on this little intracellular financier? Fine. Just don't expect me to feign enthusiasm.
Ferritin
Ferritin. It's that ubiquitous protein you find lurking in nearly every cell, inside and out, tasked with the rather vital job of storing iron and then, with infuriating precision, doling it out. Think of it as the universe’s personal banker for iron. And it’s not picky; almost all living things, from the microscopic archaea and bacteria to the grander algae, higher plants, and of course, us animals, rely on it. It’s the primary guardian of iron within our cells, ensuring this essential but potentially lethal element is kept safely contained and accessible. Without it, we'd be either starved or poisoned. A delicate balance, wouldn't you say? It buffers us against the twin perils of iron deficiency and, perhaps more dramatically, iron overload.
You'll find ferritin predominantly in the cytosol, but don't let that fool you. Small amounts manage to escape, circulating in the serum as an iron carrier. This circulating form, plasma ferritin, is particularly noteworthy because it serves as an indirect gauge of the body’s total iron reserves. Hence, its levels in the blood are crucial for diagnosing conditions like iron-deficiency anemia and, conversely, iron overload. Should ferritin become… unhinged, aggregating into a less soluble, crystalline form, it transforms into something called hemosiderin, a more permanent, albeit less accessible, iron deposit.
Structurally, ferritin is quite the architectural marvel. It’s a globular protein complex, a nanocage of sorts, composed of 24 protein subunits. This spherical structure, with its hollow interior, is designed for maximum iron interaction. When stripped of its iron cargo, it’s referred to as apoferritin.
Gene
The genetic blueprints for ferritin are remarkably well-preserved across species, a testament to its fundamental importance. In all vertebrates, the ferritin genes are structured with three introns and four exons. In humans, these introns strategically divide the coding sequence, appearing between amino acid residues 14 and 15, 34 and 35, and 82 and 83. Additionally, there are untranslated regions, a buffer of one to two hundred bases, flanking the coding sequence. A specific tyrosine at position 27 within the amino acid chain is thought to play a significant role in the process of biomineralization, the controlled deposition of minerals.
Protein structure
Imagine a microscopic, hollow sphere, about 12 nanometers in external diameter and 8 nanometers internally – that's ferritin. This intricate cage is constructed from 24 individual protein subunits, each contributing to the overall structure and function. The precise composition of these subunits, however, varies depending on the organism:
- In vertebrates, the building blocks are of two distinct types: the light (L) subunit and the heavy (H) subunit. While their molecular weights differ slightly (19 kDa for L, 21 kDa for H), their amino acid sequences show a significant degree of homology, sharing about 50% identity.
- Amphibians have a bit of a variation, sporting an additional "M" type of ferritin subunit.
- Plants and bacteria tend to simplify, employing a single type of ferritin subunit, which bears a closer resemblance to the vertebrate H-type.
- Among the gastropods, specifically the genus Lymnaea, researchers have identified two distinct ferritin types: one found in somatic cells and another in the yolk.
- The pearl oyster, Pinctada fucata, incorporates an additional subunit, akin to the Lymnaea somatic ferritin, which is implicated in shell formation.
- Even parasites like Schistosoma exhibit diversity, with two ferritin types present, differentiated between males and females.
Despite these variations, most ferritins maintain a primary sequence similarity to the vertebrate H-type. Even E. coli exhibits about a 20% similarity to human H-ferritin. Some vertebrate ferritin complexes are not uniform but are hetero-oligomers, meaning they are composed of two closely related gene products. The specific ratio of these subunits within the complex is dictated by the relative expression levels of their respective genes, fine-tuning the protein's physiological properties.
The cytosolic ferritin shell, also known as apoferritin, is a complex assembly of 24 heavy (H) and light (L) polypeptide chains. These subunits interlock to form the characteristic hollow spherical nanocage. Within this cavity, iron is stored, not as simple ions, but as crystalline aggregates of hydroxide and phosphate of ferric iron (Fe³⁺). This core structure closely resembles the mineral ferrihydrite (5Fe₂O₃·9H₂O). Each complete ferritin complex has the capacity to sequester approximately 4500 iron ions. The relative abundance of H and L subunits varies significantly between different tissues, leading to heterogeneity in ferritin's properties, particularly its charge. Ferritins richer in L subunits, typically found in the spleen and liver, tend to be more basic than those with a higher proportion of H subunits, which are prevalent in the heart and red blood cells.
Serum ferritin, the form found in the blood, is generally iron-poor and composed almost exclusively of L subunits. Its heterogeneity is further influenced by glycosylation – the addition of sugar molecules. The observed glycosylation patterns and the direct correlation between serum ferritin levels and iron stored in macrophages suggest that macrophages are the primary source of secreted serum ferritin, responding to changes in systemic iron levels.
Human mitochondrial ferritin, designated MtF, is initially synthesized as a pro-protein. Upon uptake by the mitochondria, it is processed into its mature form, structurally resembling cytosolic ferritins. It then self-assembles into functional ferritin shells. Interestingly, unlike other human ferritins, mitochondrial ferritin is a homopolymer composed solely of H-type subunits and appears to lack introns in its genetic code. Its structure, as revealed by Ramachandran plot analysis, is predominantly alpha helical with minimal beta sheet content. Elevated levels of mitochondrial ferritin are observed in the erythroblasts of individuals suffering from impaired heme synthesis.
Function
Iron storage
Ferritin is present in virtually every cell type. Its primary role is to store iron in a form that is both non-toxic and readily available for cellular use. This intricate process involves not only safe storage but also regulated release of iron to where it's needed. The specific function and structure of ferritin can vary across different cell types, largely dictated by the amount and stability of its messenger RNA (mRNA), as well as variations in mRNA storage and transcription efficiency. A common trigger for ferritin production is the simple presence of iron itself, though there are exceptions, such as the yolk ferritin found in Lymnaea sp., which lacks the typical iron-responsive element.
Free iron is a cellular menace. It’s a potent catalyst in the generation of harmful free radicals from reactive oxygen species through the infamous Fenton reaction, leading to cellular damage. To combat this, vertebrates have developed sophisticated mechanisms to bind and compartmentalize iron within various tissue environments. Within cells, iron is primarily stored in ferritin or the related, less soluble compound, hemosiderin. Apoferritin, the iron-free form, readily binds to free ferrous iron (Fe²⁺) and oxidizes it to the ferric state (Fe³⁺) for storage. As ferritin accumulates within cells of the reticuloendothelial system, particularly in the spleen and liver, it can aggregate to form hemosiderin. While iron stored in ferritin can be mobilized, hemosiderin represents a more recalcitrant iron reserve. Under normal physiological conditions, the concentration of ferritin in the blood serum closely mirrors the total iron stored in the body, making serum ferritin a convenient measure for estimating iron reserves.
Iron is also a critical component in biomineralization processes. Organisms like molluscs utilize ferritin in their shells to precisely control iron concentration and distribution, influencing shell morphology and coloration. In the haemolymph of polyplacophora, ferritin facilitates the rapid transport of iron to the site of radular teeth mineralization.
The release of iron from ferritin for cellular use is primarily achieved through the degradation of the protein, a process largely carried out by lysosomes.
Ferroxidase activity
Vertebrate ferritin subunits are categorized as L (light), H (heavy), and in some cases, M (middle), though the M subunit is primarily observed in bullfrogs. Ferritin in bacteria and archaea, however, consists of only one subunit type. The H and M subunits in eukaryotes, and all subunits in prokaryotic ferritins, possess ferroxidase activity. This crucial function allows them to convert ferrous iron (Fe²⁺) into ferric iron (Fe³⁺). This oxidation is vital for preventing the hazardous Fenton reaction between ferrous iron and hydrogen peroxide, which generates highly destructive hydroxyl radicals. The ferroxidase activity is localized to a specific diiron binding site within the core of each H-type subunit. Following the oxidation of Fe(II) to Fe(III), the ferric iron remains temporarily bound in the ferroxidase center until it is displaced by another Fe(II) ion, a mechanism believed to be conserved across ferritins from all three domains of life. The L-chain, lacking ferroxidase activity, is thought to play a role in facilitating electron transfer across the protein cage.
Immune response
Ferritin concentrations can surge significantly during infections or in the presence of cancer. Endotoxins, for instance, are known to upregulate the gene responsible for ferritin production, leading to elevated ferritin levels. Paradoxically, certain bacteria, such as Pseudomonas, despite possessing endotoxins, can cause a sharp drop in plasma ferritin within the first 48 hours of infection. This mechanism effectively deprives the invading pathogen of essential iron stores, hindering its metabolic processes.
Stress response
The concentration of ferritin has been observed to increase in response to various stresses, including anoxia. This suggests that ferritin may function as an acute phase protein, responding to cellular distress.
Mitochondria
Mitochondrial ferritin engages in a range of molecular functions, including ferroxidase activity, iron ion binding, and participation in oxidoreductase activity. Biologically, it plays roles in oxidation-reduction processes, iron ion transport across membranes, and the maintenance of cellular iron homeostasis.
Yolk
In certain snail species, the primary protein component of their egg yolk is ferritin. This yolk ferritin is genetically distinct from the somatic ferritin and is synthesized in the midgut glands before being secreted into the haemolymph for transport to the developing eggs.
Tissue distribution
In vertebrates, ferritin is predominantly found within cells, though it is also present in lower concentrations in the plasma.
Diagnostic uses
Serum ferritin levels are a standard measurement in medical laboratories as part of the workup for iron-deficiency anemia. These levels are typically reported in nanograms per milliliter (ng/mL) or micrograms per liter (μg/L), which are equivalent units.
Generally, serum ferritin levels correlate directly with the total amount of iron stored in the body. However, this correlation can be disrupted. In conditions like anemia of chronic disease, ferritin levels can be artificially elevated because ferritin acts as an inflammatory acute phase protein, rather than reflecting iron stores.
Normal ranges
The definition of normal ferritin blood levels, or the reference interval, can differ slightly between testing laboratories. However, typical ranges for adults are approximately 40–300 ng/mL (or μg/L) for males and 20–200 ng/mL (or μg/L) for females. Pediatric ranges vary by age, with infants generally having higher levels than older children.
| Normal ferritin blood levels according to sex and age |
|---|
| Adult males: 40–300 ng/mL (μg/L) |
| Adult females: 20–200 ng/mL (μg/L) |
| Children (6 months to 15 years): 50–140 ng/mL (μg/L) |
| Infants (1 to 5 months): 50–200 ng/mL (μg/L) |
| Neonates: 25–200 ng/mL (μg/L) |
Deficiency
A ferritin level below 30 ng/mL is generally indicative of iron deficiency, while a level below 10 ng/mL suggests iron-deficiency anemia. More specific guidelines exist, with the World Health Organization suggesting that ferritin levels below 12 ng/mL in children under 5 and 15 ng/mL in individuals aged 5 and over indicate iron deficiency in otherwise healthy populations.
Some research indicates that women experiencing fatigue with ferritin levels below 50 ng/mL may experience relief after iron supplementation.
In the context of anemia, a low serum ferritin level is the most specific laboratory finding for iron-deficiency anemia. However, its sensitivity can be compromised. Since ferritin levels rise during infection or inflammation (as an acute phase reactant), these conditions can mask true iron deficiency by elevating ferritin levels into the normal range. Therefore, a low ferritin level carries more diagnostic weight than a normal one. Falsely low ferritin readings are uncommon but can occur in extreme cases due to a hook effect in the assay methodology.
Low ferritin may also be associated with hypothyroidism, vitamin C deficiency, or celiac disease.
Low serum ferritin levels are also observed in some patients with restless legs syndrome, even in the absence of anemia, potentially reflecting depleted iron stores.
Interestingly, vegetarianism itself is not inherently a cause of low serum ferritin. Position statements from organizations like the American Dietetic Association indicate that while vegetarians may have lower iron stores, their serum ferritin levels are typically within the normal range, and the incidence of iron-deficiency anemia is similar to that of non-vegetarians.
Excess
Elevated ferritin levels can signify either an excess of iron in the body or an acute inflammatory response where ferritin production is increased without a corresponding iron surplus. For example, high ferritin levels during an infection do not necessarily indicate iron overload.
Ferritin serves as a key marker for iron overload disorders such as hemochromatosis and hemosiderosis. Abnormally high ferritin levels can also be seen in conditions like Adult-onset Still's disease, certain porphyrias, and hemophagocytic lymphohistiocytosis/macrophage activation syndrome.
Given that ferritin is also an acute-phase reactant, its elevation can occur during various disease states. A normal C-reactive protein level can help rule out elevated ferritin due to acute phase reactions.
Elevated ferritin has been noted in some cases of COVID-19 and may correlate with a more severe clinical outcome. Ferritin, along with IL-6, is considered a potential immunological biomarker for severe and fatal COVID-19 cases. Combined with C-reactive protein, ferritin might serve as a screening tool for the early diagnosis of systemic inflammatory response syndrome in COVID-19 patients.
Studies on patients with anorexia nervosa suggest that ferritin can be elevated during periods of acute malnourishment. This may be due to iron being shifted into storage as intravascular volume decreases, leading to a relative reduction in red blood cell count. The catabolic nature of anorexia nervosa might also lead to the release of isoferritins, contributing to an overall increase in measured ferritin. Moreover, ferritin has functions beyond iron storage, including protection against oxidative damage. Assays measuring ferritin via immunoassay or immunoturbidimetry might detect these isoferritins, thus not providing a precise reflection of iron storage status.
Research indicates that a transferrin saturation exceeding 60% in men and 50% in women is highly accurate (approximately 95%) in identifying iron metabolism abnormalities, particularly hereditary hemochromatosis. This finding aids in the early diagnosis of hereditary hemochromatosis, even when serum ferritin levels remain relatively low. In hereditary hemochromatosis, iron primarily accumulates in parenchymal cells, with reticuloendothelial cell involvement occurring late in the disease, unlike in transfusional iron overload where the pattern is reversed. This explains the paradox of high transferrin saturation with relatively low ferritin in hereditary hemochromatosis.
In chronic liver diseases
Hematological abnormalities are frequently associated with chronic liver diseases. Both iron overload and iron deficiency anemia can occur. Iron overload is often linked to reduced hepcidin levels due to the liver's diminished synthetic capacity, while iron deficiency can result from chronic bleeding caused by portal hypertension. Inflammation is also a common feature in advanced chronic liver disease. Consequently, elevated hepatic and serum ferritin levels are consistently reported in these conditions.
Studies have demonstrated a link between high serum ferritin levels and an increased risk of short-term mortality in cirrhotic patients experiencing acute decompensation and acute-on-chronic liver failure. Similarly, high serum ferritin has been associated with an increased risk of long-term mortality in both compensated and decompensated cirrhotic patients. The same research suggested that elevated serum ferritin could predict the onset of bacterial infections in stable decompensated cirrhotic patients. Conversely, in compensated cirrhotic patients, low serum ferritin levels were associated with a higher incidence of the first episode of acute decompensation, a finding potentially explained by the link between chronic bleeding and increased portal pressure.
Discovery
The discovery of ferritin is credited to the Czechoslovakian scientist Vilém Laufberger in 1937. Later, in 1942, Sam Granick and Leonor Michaelis successfully produced apoferritin.
Applications
Ferritin finds utility in materials science as a precursor for synthesizing iron nanoparticles used in carbon nanotube growth via chemical vapor deposition. It has also demonstrated the ability to store electrons for extended periods and facilitate electron tunneling under ambient conditions, properties that may be relevant in biological contexts.
The cavities formed by ferritin and its smaller counterpart, mini-ferritin (Dps) proteins, have been effectively employed as reaction chambers for fabricating metal nanoparticles. The protein shells act as templates, controlling particle size and preventing aggregation. By utilizing different sizes of protein shells, nanoparticles of various dimensions can be synthesized for diverse chemical, physical, and biomedical applications.
Experimental COVID-19 vaccines have been developed utilizing ferritin nanoparticles engineered to display the receptor binding domain of the spike protein.
Apoferritin is also a standard choice for assessing the resolution of single-particle analysis in Cryogenic Electron Microscopy (Cryo-EM), owing to its highly symmetrical structure and ease of sample preparation. Apoferritin currently holds the record for the highest resolution structure determined by cryo-EM, achieving resolutions around 1.1-1.2 Å.
Notes
The primary peptide sequence of human ferritin is:
MTTASTSQVR QNYHQDSEAA INRQINLELY ASYVYLSMSY YFDRDDVALK NFAKYFLHQS HEEREHAEKL MKLQNQRGGR IFLQDIKKPD CDDWESGLNA MECALHLEKN VNQSLLEFPS PISPSPSCWH HYTTNRPQPQ HHLLRPRRRK RPHSIPTPIL IFRSP.