From Proteopedia

Ferritin

Function

The primary function of ferritin is it acts as an iron storage mechanism and is used for iron homeostasis. Another reason for the amount of ferritin in the blood is how it's stored and how well the protein is transcribed. Ferritin is found in both eukaryotes and prokaryotes; nearly all living things produce ferritin within the bloodstream to keep iron. As iron storage is precious to living species, it is vital to conserve the protein. Another function is that it makes iron available for critical cellular processes as it protects lipids, DNA, and other proteins from the toxic effects of iron. ^[1] Although ferritin is widely known for storing and releasing iron within the bloodstream, it can also keep and release other metal ions. The protein is capable of binding silver ^[2], beryllium, zinc, aluminum, and copper.^[3]

Disease

Besides storing iron, ferritin plays a role in different conditions, including malignant diseases, neurodegenerative, and inflammatory. The importance of ferritin in your bloodstream lets your doctor know if your iron levels are too low or high. Introns are laid out between three amino acid residues in the structure to regulate iron levels. It's a critical marker for allowing the patient to see if they have an iron deficiency or overload with a ferritin test. The causes for having iron deficiency lead to having fewer red blood cells as this can come from blood loss, a poor diet, or that the patient's body doesn't absorb iron regularly from food. The low amounts of iron can also lead to a lower hemoglobin level as the production of hemoglobin depends on iron stored. The causes of having too high iron levels can proceed towards your pancreas, liver, heart, and joints.^[4]

Binding

For the oxidation of ferritin, the protein binds a Fe2+ ion. Glutamic acid (Glu), histidine (His), and glutamine (Gln) residues are the affective binding sites for the ion to attach itself to the H-chain ^{[5] [6]}. The binding sites glutamic acid and glutamine are complex bases, while histidine is a borderline base. Fe2+ is categorized as a borderline acid and binds well with rigid and borderline characters like itself. Some oxidizing agents that all have hard characters are water molecules, oxygen molecules, and hydrogen peroxide. The iron ion was shown to act similar to other metal ions for the ligand like zinc. The zinc ion is not as hard as the iron ion, so it would not be efficient with the glutamic acid and glutamine binding sites. Besides the binding of Fe2+, Fe3+ is another hard acid but is not preferable compared to Fe2+. It can still bind to the glutamic acid and glutamine residues in the L-chain and the oxide species as it prefers hard acids for binding. Chelation affects the binding of metal ions on the binding sites as it prefers hard acids through multiple donor atoms. The binding site of Fe2+ on ferritin is considered to be a chelator.

Structural highlights

Ferritin's distinct structure creates a spherical shell that helps iron to be stored as iron (III) in a crystalline mineral. The intersection of ferritins depends on forming two channels: the polar 3-fold channel and the nonpolar 4-fold channel . The two forms of channels are tiny cavities that allow specific molecules or ions to travel through within the protein structure. The three-fold and four-fold channels are crucial for releasing iron into the bloodstream in a controllable system. The four-fold channels are at the convergence of four peptides, and the three-fold channels are at the crossing of three peptides for the making of the channel. Each channel has different chemical properties, leading to both of their functions for storing and releasing iron.^[7] The three-fold ferritin channels are made up of glutamate (Glu) and aspartate (Asp), representing this particular channel to be polar. The side chains within a protein can only influence whether the channel is considered polar or nonpolar. The side chains' polarity is introduced from the drastic changes in electronegativity between side-by-side atoms in a molecule. Although there are polar groups on the peptide's backbone, they do not share responsibility for the polarity of the side chain amino acids. Glu and Asp are both polar amino acids, so the three-fold channel is polar as it has a favorable interaction with Fe2+ ions and with water. The positive pole of water and the ions' positive charges attract the side chains' opposite. This supportive interchange lets the Fe2+ ions proceed comfortably through the three-fold channel of ferritin. For the four-fold channels, leucine, a nonpolar amino acid, lines these pathways (Leu). Leucine's side chain comprises only hydrogen and carbon atoms, which are relatively the same electronegativities. The functions are different for four-fold channels as the polarity is opposite to the three-fold channels. Because of this channel being nonpolar, the Fe2+ ions do not interact favorably in this channel and don't leave the ferritin shell through this channel as well. Four-fold channels are the site for electron transfer for Fe3+ ions to be reduced to Fe2+. Electrons are transmitted through this channel to decrease the iron (III) in the proteins lattice to iron (II), making the iron soluble and permitting it to be let go from ferritin by the other channel. Although we know this channel to be the site for electron transfer, the mechanism is not well understood.^[8]

References

1. ↑ Stevens, Ruth; Edward Stiefel, chemist who bridged domains, dies at age 64 [1]
2. ↑ Ilari, A., Fiorillo, A., Ceci, P., The X-ray structure of Ferritin from Pyrococcus furiosus loaded with Ag(I) DOI: 10.2210/pdb2X17/pdb
3. ↑ Torti, Frank M., Torti, Suzy V., Regulation of ferritin genes and protein (2002) 99: 3505-3516. DOI: 10.1182/blood.V99.10.3505
4. ↑ Knovich, Mary Ann, Storey, Jonathan A., Coffman, Lan G., and Torti, Suzy V., Ferritin for the Clinician 2009 May ; 23(3): 95–104.doi: 10.1016/j.blre.2008.08.001.
5. ↑ Hempstead, P.D., Artymiuk, P.J., Harrison, P.M., HUMAN H CHAIN FERRITIN DOI: 10.2210/pdb2FHA/pdb
6. ↑ Artymiuk, P.J., Harrison, P.M., SOLVING THE STRUCTURE OF HUMAN H FERRITIN BY GENETICALLY ENGINEERING INTERMOLECULAR CRYSTAL CONTACTS DOI: 10.2210/pdb1FHA/pdb
7. ↑ Tri-iodide derivative of apoferritin DOI: 10.2210/pdb1GWG/pdb
8. ↑ Casiday, Rachel and Frey, Regina; Iron Use and Storage in the Body: Ferritin and Molecular Representations Iron in Biology: Study of the Iron Content in Ferritin, The Iron-Storage Protein. [2]