ferritins

Ferritins are iron-storage proteins that sequester iron in a non-toxic, bioavailable form. In vertebrates, ferritin forms a hollow nanocage assembled from 24 subunits, creating an inner cavity that can store up to about 4,500 iron atoms as ferric oxide mineral cores. The cage measures roughly 12 nm in diameter externally and about 8 nm internally.

Subunits are mainly H (heavy) and L (light). The H subunit contains a ferroxidase center that converts Fe2+ to Fe3+, accelerating oxidation, while the L subunit stabilizes and optimizes long-term iron storage. The H:L ratio varies by tissue to tune storage and release.

In bacteria and some archaea, ferritin-like proteins include bacterioferritins, which incorporate heme, and DPS (DNA-binding proteins from starved cells) that store iron and also protect DNA from oxidative damage.

Iron is imported as Fe2+, oxidized at the ferroxidase center, and deposited as a ferrihydrite-like core. Release occurs when iron is needed, usually via reduction and transport by cellular export pathways such as ferroportin in animals.

Regulation ties ferritin to cellular iron status. Iron regulatory proteins bind iron-responsive elements in ferritin mRNA, repressing translation under iron deficiency and permitting synthesis when iron is abundant.

Clinically, serum ferritin serves as a marker of body iron stores and behaves as an acute-phase protein, rising with inflammation and decreasing in iron deficiency.