Ferritins
 
Ferritins and transferrins are nonheme iron-transport proteins that provide the crucial link between dietary iron and organismal heme-iron pools (hemoglobin, myoglobin, and hemosiderin) in maintaining iron homeostasis in mammals. Ferritin is a large protein (ca. 500 kDa) that consists of an aggregate of so-called heavy and light chains, which form a hollow shell in the apo form. Each apoferritin molecule can take up 4500 iron atoms, effectively shielding them from harmful reactions with peroxide and superoxide.
 Transferrins are much smaller proteins that take up iron by binding to the transferrin receptor (TfR( at the cell surface. The complex is subsequently internalized by endocytosis, and then the iron dissociates into the cytoplasm, where it is free to be taken up by ferritin and other cellular proteins.
 The messenger RNAs for transferrin receptor and for both ferritin subunits contain so-called iron-response elements (IREs), which are RNA structures containing a stem and a loop that has the consensus sequence CAGUGX (1). This regulates the translation of the mRNAs.
 
References
1. M. W. Hentze and L. C. Kühn (1996) Proc. Natl. Acad. Sci. USA 93, 8175–8182.