The ferritin complex is an oligomer of 24 subunits with light and heavy chains. The structural features of ferritin arise from the combination in various ratios of two subunits, H and L, which differ in size, amino acid composition, surface charge, and immunoreactivity. A corollary related differences in ferritin iron content to the functional efficiency of one of the two subunits for storing iron. In humans the H subunit is associated with a lower pI and lower iron content, and predominates in heart tissue, whereas the L subunit is associated with a higher pI and higher iron content, and predominates in the liver.
The functional molecule forms a roughly spherical shell with a diameter of 12 nm and contains a central cavity into which the insoluble mineral iron core is deposited. Iron metabolism provides a useful example of gene expression translational control. Increased iron levels stimulate the synthesis of the iron-binding protein, ferritin, without any corresponding increase in the amount of ferritin mRNA. The 5'-UTR of both ferritin heavy chain mRNA and light chain mRNA contain a single iron-response element (IRE), a specific cis-acting regulatory sequence which forms a hairpin structure.