Ferritin is made of apoferritin and iron. Apoferritin is a protein shell made of 24 subunits that is 13nm in diameter. The shell has six channels for entry of ions. The core of ferritin holds ferric-hydroxyl phosphate. Ferritin can hold 4500 atoms of iron but usually close to 2000 atoms are present.
Apoferritin is made of two units, the H and L units. About twenty types of ferritins (isoferritins) having different ratios subunits are been found in humans. The H (heavy or heart) subunit has a molecular weight of 21000 Da. The gene for the subunit is present at 11q12.3. It has a ferroxidase centre that is responsible for oxidation of Fe2+ ion to Fe3+ ion before incorporation into ferritin. The L (Liver or light) subunit has a molecular weight of 19000 Da. The gene for the L subunit is at 19q13.33. Ferritins rich in H unit acquire iron rapidly where as those rich in the L subunits are more stable.
Synthesis of ferritin begins with the apoferritin shell. Iron in the ferrous form then passes to the core through the channels, is oxidized by the ferroxidase to the ferric form and incorporated in the ferritin. Fully assembled ferritin has molecular weight of 400-500kDa. Iron promotes ferritin synthesis.
Mitochondrial ferritin is transcribed from a different gene situated on chromosome 5q23. Its function is not fully understood. As mitochondrial ferritin is highly conserved, it must have an important role.
Hemosiderin, unlike ferritin, is not s defined chemical entity. It is formed by the partial degradation of ferritin. It contains more iron than ferritin and is less soluble than ferritin. This iron however is not as freely available as iron from ferritin. Hemosiderin only releases iron after prolonged or severe iron depletion.