The protein transferrin (679 amino acids) is required for the transport and uptake of iron into cells. Each transferrin molecule can carry up to two iron ions, each coupled with an anion (preferably CO32-) ligand to from an iron-transferrin complex. The cofactor acts as bridging ligand between metal and protein, therefore, without the anion chelate, iron is unable to bind to transferrin. The protein consists of two lobes; each lobe is able to form four bonds to Fe3+ ion and locking it in place. Transferrin could exist in three different forms; apotransferrin, monoferric transferrin, and diferric transferrin (binding 0, 1, or 2 Fe3+ ions respectively). Once the iron-transferrin complex is formed, it enters and flows through the bloodstream until it finds a transferrin receptor on the surface of a cell. The majority of the iron bound to circulating transferrin is delivered to the bone marrow (erythrocyte production), the liver (iron storage in ferritin) and the spleen where it binds to the receptor.After iron-transferrin binds tightly to the receptor, it is invaginated by clathrin-coated pits to form endocytic vesicles (an endocytosis process). The cell then acidifies the endosome via ATP-dependent proton pumps which lowers the pH to about 5.5. The lowered pH weakens the bond between the iron and transferrin, thus releasing the iron. The receptor-bound apotransferrin are then recycled back to the outside surface of the cell (an exocytosis process), and dissociated (via the neutral (7.4) pH) to enable transferrin to continue gathering and transporting iron around the body.

The affinity of transferrin for iron increases as the number of binding Fe3+ ions increases (i.e. diferric transferrin has a higher affinity for iron than monoferric transferrin).

Transferrin Structure". St. Edward's University. 2005-07-18. Retrieved 2009-04-24.