Transferrin is a monomeric glycoprotein found in plasma at an average concentration of 250 mg/100ml. Transferrin (TF) belongs to the family of bilobal glycoproteins which tightly bind ferric iron. Each of the homologous N- and C-lobes of the protein contains a single iron-binding site present in a deep cleft.
Transferrin (TF) serves as the iron transport protein in the blood. Iron is transported in the serum by binding to circulating transferrin, which in turn binds to receptors on the cell surface. Stored in bone marrow as TF-bound iron, it also possesses bacteriostatic and fungistatic activity. At the alkaline extracellular pH of 7.4, TF binds one or two ferric ions and two iron-bound TF molecules can bind the dimeric transferrin receptor (TfR). At this pH, iron-free transferrin is not recognized by TfR. This is followed by an endocytotic pathway involving the TfR, where the entire complex is internalized by endocytosis. When the pH is reduced in the cell, iron is released from TF leading to a conformational change in each lobe. Two critical lysines in the N-lobe of TF aid this conformational change allowing entry of a chelator to capture the iron. Finally, the complex gets back to the cell surface and the extracellular pH causes the dissociation of the apo-TF molecules from the receptor. Clinically, decreases in transferrin are observed in congenital disorders, newborns, inflammatory diseases, hypo-proteinemias and nephritic syndrome; increases are found in pregnancy, iron-deficiency anemias and inoculation hepatitis. Transferrin is required by all types of cells in cultures for maximal growth. It is, therefore, an important factor used in defined culture media.