Transferrins are glycoproteins found in vertebrates which bind and consequently mediate the transport of iron (Fe) through blood plasma. They are produced in the liver and contain binding sites for two Fe ions. Human transferrin is encoded by the TF gene and produced as a 76 kDa glycoprotein.
Transferrin glycoproteins bind iron tightly, but reversibly. Although iron bound to transferrin is less than 0.1% (4 mg) of total body iron, it forms the most vital iron pool with the highest rate of turnover (25 mg/24 h). Transferrin has a molecular weight of around 80 kDa and contains two specific high-affinity Fe(III) binding sites. The affinity of transferrin for Fe(III) is extremely high (association constant is 10 M at pH 7.4) but decreases progressively with decreasing pH below neutrality. Transferrins are not limited to only binding to iron but also to different metal ions. These glycoproteins are located in various bodily fluids of vertebrates. Some invertebrates have proteins that act like transferrin found in the hemolymph.
When not bound to iron, transferrin is known as "apotransferrin" (see also apoprotein).