Transferrin

Transferrins are iron-binding blood plasma glycoproteins that control the level of free iron (Fe) in biological fluids. Human transferrin is encoded by the TF gene. 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 1020 M−1 at pH 7.4) but decreases progressively with decreasing pH below neutrality. When not bound to iron, transferrin is known as "apotransferrin".