Transferrins are glycoproteins that are often found in biological fluids of vertebrates. When a transferrin protein loaded with iron encounters a transferrin receptor on the surface of a cell, e.g., erythroid precursors in the bone marrow, it binds to it and is transported into the cell in a vesicle by receptor-mediated endocytosis. The pH of the vesicle is reduced by hydrogen ion pumps ( ATPases) to about 5.5, causing transferrin to release its iron ions. Iron release rate is dependent on several factors including pH levels, interactions between lobes, temperature, salt, and chelator. The receptor with its ligand bound transferrin is then transported through the endocytic cycle back to the cell surface, ready for another round of iron uptake.
Each transferrin molecule has the ability to carry two iron ions in the ferric form ().