Figure 2 | The transferrin cycle.   HOLOTRANSFERRIN (HOLO-TF) binds to transferrin receptors (TFR) on the cell surface. The complexes localize to clathrin-coated pits, which invaginate to initiate endocytosis. Specialized endosomes form, and become acidified through the action of a proton pump. Acidification leads to protein conformational changes that release iron from transferrin. Acidification also enables proton-coupled iron transport out of the endosomes through the activity of the divalent metal transporter 1 protein (DMT1). Subsequently, APOTRANSFERRIN (APO-TF) and the transferrin receptor both return to the cell surface, where they dissociate at neutral pH. Both proteins participate in further rounds of iron delivery. In non-erythroid cells, iron is stored as ferritin and haemosiderin.

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