Practice question

Vertebrates transport ferric iron safely via transferrin family to avoid oxidative Fenton radical damage and precipitation. Hepatocytes secrete apo-transferrin 80 kDa bilobed glycoprotein serum loads two Fe3+ each coordinated synergistically bidentate carbonate anion plus four protein ligands two tyrosines histidine aspartate each lobe producing salmon pink complex termed ferrotransferrin diferric transferrin. Doubly loaded species binds transferrin receptor TfR1 homodimer dissociation constant 1-10 nM neutral pH versus 50-fold lower affinity monoferric negligible apo providing selection iron loaded species for uptake efficiency. After clathrin-mediated endocytosis acidification pH 5.5 protonates carbonate triggering iron release facilitated ferrireductase STEAP3 converting Fe3+ to Fe2+ transported via DMT1 SLC11A2 divalent transporter. Ferritin 24-subunit cytosolic cage storing 4500 atoms ferrihydrite mineral core storage not serum transport. Heme carries Fe2+ within protoporphyrin oxygen transport hemoglobin myoglobin. Lysosome degrades iron proteins not physiological carrier. Ferrotransferrin therefore represents circulating iron import form enabling receptor specificity regulated high-affinity uptake supporting erythropoiesis respiration enzyme activities and DNA synthesis.