Iron uptake in ferritin is blocked by binding ofCr(TREN)(H2O)(OH)2+ a slowdissociating model forFe(H2O)62+

摘要

Ferritin concentrates iron as a hydrous ferric oxide in a protein cavity (8 nm in diameter) by using eight pores along the threefold symmetry axes of the octahedral supramolecular structure. The role of ligand exchange in the entry of Fe(II) hexahydrate into ferritin protein has been studied with [Cr(TREN)(H2O)(OH)]²⁺ [TREN = N(CH2CH2NH2)3], a model for Fe(H2O)6²⁺ with only two exchangeable ligands. The results show that five different ferritin proteins, varying in pore structure, oxidation sites, and nucleation sites, bind Cr(TREN) at functional protein sites, based on inhibition of iron mineralization and oxidation. Properties of Cr(TREN)–ferritin adducts include an increased isoelectric point, a shift in the Cr(TREN) UV/vis spectrum consistent with exchange of water for protein carboxylate or thiolate ligands, binding affinities of 50–250 μM, and a slow rate of dissociation (k = 4 × 10⁻⁶ sec⁻¹). The relationship of Cr(TREN)inhibition of iron oxidation and mineralization by Cr(TREN) to theknown structures of the various ferritins tested suggests that Cr(TREN)plugs the ferritin pores, obstructing Fe(II) entry in folded andunfolded pores. Because only two exchangeable waters are sufficient forpore binding of Cr(TREN), the physiological Fe(II) donor must bind tothe pore with few exchangeable ligands. These results show theadvantage of using stable model complexes to explore properties oftransient Fe–protein complexes during Fe mineralization in ferritin.