The universal mechanism for iron translocation to the ferroxidase site in ferritin, which is mediated by the well conserved transit site

Ferritins are ubiquitous iron storage proteins. Recently, we identified a novel metal-binding site, transit site, in the crystal structure of phytoferritin. To elucidate the function of the transit site in ferritin from other species, we prepared transit-site-deficient mutants of human H ferritin, E140A and E140Q, and their iron oxidation kinetics was analyzed. The initial velocities of iron oxidization were reduced in the variants, especially in E140Q. The crystal structure of E140Q showed that the side chain of the mutated Gln140 was fixed by a hydrogen bond, whereas that of native Glu140 was flexible. These results suggest that the conserved transit site also has a function to assist with the metal ion sequestration to the ferroxidase site in ferritins from vertebrates.

Research highlights
► Ferritin is a ubiquitous iron storage protein.
► Ferrous iron are incorporated from the entry channel, and then sequestered to the ferroxidase site of ferritin.
► A conserved glutamate residue, Glu140 in human H chain ferritin, functioned as transit site from the entry channel to the ferroxidase site.
► This metal translocation pathway might be the universal among ferritin form various species.