Anion deposition into ferritin

When the iron core of equine spleen ferritin is reduced, anions in solution cross the protein shell and enter the ferritin interior as part of a charge balancing reaction. Anion sequestration inside ferritin during iron core reduction was monitored using ion selective electrodes, inductively coupled plasma emission, and energy-dispersive X-ray spectroscopy. The requirement for anion translocation to the ferritin interior occurs because upon iron core reduction, two OH− ions per iron are released or neutralized inside ferritin leaving a net positive charge. Halides and oxoanions were tested as anionic substrates for this reaction. A general trend for the halides showed that the smaller halides accumulated inside ferritin in greater abundance than larger halides, presumably because the protein channels restrict the transfer of the larger anionic species. In contrast, oxoanion accumulation inside ferritin did not show selectivity based on size or charge. Vanadate and molybdate accumulated to the highest concentrations and nitrate, phosphate and tungstate showed poor accumulation inside ferritin. Fe(II) remains stably sequestered inside ferritin, as shown by electron microscopy and by column chromatography. Upon oxidation of the iron core, the anions are expelled from ferritin, and OH− ions coordinate to the Fe(III) to form the original Fe(O)OH mineral. Anion transport across the ferritin protein shell represents an important mechanism by which ferritin maintains proper charge balance inside the protein cavity.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Reduction of the ferritin iron core causes hydroxide ions to exit the ferritin interior in a charge balance reaction. ► In the presence of external anions, two hydroxide ions exit ferritin and the external anions enter ferritin to balance charge. ► Halides and oxo-anions can be sequestered inside ferritin using this reaction.