Functional properties of threefold and fourfold channels in ferritin deduced from electrostatic calculations

The iron storage protein ferritin contains threefold and fourfold symmetric channels that are thought to provide pathways for the transfer of Fe²⁺ ions in and out of the protein. Using the known crystal structure of the ferritin protein, we perform electrostatic potential energy calculations to elucidate the functional properties of these channels. The threefold channel is shown to be responsible for the transit of Fe²⁺ ions. Monovalent ions can also diffuse through the threefold channel but presence of divalent ions in the pore retards this process leading to a selectivity mechanism similar to the one observed in calcium channels. The fourfold channel is found to be impermeant to all cations with the possible exception of protons. Because proton transfer is essential to maintain the electroneutrality of the protein during iron deposition, we suggest that the function of the fourfold channel is to form a "proton wire" that facilitates their transfer in and out of ferritin.