Release of Iron from Ferritin
The iron in the ferritin core is stored as Fe(III) in a crystalline solid that has the chemical formula [FeO(OH)]8[FeO(H2PO4)]. The best model for ferritin's core is the mineral ferrihydrite (Figure 10). A crystalline solid is a three-dimensional structure in which the constituents (i.e., atoms, ions, or molecules) are arranged in a definite repeating pattern. The positions of the constituents' centers are represented by points on a three-dimensional lattice. The smallest repeating unit of the lattice is called the unit cell. The unit cell for ferrihydrite is shown in Figure 10a. This unit cell is repeated in a specific pattern to form an extended nonmolecular structure (see Figure 10b).
In the mineral ferrihydrite (Figure 10b), every Fe(III) ion is coordinated to six O(II) ions. However, in ferritin, the mineral core has approximately 10% of the Fe(III) ions coordinated to five O(II) ions and 1 phosphate group. Most of the phosphate groups that are coordinated to the iron ions lie on the outside of the crystalline structure, and are used to bind the mineral to the residues on the inside of the ferritin shell (the protein).
As long as a lattice remains intact, the atoms in the lattice are not soluble because they form part of the lattice's continuous structure. (Recall the solvation process for a crystalline solid as described in the "Treating the Public Water Supply: What Is In Your Water, and How Is It Made Safe to Drink?" tutorial.) Thus, in order for iron to be released from ferritin, the mineral lattice must be dissolved (i.e., to allow the iron atoms to break away from the lattice structure). This removal is accomplished by reducing iron from the Fe(III) (ferric) oxidation state to the Fe(II) (ferrous) oxidation state. In the Fe(II) state, iron breaks away from the lattice as the Fe2+ ion. The positive charge of the Fe2+ ion attracts the electronegative oxygen atoms of water, and so a water "cage" forms around the ion. (In the water cage, six water molecules surround the ion at close range.) Thus, iron becomes soluble as a hydrated Fe2+ ion, Fe(H2O)62+, and can be released from the ferritin protein via the channels in the spherical shell.
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This page created by Matt Traverso, Washington University in St Louis.
© 2004, Washington University.
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