Ferritin Biomineralization

Since ferritin subunits spontaneously self-assemble in vivo as well as in vitro to yield the complete apoferritin shell, it has been possible to study mineral deposition within the ferritin cavity under various experimental conditions.

Mineral formation within the ferritin cavity proceeds via two different pathways (Figure 2): At low iron content, hydrated Fe(II) ions are taken up from the external medium by the H-chain ferroxidase centre where rapid oxidation takes place. Hydrated Fe(III) ions are released from the ferroxidase centre to

Figure 2 Iron mineral formation in ferritin. A self-assembled heteropolymeric shell forms a spatially confined mineralization compartment composed of H- and L-chain subunits that possess different roles in mineralization. The H-chain contains a pre-organized arrangement of coordinating amino acid residues (ferroxidase centre) which is able to take up two Fe(II) ions and to catalyse their oxidation to Fe(III) ions. The L-chain possesses four highly conserved glutamic acid residues, that are assumed to play a decisive role in Fe(III) mineral nucleation.

Figure 2 Iron mineral formation in ferritin. A self-assembled heteropolymeric shell forms a spatially confined mineralization compartment composed of H- and L-chain subunits that possess different roles in mineralization. The H-chain contains a pre-organized arrangement of coordinating amino acid residues (ferroxidase centre) which is able to take up two Fe(II) ions and to catalyse their oxidation to Fe(III) ions. The L-chain possesses four highly conserved glutamic acid residues, that are assumed to play a decisive role in Fe(III) mineral nucleation.

enter the ferritin cavity (path A). The inner apofer-ritin surface bears a multitude of primarily hy-drophilic and anionic amino acid residues which point towards the central cavity. In particular, each L-chain subunit contains a distinct array of four potentially coordinating glutamate residues (Figure 1A) that could accumulate a small number of hydrated Fe(III) ions. The immobilized Fe(III) ions start to polymerize, leading to an OH"-bridged multinuclear Fe(III) oxyhydroxide cluster. Further cluster growth proceeds via addition of Fe(III) hexa-qua cations to the cluster surface with concomitant loss of an H2O ligand for each established coor-dinative bond.

On reaching a critical crystallite size, autocatalytic Fe(II) oxidation at the surface of the mineral nucleus outweighs ferroxidase-induced oxidation, and mineral growth continues until most of the cavity is filled by one (or several) Fe(III) oxyhydroxide nanocrys-tal(s) (path B). The pathways of iron transport into the cavity, as well as the primary product(s) of iron(II) oxidation, are not yet fully established.

Since hydrated Fe(III) ions form polycationic oligomers at the initial stages of polymerization, the general consensus is that L-chain glutamate residues contribute to a negative surface potential on the inside of the apoferritin shell in order to create a ther-modynamic sink for Fe(III) oxyhydroxide deposition. In addition to this general effect, a more specific template effect on iron mineral formation has been proposed which takes into account the special arrangement of glutamate residues and the fact that the sequence motif is highly conserved among ferritins from different classes of organisms. The poor crystallinity of the ferritin mineral on the other hand, as well as the fact that nanosized Fe(III) oxyhydroxide particles spontaneously grow from Fe(III)-containing aqueous solutions, challenges this interpretation.

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