Biomineralization

Table 1 Characteristics of ferritins

Iron storage protein Source Composition Physiological functions

Ferritins

Vertebrates Invertebrates

24-mer, predominantly heteropolymeric composed of H-, L- and M-chains Core of crystalline ferrihydrite (polydisperse) Fe/P ratio > 10:1, 1000-3000 Fe(III)/core

Mobile iron storage Iron detoxification Prevention from hydroxyl radical formation

Bacterioferritins

Eubacteria Fungi

24-mer, predominantly homopolymeric Up to 12 haem (cytochome b557) groups Core of amorphous hydrous ferric phosphate Fe/P ratio 1.1 : 1-1.9: 1, 600-2300 Fe(III)/core

Precursor to magnetite in magnetotactic bacteria

Adapted with permission from Le Brun N, Thomson AJ and Moore GR (1997) Metal centres of bacterioferritins or non haem-iron-containing cytochromes b557. Structure andBonding 88: 103-138

Adapted with permission from Le Brun N, Thomson AJ and Moore GR (1997) Metal centres of bacterioferritins or non haem-iron-containing cytochromes b557. Structure andBonding 88: 103-138

prokaryots (Table 1). Sequence similarities of haem-free ferritins and haem-containing bacterioferritins may fall below 20%. However, their quaternary protein structures are almost identical, suggesting that a convergent molecular evolution within different groups of organisms has independently led to an optimal solution for the availability of a mobile temporary iron storage.

While the general physiological effect of ferritin originating from different organisms may differ, it clearly functions as an iron deposit on the molecular scale, owing to several remarkable features:

• the apoferritin creates a confined space which ultimately restricts the maximum size of the inwardly growing mineral phase;

• several anionic residues induce a net negative charge on the inner protein surface which compensates for the positive surface charges of initially formed polycationic Fe(III) oxyhydroxy species;

• the H-chain ferritin subunit contains a ferroxidase centre that catalyses the oxidation of Fe(II) by molecular oxygen to yield Fe(III);

• the L-chain ferritin subunit bears glutamic acid residues in close proximity which point towards the central cavity, thus possibly acting as an active site for crystal nucleation;

• the apoferritin supports long range electron transfer across the protein coat, enabling fast reductive release of Fe(II) ions from the highly insoluble Fe(III) oxyhydroxide mineral.

Apoferritin is built up by 24 structurally complementary subunits that self-assemble to form a hollow shell of an approximate outer diameter of 11 nm. An individual subunit consists of a long 4-a-helix bundle with an additional short a-helix lying at an angle of about 60° to the bundle axis at the C-terminal side of the amino acid chain (Figure 1A). At the beginning of apoferritin self-assembly dimers form mainly through a multitude of hydrophobic contacts along the juxtaposed 4-a-helix bundles. The complete apoferritin shell is composed of 12 dimers that form the faces of an imaginary rhombic dodecahedron (Figure 1B). The protein shell encloses

Figure 1 (A) Single apoferritin subunit; a-helix regions of the secondary protein structure are represented as cylinders. The arrow marks a putative mineral nucleation site (here: Glu 57, 60, 61 and 64 of L-chain horse apoferritin, PDB code: 1AEW). (B) Supramolecular architecture of the apoferritin protein shell: 12 subunit dimers form the faces of an imaginary rhombic dodecahedron. (C, D) View along the three-fold channels (the four-fold channels, respectively) of the apoferritin structure. Adapted from Harrison and Arosio (1996).

Figure 1 (A) Single apoferritin subunit; a-helix regions of the secondary protein structure are represented as cylinders. The arrow marks a putative mineral nucleation site (here: Glu 57, 60, 61 and 64 of L-chain horse apoferritin, PDB code: 1AEW). (B) Supramolecular architecture of the apoferritin protein shell: 12 subunit dimers form the faces of an imaginary rhombic dodecahedron. (C, D) View along the three-fold channels (the four-fold channels, respectively) of the apoferritin structure. Adapted from Harrison and Arosio (1996).

a nearly spherical cavity of an approximate diameter of 8 nm. The central cavity is accessible through channels of three-fold and four-fold symmetry which are situated at the vertices of the rhombic dodecahedron (Figure 1C, D). While the three-fold channels possess a hydrophilic surface, the four-fold channels are more hydrophobic in nature. The transport characteristics of the ferritin channels are still a matter of controversy. The three-fold channels are likely to be involved in the uptake and release of iron ions as well as in the regulation of the ferritin water content. For the four-fold channels an active role in the uptake of dioxygen has been proposed.

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