Biomineralisation - Biological Inorganic Chemistry - page 366

FIGURE 19.8 Ferroxidase centre reaction intermediates inH-chain ferritin. (a) apoprotein, (b) m -peroxo di-iron(III) complex, (b 0 ) m -hydroperoxo
di-iron(III) complex, and (c) m -oxo di-iron(III) complex. (From Bou Abdallah, 2010 . Copyright 2010 with permission from Elsevier.)
While core formation during hydrolysis of Fe(III) produces electrically neutral ferrihydrite, it also produces
protons: two per Fe(II) oxidised and hydrolysed, whether due to iron oxidation and hydrolysis at the ferroxidase
centre, followed by further hydrolysis and migration to the core nucleation sites or by direct Fe(II) oxidation and
hydrolysis on the mineral surface of the growing core. These protons must either be evacuated from the cavity or
else their charges must be neutralised by incoming anions, and it seems likely that both mechanisms are employed.
In most ferritin molecules, some hydroxyl ions of the core (mostly on the core surface) are replaced by phosphate
ions, while electrostatic calculations suggest that expulsion of protons (as well as Fe 3 þ or Fe 2 þ ions) or uptake of
orthophosphate (or other anions such as chloride) would be facilitated by the electrostatic field gradient through
the four-fold channels in human H-chain ferritin.
Why mammalian ferritin cores contain ferrihydrite-like structures rather than some other mineral phase is
less easy to understand, and presumably reflects the way in which the biomineral is built up within the interior
of the protein shell together with the geometry of the presumed nucleation sites. The phosphate content in the
intracellular milieu can readily be invoked to explain the amorphous nature of the iron core of bacter-
ioferritins and plants. Indeed, when the iron cores of bacterioferritins are reconstituted in the absence of
phosphate, they are found to be more highly ordered than their native counterparts, and give electron
diffraction lines typical of the ferrihydrite structure. The 12 subunit ferritin-like Dps protein, discussed in
Chapter 8, forms a ferrihydrite-like mineral core, which would seem to imply that deposition of ferric
oxyhydroxides within a hollow protein cavity (albeit smaller) leads to the production of this particular mineral
form.