Abstract
The formation of Mn(III) oxyhydroxide (MnOOH) cores within the nanoscale
cavity of the iron storage protein ferritin has been investigated
by electron microscopy and visible absorption spectroscopy. At pH
8.9, discrete amorphous MnOOH cores were formed within horse spleen
apoferritin at a range of metal:protein ratios, as well as in ferritin
molecules seeded with a small ferrihydrite nucleus. Analysis of the
resultant core size distributions showed that the reconstitution
of horse spleen apoferritin with Mn(II) was similar to that observed
previously for Fe(II) reconstitution in recombinant human L-chain
ferritin, suggesting that horse spleen apoferritin does not exhibit
Mn(II) oxidase activity at pH 8.9. Reconstitution with MnOOH shows
essentially ''all-or-nothing'' behavior in which many protein molecules
remain unmineralized whilst others are loaded to maximum capacity.
Kinetic studies showed no significant differences between horse spleen
ferritin, recombinant H- and L-chain homopolymers, and H-chain variants
containing site-directed modifications at the ferroxidase and putative
Fe nucleation centers. Our results indicate that the reconstitution
of ferritin with MnOOH cores proceeds by a nonspecific pathway. We
propose that the outer surface of the protein inhibits the development
of MnOOH nuclei in bulk solution whereas the inner surface is inactive,
enabling nucleation and growth to proceed unperturbed within the
cavity. One possibility is that differences in the general polyelectrolyte
properties of these two surfaces, rather than site-specific charges,
account for the ''Janus'' behavior of the molecule. A similar mechanism
might also increase the specificity of iron oxide mineralization
in ferritins that lack ferroxidase centers.
Users
Please
log in to take part in the discussion (add own reviews or comments).