# The Mechanism of Mineral Nucleation and Growth in a Mini-Ferritin

## TL;DR

Scientists used cryo-EM to observe how iron minerals form inside a mini-ferritin protein, revealing detailed steps of nucleation and growth.

## Contribution

The first pseudoatomic-level observation of mineral nucleation and growth in a ferritin protein.

## Key findings

- A conserved glutamate crucible catalyzes iron nucleation at acidic pores in mini-ferritin.
- Main-chain carbonyls later support crystalline mineral growth toward the particle center.
- The observed mineral structure aligns with one model of ferrihydrite.

## Abstract

Iron is an enigmatic element. While necessary for life,
as Fe­(II)
it also catalyzes formation of reactive oxygen species. To mitigate
this, cellular life has evolved the ferritin protein superfamily,
which includes the 24 subunit ferritins and bacterioferritins, and
12 subunit mini-ferritins (DPS). Each catalyze the oxidation of Fe­(II)
to ferric oxyhydroxide, which is then sequestered within the hollow
protein shell. While there is a wealth of structural information on
unmineralized ferritins, high resolution information on iron loaded
ferritins is lacking, and the mechanism of iron mineralization is
poorly understood. To address this, we followed iron loading in a
mini-ferritin by cryo-EM. We determined a 1.86 Å structure in
the unmineralized state, as well as a 1.91 Å structure of an
early, iron loading state in which the mini-ferritin catalyzes nucleation
of ferric oxyhydroxide at the acidic 3-fold pores. Mechanistically,
a conserved crucible of precisely positioned glutamates and unsaturated
main chain carbonyls are employed as a template to catalyze nucleation.
A 2.4 Å structure at a later time point was also determined,
revealing the role of a second constellation of main-chain carbonyls
on the interior surface that subsequently supports crystalline mineral
growth, that then proceeds into the center of the particle. Notably,
the visualized mineral is consistent with one of two competing structural
descriptions for ferrihydrite. This study provides the first pseudoatomic
level observation of controlled mineral nucleation and growth in any
member of the ferritin superfamily, and informs general mechanisms
of nucleation and biomineralization.