Opening protein pores with chaotropes enhances Fe reduction and chelation of Fe from the ferritin biomineral

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FONTE

The National Academy of Sciences

RESUMO

Iron is concentrated in ferritin, a spherical protein with a capacious cavity for ferric nanominerals of <4,500 Fe atoms. Global ferritin structure is very stable, resisting 6 M urea and heat (85°C) at neutral pH. Eight pores, each formed by six helices from 3 of the 24 polypeptide subunits, restrict mineral access to reductant, protons, or chelators. Protein-directed transport of Fe and aqueous Fe3+ chemistry (solubility ≈10−18 M) drive mineralization. Ferritin pores are “gated” based on protein crystals and Fe chelation rates of wild-type (WT) and engineered proteins. Pore structure and gate residues, which are highly conserved, thus should be sensitive to environmental changes such as low concentrations of chaotropes. We now demonstrate that urea or guanidine (1–10 mM), far below concentrations for global unfolding, induced multiphasic rate increases in Fe2+-bipyridyl formation similar to conservative substitutions of pore residues. Urea (1 M) or the nonconservative Leu/Pro substitution that fully unfolded pores without urea both induced monophasic rate increases in Fe2+ chelation rates, indicating unrestricted access between mineral and reductant/chelator. The observation of low-melting ferritin subdomains by CD spectroscopy (melting midpoint 53°C), accounting for 10% of ferritin α-helices, is unprecedented. The low-melting ferritin subdomains are pores, based on percentage helix and destabilization by either very dilute urea solutions (1 mM) or Leu/Pro substitution, which both increased Fe2+ chelation. Biological molecules may have evolved to control gating of ferritin pores in response to cell iron need and, if mimicked by designer drugs, could impact chelation therapies in iron-overload diseases.

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