Catalysis of iron core formation in Pyrococcus furiosus ferritin |
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Authors: | Kourosh Honarmand Ebrahimi Peter-Leon Hagedoorn Jaap A Jongejan Wilfred R Hagen |
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Institution: | (1) Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands; |
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Abstract: | The hollow sphere-shaped 24-meric ferritin can store large amounts of iron as a ferrihydrite-like mineral core. In all subunits
of homomeric ferritins and in catalytically active subunits of heteromeric ferritins a diiron binding site is found that is
commonly addressed as the ferroxidase center (FC). The FC is involved in the catalytic Fe(II) oxidation by the protein; however,
structural differences among different ferritins may be linked to different mechanisms of iron oxidation. Non-heme ferritins
are generally believed to operate by the so-called substrate FC model in which the FC cycles by filling with Fe(II), oxidizing
the iron, and donating labile Fe(III)–O–Fe(III) units to the cavity. In contrast, the heme-containing bacterial ferritin from
Escherichia coli has been proposed to carry a stable FC that indirectly catalyzes Fe(II) oxidation by electron transfer from a core that oxidizes
Fe(II). Here, we put forth yet another mechanism for the non-heme archaeal 24-meric ferritin from Pyrococcus furiosus in which a stable iron-containing FC acts as a catalytic center for the oxidation of Fe(II), which is subsequently transferred
to a core that is not involved in Fe(II)-oxidation catalysis. The proposal is based on optical spectroscopy and steady-state
kinetic measurements of iron oxidation and dioxygen consumption by apoferritin and by ferritin preloaded with different amounts
of iron. Oxidation of the first 48 Fe(II) added to apoferritin is spectrally and kinetically different from subsequent iron
oxidation and this is interpreted to reflect FC building followed by FC-catalyzed core formation. |
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