Reaction of O ₂ with a diiron protein generates a mixed-valent Fe ²⁺ /Fe ³⁺ center and peroxide

Abstract

The gene encoding the cyanobacterial ferritinSynFtn is up-regulated in response to copper stress. Here, we show that, whileSynFtn does not interact directly with copper, it is highly unusual in several ways. First, its catalytic diiron ferroxidase center is unlike those of all other characterized prokaryotic ferritins and instead resembles an animal H-chain ferritin center. Second, as demonstrated by kinetic, spectroscopic, and high-resolution X-ray crystallographic data, reaction of O2with the di-Fe2+center results in a direct, one-electron oxidation to a mixed-valent Fe2+/Fe3+form. Iron–O2chemistry of this type is currently unknown among the growing family of proteins that bind a diiron site within a four α-helical bundle in general and ferritins in particular. The mixed-valent form, which slowly oxidized to the more usual di-Fe3+form, is an intermediate that is continually generated during mineralization. Peroxide, rather than superoxide, is shown to be the product of O2reduction, implying that ferroxidase centers function in pairs via long-range electron transfer through the protein resulting in reduction of O2bound at only one of the centers. We show that electron transfer is mediated by the transient formation of a radical on Tyr40, which lies ∼4 Å from the diiron center. As well as demonstrating an expansion of the iron–O2chemistry known to occur in nature, these data are also highly relevant to the question of whether all ferritins mineralize iron via a common mechanism, providing unequivocal proof that they do not.

Routes of iron entry into, and exit from, the catalytic ferroxidase sites of the prokaryotic ferritin SynFtn

“…A bacterial ferritin, called SynFtn, isolated from the coastal marine cyanobacterium Synechococcus strain CC9311 was recently shown to be unusual in that it more closely resembles eukaryotic H-chain ferritins than previously studied Ftn proteins. 18 This study demonstrated that SynFtn oxidises iron via a mechanism that is unique among diiron proteins characterised to date. The protein as isolated was free from metal ions.…”

Genomic mosaicism underlies the adaptation of marine Synechococcus ecotypes to distinct oceanic iron niches

“…We suggest that ferritin paralogs likewise have distinct functions such as managing intracellular, extracellular and/or diel fluctuations in Fe levels (Botebol et al ., ). For example, ferritin paralog GC_00028B in coastal strain CC9311 was recently suggested to play a novel role in metal detoxification based on structural and biochemical analysis (Bradley et al ., ). Consistent with this, the paralog was most abundant at sites with intermediate levels of Fe (0.3–1 nM) that also likely have higher concentrations of other toxic metals.…”

Facile synthesis of amino‐functionalized polyphosphazene microspheres and their application for highly sensitive fluorescence detection of Fe3+

“…Metal ions play a crucial role in environmental and biological systems. Fe 3+ , one of the most essential transition metal ions, is indispensable for biochemical processes at cellular level . Excess and deficiency of Fe 3+ would impair biological systems seriously .…”

“…Fe 3+ , one of the most essential transition metal ions, is indispensable for biochemical processes at cellular level . Excess and deficiency of Fe 3+ would impair biological systems seriously . Iron deficiency would cause anemia, decreased immunity, liver, heart and kidney damages, while Parkinson's and Alzheimer's diseases are related to iron surplus .…”