Abstract
Bacterioferritin (BFR) from Escherichia coli is a member of the ferritin family of iron storage proteins and has the capacity to store very large amounts of iron as an Fe(3+) mineral inside its central cavity. The ability of organisms to tap into their cellular stores in times of iron deprivation requires that iron must be released from ferritin mineral stores. Currently, relatively little is known about the mechanisms by which this occurs, particularly in prokaryotic ferritins. Here we show that the bis-Met-coordinated heme groups of E. coli BFR, which are not found in other members of the ferritin family, play an important role in iron release from the BFR iron biomineral: kinetic iron release experiments revealed that the transfer of electrons into the internal cavity is the rate-limiting step of the release reaction and that the rate and extent of iron release were significantly increased in the presence of heme. Despite previous reports that a high affinity Fe(2+) chelator is required for iron release, we show that a large proportion of BFR core iron is released in the absence of such a chelator and further that chelators are not passive participants in iron release reactions. Finally, we show that the catalytic ferroxidase center, which is central to the mechanism of mineralization, is not involved in iron release; thus, core mineralization and release processes utilize distinct pathways.
Highlights
The insolubility and potential toxicity of the essential metal iron led to the evolution of the ferritin family of proteins, which function to maintain cellular stores of iron in a nontoxic form [1, 2]
We show that the bis-Met-coordinated heme groups of E. coli BFR, which are not found in other members of the ferritin family, play an important role in iron release from the BFR iron biomineral: kinetic iron release experiments revealed that the transfer of electrons into the internal cavity is the rate-limiting step of the release reaction and that the rate and extent of iron release were significantly increased in the presence of heme
This behavior is similar to that previously reported for Azotobacter vinelandii BFR, for which reductant saturation kinetics were observed, and under excess dithionite, the majority of iron release was associated with the first exponential phase [11]
Summary
The insolubility and potential toxicity of the essential metal iron led to the evolution of the ferritin family of proteins, which function to maintain cellular stores of iron in a nontoxic form [1, 2]. In which the high affinity Fe2ϩ chelator ferrozine (see supplemental Fig. S1A) was present (at 1 mM), demonstrated that Fe2ϩ was not available for complexation in iron-loaded BFR samples in the absence of a reductant, as evidenced by the complete lack of absorbance increase at 562 nm, which characterizes the [Fe(ferrozine)3]4Ϫ complex.
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