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Mechanistic studies of iron uptake into, and release from, Ferritins

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  • Biology


Abstract Iron is the most common and perhaps the most crucial transition metal. It is essential for virtually all of life yet it is also potentially extremely toxic; therefore, how organisms handle iron (iron metabolism) is of critical importance. Ferritins constitute a large family of iron storage proteins that are found throughout life, in bacteria, archaea, plants and animals. The newly discovered ferritin from the bloom-forming pennate diatom Pseudo-nitzschia multiseries is similar to other eukaryotic ferritins that contains a ferroxidase centre but also contains a third site (site C) only previously found in non-heme bacterial ferritins. The mechanism of iron uptake in P. multiseries ferritin and the effect of substitution of residues at site C has been studied using rapid reaction kinetic methods. These revealed that oxidation in PmFTN is very rapid suggesting a distinct mechanism of iron uptake. Bacterioferritin (BFR) is a unique heme-containing bacterial member of the ferritin family that stores up to 2700 iron ions as a ferric oxyhydroxide phosphate mineral within its central cavity. This core is surrounded by 24 identical protein subunits, each of which possesses a dinuclear iron centre that catalyses the oxidation of Fe(II) to Fe(III). The heme-binding sites are sandwiched between pairs of subunits and coordinated by two methionine residues (one from each subunit). The heme groups play an important role in iron release, though understanding of this has been hampered by the difficulty in obtaining fully heme-loaded protein that is isolated from overexpressing bacterial cultures. Here, an in vitro heme-loading method is described and used to generate BFR containing variable amounts of heme. Studies of iron release indicate increased heme levels only marginally increases the rate of iron release and actually lowers the extent of iron release. These results suggest that the reconstituted heme-loaded protein behaves differently from naturally heme-loaded protein.

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