Chemistry at the protein-mineral interface

The nucleation site of iron mineral in human L ferritin revealed by anomalous -ray diffraction

Iron ions have crucial functions in every living organism being essential for cellular respiration, DNA synthesis, detoxification of exogenous compounds, just to provide a few examples. However, the redox properties of iron ions can also cause the occurrence of deleterious free-radicals. For these reasons, when unnecessary, iron must be kept in appropriate forms unable to cause damage. Nature evolved a special protein cage, called ferritin, consisting of 24 subunits arranged to form a hollow sphere with an internal diameter of about 80 Å where mineralized iron is stored, generally under the form of insoluble ferric oxides.

In mammals, two types of subunits build-up the 24-mer ferritins: the ‘heavy’ (H) and the ‘light’ (L). These subunits differ not only in molecular weight (21.2 kDa for H and 20.0 kDa for L) but, mainly, in function. The H subunit is able to catalyze the rapid oxidation of Fe2+ to Fe3+ followed by transfer in the storage cavity. On the contrary, the L-chain does not possess catalytic activity, but it is still able to mineralize ferric ions upon spontaneous oxidation by dioxygen of captured Fe2+. Despite the intensive research on ferritin chemistry, the mechanisms of iron oxidation and storage to form mineral nanoparticles inside the ferritin cavity are still to be fully established.

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