Effect of the point mutation H54N on the ferroxidase process of Rana catesbeiana H' ferritin.

Human H and Rana catesbeiana H' subunits in vertebrate ferritin protein cages catalyze the Fe(II) oxidation by molecular oxygen and promote the ferric oxide biomineral synthesis. By depositing iron biomineral, ferritins also prevent potentially toxic reactions products from Fe(II)-based Fenton chemistry. Recent work from our laboratory was aimed to describe the iron pathways within ferritin, from entrance into the cage to the ferroxidase site, and to understand the role played by amino-acid residues in iron trafficking and catalysis. Our approach exploits anomalous X-ray diffraction from ferritin crystals, exposed to a ferrous salt, to track transient iron binding sites along the path towards a well-defined di-iron site where they get oxidized by oxygen. Coupling structure determination with solution kinetic measurements on selected variants, allows validating the role played by key residues on the catalytic iron oxidation. Our previous studies on H' ferritin indicated the regulatory role played by His54, and by its human counterpart Gln58, on guiding Fe(II) ions to the catalytic site. Here, we have investigated the effects induced by substituting the wild type His54 with Asn54, having different iron coordination properties. We have obtained a series of atomic-resolution crystal structures that provide time-dependent snapshots of iron bound at different locations in the H' ferritin H54N variant. The comparison with H' ferritin and H' ferritin H54Q variant leads to identify a new iron binding site. Our kinetic and structural data support the role of H' ferritin residue 54 in regulating the access of Fe(II) ions to the catalytic site.