THE FUNCTION OF H AND L CHAINS IN IRON SEQUESTRATION IN FERRITIN

In order to understand the iron sequestering mechanism within the protein shell in ferritins, Mossbauer studies were performed on horse spleen ferritin (about 15% H and 85% L chains) and variants of human H chain ferritins in which putative oxidation and nucleation site ligands have been changed by site directed mutagenesis and which were loaded with small amounts of iron and frozen at short times after iron loading. It was found that the catalysis by apoferritin of Fe(II) oxidation is associated with residues within the H chains, that both Fe(III) monomers and dimers are located on H chains and that these dimers form at ferroxidase centers. Nevertheless iron core formation does occur also in ferritins that lack the ferroxidase center of the H chains, though initial Fe(II) oxidation is slower.