NUCLEATION OF THE IRON CORE OCCURS AT THE 3-FOLD CHANNELS OF HORSE SPLEEN APOFERRITIN - AN EXAFS STUDY ON THE NATIVE AND CHEMICALLY-MODIFIED PROTEIN

Extended X-ray absorbance fine structure measurements have been carrie d out on the initial Fe(III)-apoferritin complex at a Fe/subunit ratio of 2 in native and modified horse spleen apoferritin. Analysis of the data indicates that in the native protein the iron forms a protein-bo und polynuclear cluster (Fe-Fe distance 3.4 angstrom) with a first coo rdination sphere constituted by 5-6 low-Z atoms, e.g., nitrogen atoms, carboxylate-like ligands or oxo bridges between the iron atoms. Modif ication of Cys-126, a residue localized on the outer surface of the hy drophilic three-fold channels, with p-chloromercuribenzoate (PMB) or p henylmercuric acetate (PMA) brings about distinctive differences. In p articular, in the PMB-reacted protein the feature assigned to the iron -iron interaction disappears from the spectrum, whilst in the PMA-reac ted protein the main differences with respect to the native protein ar e observed at the level of the first coordination sphere. These result s confirm the formation of protein-Fe(III)-clusters and localize these sites at the hydrophilic three-fold channels of horse spleen apoferri tin.