Iron Binding to Horse Spleen Apoferritin: a Vanadyl ENDOR Spin Probe Study

The role of the protein shell in the formation of the hydrous ferric oxide core of ferritin is poorly understood. A VO2+ spin probe study was undertaken to characterize the initial complex of Fe2+ with horse spleen apoferritin (96% L-subunits). A competitive binding study of VO2+ and Fe2+ showed that the two metals compete 1:1 for binding at the same site or region of the protein. Curve fitting of the binding data showed that the affinity of VO2+ for the protein was 15 times that of Fe2+. Electron nuclear double resonance (ENDOR) measurements on the VO(2+)-apoferritin complex showed couplings from two nitrogen nuclei, tentatively ascribed to the N1 and N3 nitrogens of the imidazole ligand of histidine. The possibility that the observed nitrogen couplings are from two different ligands is not precluded by the data, however. A pair of exchangeable proton lines with a coupling of approximately 1 MHz is tentatively assigned to the NH proton of the coordinated nitrogen. A 30-40% reduction in the intensity of the 1H matrix ENDOR line upon D2O-H2O exchange indicates that the metal-binding site is accessible to solvent and, therefore, to molecular oxygen as well. The ENDOR data provide the first evidence for a principle iron(II)-binding site with nitrogen coordination in an L-subunit ferritin. The site may be important in Fe2+ oxidation during the beginning stages of core formation.