Magnetic susceptibility studies on the diiron forms of the metalloprotein purple acid phosphatase from bovine spleen and kidney bean

Temperature dependent magnetic susceptibility measurements of the reduced and of the oxidized diiron forms of purple acid phosphatase from kidney bean (KBP) and from bovine spleen (BSP) have been carried out in the temperature range 4.2-220 K in order to detect the strength of the exchange coupling between the two metal centers in the active site. Investigations of BSP were additionally carried out at different pH values. For the reduced forms with the Fe(III)-Fe(II) center, values for the exchange coupling constant in the range of J = -5 to -15 cm(-1) were obtained for KBP (pH = 6) and BSP (pH = 3.9, 4.9 and 5.6), based on the Hamiltonian (H) over cap = -2JS(1)S(2). In the presence of phosphate at pH = 3.6 and 5.6 a significant reduction of the strength of the exchange coupling was observed. The analysis of the magnetic data on the oxidized, diferric metalloproteins yielded coupling constants with similar values as found for the reduced forms. After addition of phosphate to the oxidized BSP. a reduction of the exchange coupling was also observed, indicating a similar change in the magnetic properties as found for the reduced form.