Zn‐exchange and Mössbauer studies on the [Fe‐Fe] derivatives of the purple acid Fe(III)‐Zn(II)‐phosphatase from kidney beans

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Abstract
In order to perform Mössbauer studies, Zn(II) in the Fe(III)‐Zn(II) purple acid phosphatase of the red kidney bean has been exchanged by incubating the semiapoenzyme with 57Fe(II). The resulting Fe(III)‐57Fe(II) enzyme has 125% activity, compared with that of the Zn(II) enzyme. It can be oxidized by H2O2 or peroxydisulfate to the Fe(III)‐57Fe(III) species with a 30‐times lower activity. Incubation of the metal‐free apoenzyme with 57Fe(II) in the presence of O2 leads to the 57Fe(III)‐57Fe(II) species which is stable in dilute solutions, but partially oxidized during the concentration procedure to the 57Fe(III)‐57Fe(III) enzyme. Limited reduction of the oxidized enzyme with ascorbate delivers a mixture of the Fe(II)‐Fe(II)/Fe(III)‐Fe(III) species, but not the mixed valent Fe(III)‐Fe(II) species, indicating that after the transfer of the first electron the second electron of the ascorbate radical is immediately transferred to the second Fe(III).The Mössbauer spectra of the oxidized species show at 4.2 K two quadrupole doublets with δ of 0.51 mm/s and 0.53 mm/s and E of 1.46 and 0.96 mm/s indicating high spin Fe(III) in two different binding sites, obviously with a higher asymmetry in the chromophoric Fe(III) site. The values are too low for a μ‐oxo bridge. The mixed‐valent Fe(III)‐Fe(II) species shows two quadrupole doublet with δ values of 0.55 mm/s and 1.14 mm/s and E values of 1.43 mm/s and 3.01 mm/s at 70 K for high spin Fe(II) and Fe(III), but the signal of the Fe(II) component shows magnetic patterns at 4.2 K indicating a half‐integer spin system with antiferromagnetic coupling. The Fe(II)‐Fe(II) system exhibits two quadrupole doublets with δ values of 1.18 mm/s and 1.22 mm/s and with E values of 3.69 mm/s and 2.68 mm/s again indicating a higher asymmetry in the originally chromophoric Fe(III)‐binding site. Addition of phosphate shows only minor differences in the oxidized enzyme and in the mixed valent Fe(III)‐Fe(II) system. Interaction with O2 is discussed.

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