Moessbauer analysis of the binuclear iron site in purple acid phosphatase from pig allantoic fluid

Abstract

We present a quantitative analysis of the Mossbauer spectra of the spin-coupled two-iron center in porcine purple acid phosphatase. The active enzyme contains a high-spin Fe(III)-Fe(II) pair with a ground state of effective spin S = /and g tensor g = (1.56, 1.72, 1.93), while the oxidized, inactive enzyme contains a high-spin Fe(III)-Fe(III) pair with a diamagnetic ground state. The Mossbauer spectra of reduced enzyme recorded at 4.2 K show complex magnetic hyperfine splittings, which have been parametrized in terms of the spin Hamiltonian H =βsgb + ∑SAI + ∑IPI-∑iiβNgniibis = /, where i = A and B labels the ferric and ferrous site, respectively. The fitting parameters g and Acan, in turn, be related to the intrinsic tensors gand aof the ferric and ferrous ions, respectively, assuming the coupling H =-2JSS + ∑SDS between the intrinsic spins S = /and S = 2. With the estimate-2J = 20 cm (Lauffer et al. J. Biol. Chem. 1983, 258, 14212–14218) and reasonable zero-field splittings Dthe model explains the low g values and the unusual anisotropy of the ferric hyperfine tensor aain the coupled representation. Explicit expressions for the tensors g and A are given to first order in D/J. this intermediate coupling model, |D/ J ≲ 1, is likely to apply to semimethemerythrins, methane monooxygenase, and other high-spin Fe(III)-Fe(II) couples with g values differing widely from g = 2 and to hold the key to a quantitative interpretation of susceptibility, ENDOR, and other magnetic properties. Preliminary data for metal-substituted and differentially enriched phosphatase are also presented. The parameters of the iron in the Fe-Zn enzyme are found to be close to the intrinsic parameters deduced from the analysis of the native Fe(III)-Fe(II) enzyme, a result that not only corroborates our model but also indicates little change in the iron environment on substitution of zinc. This result further suggests that in mixed-metal clusters the intrinsic properties of each iron site can be studied without the complexities arising from the exchange coupling.