Exchange model of the {[Fe4S4]Fe} active site of sulfite reductase Original Research Article

The exchange model of the {[4Fe4S]2+Fe} iron center of the active site of sulfite reductase is developed. Analytical solutions of the exchange and hyperfine problems were obtained. The ground state of the {[4Fe4S]2+Fe} center with total spin Sc is characterized by the mixture of intermediate spins St = 0 and St = 1 of the tetramer [4Fe4S]2+ moiety. The St mixing depends on the tetramer-siroheme coupling Jth and on the inter-pair interaction J in the Fe/S tetramer. The expectation values 〈siz〉 for individual ions of the [4Fe−4S]2+ fragment are non-zero: 〈s1z(3z)≈ + 1.63(Jth/J), 〈s2z(4z)〉 ≈ (−)+ 2.04(Jth/J). At the same time, the tetramer ground state spin is equal to zero: 〈Stz〉 = Σi = 14〈siz〉 ≈ 0. The tetramer-siroheme coupling induces individual magnetic moments μi(= −gi〈siz〉μB) for the iron ions of the [4Fe4S]2+ moiety of the active site. In the ground state of the system, the exchange-induced moments μ1(2) and μ3(4) have the same magnitudes in pairs and opposite directions, as a result, the tetramer fragment of the active site is diamagnetic as a whole. The exchange-induced paramagnetism of individual ions results in the internal hyperfine fields Hi(∼ 〈siz〉) on iron nuclei of the diamagnetic [4Fe−4S] tetramer. The effective hyperfine constants Ai of the iron ions of the Fe/S tetramer are proportional to Jth/J. Antiferromagnetic Heisenberg exchange results in the same magnitudes and the sign alteration for Ai constants in pairs. The contribution of the tetramer moiety g factor gt to the system molecular g factor gef is proportional to (Jth/J)2 and is very small since Jth⪡J. The proposed model of the {[4Fe4S]Fe} center explains t following experimentally observed effects: (i) paramagnetism of the [4Fe4S]2+ moiety of the active site, which was detected in Mössbauer, ENDOR and 1H NMR spectroscopy, (ii) at the same time, the EPR silence of this tetramer component and the absence of the tetramer μt contribution to total magnetic moment μc of the active center, (iii) the opposite signs of hyperfine Ai constants of Fe/S tetramer observed in Mössbauer spectra. The exchange parameters Jth/J of the active sites of SiR0 · CN and SiR1− · NO complexes were found.