Comparison and characterization of the [Fe₄S₄]^2+/3+ centre in the wild-type and C77S mutated HiPIPs from Chromatium vinosum monitored by Mössbauer, ⁵⁷Fe ENDOR and EPR spectroscopies

Abstract.

Mössbauer, ⁵⁷Fe ENDOR, CW and pulsed EPR experiments were performed on the reduced and the oxidized high-potential iron proteins (HiPIPs) of the wild type (WT) and the C77S mutant from Chromatium vinosum. The EPR spectra of the oxidized WT and mutant show three species respectively having nearly the same g-values but strongly changed spectral contributions. Relaxation times were estimated for oxidized WT and mutant at T=5 K with pulsed EPR. A-tensor components of both iron pairs were obtained by ⁵⁷Fe ENDOR, proving a similar magnetic structure for the WT and the mutant. Electronic relaxation has to be taken into account at T=5 K in native and mutated oxidized HiPIPs to achieve agreement between Mössbauer and ⁵⁷Fe ENDOR spectroscopies. The Mössbauer spectroscopy shows that the oxidized cluster contains a pure ferric and a mixed-valence iron pair coupled antiparallel. While all cluster irons from reduced C. vinosum WT are indistinguishable in the Mössbauer spectrum, the reduced C77S mutant shows a non-equivalence between the serine-bound and the three cysteine-ligated iron ions. The Mössbauer parameters confirm a loss of the covalent character of the iron bond when S is replaced by O and indicate a shift of the cluster's electron cloud towards the serine. Mössbauer spectra of the oxidized mutant can be simulated with two models: model I introduces a single electronic isomer with the serine always ligated to a ferric iron. Model II assumes two equally populated electronic isomers with the serine ligated to a ferric iron and a mixed-valence iron, respectively. The latter model is in better agreement with EPR and NMR.