Peptide-based FeS4 complexes: the zinc ribbon fold is unsurpassed to stabilize both the FeII and FeIII states

Abstract

Whereas Zn(Cys)₄ zinc fingers exist with different protein folds, only the zinc ribbon fold is found in rubredoxin Fe(Cys)₄ sites. To assess the significance of this observation, we have investigated the binding and stability of Fe²⁺ and Fe³⁺ ions by a set of four peptides designed to model Zn(Cys)₄ zinc fingers with various folds, i.e. zinc ribbon, treble clef and a loosened zinc ribbon fold. All peptides were shown by means of UV-Vis and CD spectroscopies to form stable 1 : 1 Fe^II/peptide complexes with binding constants higher than 10⁷ M⁻¹ at pH 7. Their oxidation into Fe^III complexes and the stability of the latter were compared. The UV-Vis absorption and CD spectroscopic properties of the Fe^II and Fe^III complexes were analysed with respect to the structures of the zinc analogues in order to get insight into the local arrangement of the Fe(Cys)₄ core around the metal ion. The chemical stability of these complexes was rationalized according to the shielding from the solvent provided by the various peptide folds to the FeS₄ core. In addition, we showed that whereas UV-visible spectra inform only on the FeS₄, the information derived from the corresponding CD spectra extend to the C_β orientation and the peptide fold. The results presented here demonstrate that while the zinc ribbon fold is not strictly required to obtain a Fe(Cys)₄ site, it affords a drastically superior protection of the site toward external redox agents. This finding brings new clues to engineer stable and redox-active Fe(Cys)₄ sites in de novo proteins.