Iron—sulphur clusters with labile metal ions

Abstract

A study has been carried out of the redox-linked metal ion uptake processes of the iron—sulphur cluster [3Fe-4S] in the bacterial ferredoxin, Fd III from Desulphovibrio africanus using a combination of electron paramagnetic resonance (EPR) and low-temperature magnetic circular dichroism (MCD) spectroscopy and direct, unmediated electrochemistry of the Fd in a film deposited at a pyrolytic graphite electrode. Reduction of the three-iron cluster is required before a divalent metal ion becomes bound as in the reaction sequence [3Fe-4S 1+ ⇌ [3Fe-4S] 0 ⥯M(II) [M3Fe-4S] 2+ ⇌ [M3Fe-4S] 1+ The redox potentials of these processes and the metal binding constants have been determined. The affinities of the [3Fe-4S] 0 cluster for divalent ions lie in the sequence Cd > Zn ⪢ Fe. In addition, specific binding of a monovalent ion, Thallium(I), is detected for [3Fe-4S] 1+ as well as for [3Fe-4S] 0. The results provide a clear and quantitative demonstration of the capability of the open triangular tri-μ 2-sulphido face of a [3Fe-4S] cluster to bind a variety of metal ions if the protein environment permits. In each case the entering metal ion is coordinated by at least one additional ligand which may be from solvent (H 2O or OH −) or from a protein side chain (e.g., carboxylate from aspartic acid). Hence the [3Fe-4S] core can be a redox-linked sensor of divalent metal ions, Fe(II) or Zn(II), that may trigger conformational change.