Iron-sulphur clusters with labile metal ions.

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 formula: see text] 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 greater than Zn much greater than 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-mu 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 (H2O 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.