ELECTRONIC, MAGNETIC, REDOX, AND LIGAND-BINDING PROPERTIES OF [MFE(3)S(4)] CLUSTERS (M=ZN, CO, MN) IN PYROCOCCUS-FURIOSUS FERREDOXIN

The ground and excited state properties and ligand-binding capabilitie s of [ZnFe3S4](2+,+), [CoFe3S4](2+,+), and [MnFe3S4](+) clusters in Py rococcus furiosus Fd have been investigated by the combination of EPR, variable-temperature magnetic circular dichroism (VTMCD), and MCD mag netization studies. The ground state spins, S = 5/2 for [ZnFe3S4](+), S = 2 for [ZnFe3S4](2+), S = 1 for [CoFe3S4](+), S = 1/2 for [CoFe3S4] (2+), and S = 0 for [MnFe3S4](+), are consistent with a simple couplin g scheme involving antiparallel interaction between the high-spin diva lent metal ion and an S = 5/2 [Fe3S4](+) or S = 2 [Fe3S4](0) cluster f ragment. Redox potentials (vs NHE) were determined by dye-mediated EPR titrations at pH 7.6, E(m) -241 +/- 20 mV for [ZnFe3S4](2+,+) E(m) = -163 +/- 10 mV for [CoFe3S4](2+,+), and E(m) > -100 mV for [MnFe3S4](2 +,+), indicating that the potentials for [MFe(3)S(4)](2+,+) clusters i n P. furiosus Fd are ordered M = Fe < Zn < Co < Mn or Ni. On the basis of changes in EPR and/or VTMCD spectra, evidence is presented for cya nide binding at the unique metal site of the [ZnFe3S4](+) and [CoFe3S4 ](+) clusters and for 2-mercaptoethanol binding at the Zn site of the [ZnFe3S4](+) cluster. The ground and excited state properties of the t hiolate-bound [ZnFe3S4](+) cluster in P. furiosus Fd are very similar to those of the equivalent cluster in Desulfovibrio gigas FdII, indica ting coordination of the Zn by the indigenous cysteinate in this prote in. The site specific reactivity, intracluster magnetic interactions, and redox properties of heteronuclear cubanes in general are discussed in light of these results.