ELECTROCHEMICAL POTENTIAL AND PH DEPENDENCES OF [3FE-4S][--][M3FE-4S]CLUSTER TRANSFORMATIONS (M = FE, ZN, CO, AND CD) IN FERREDOXIN-III FROM DESULFOVIBRIO-AFRICANUS AND DETECTION OF A CLUSTER WITH M = PB

The 7Fe ferredoxin from Desulfovibrio africanus contains a reactive 3F e cluster which incorporates Fe and other metals to form cubanes [M3Fe -4S] having non-cysteine ligation to the added metal (M). These reacti ons are addressed at controlled potentials in protein molecules adsorb ed on a graphite electrode, thereby facilitating an appraisal of facto rs that control cluster interconversions. Electrochemical and spectros copic methods have been used to establish connectivities among interco nversions with different M, explore the influence of pH, and determine reactivities of specific cluster oxidation levels. Formation of clust ers with M = Fe, Zn, and Co depends on ionization of a group with pK = 5.5 + 0.3, thus supporting congruence among products and suggesting c ommon involvement of aspartate-14 which replaces a cysteine normally p resent in [4Fe-4S] binding motifs. The influence of potential is compl ex: rapid and reversible interconversion (M = Fe, Zn) occurs only betw een the states [M3Fe-4S](2+) and [3Fe-4S](0), with [3Fe-4S](1+) having little affinity for M. The [M3Fe-4S](1+) cubanes and hyperreduced [3F e-4S](2-) are relatively unreactive. Uptake and release are significan tly more rapid for M = Zn compared to Fe. Among potentially intrusive metals, Pb has a particularly high affinity for the [3Fe-4S] cluster, but the product undergoes subsequent irreversible reactions. The studi es provide complementary perspectives on factors influencing cluster s tability or reactivity and on the feasibility and consequences of meta l substitutions.