Formation and properties of [4Fe-4S] clusters on the IscU scaffold protein.

Rapid and quantitative reductive coupling of two [2Fe-2S]2+ clusters to form a single [4Fe-4S]2+ cluster on the homodimeric IscU Fe−S cluster scaffold protein has been demonstrated by UV−visible absorption, Mossbauer, and resonance Raman spectroscopies, using dithionite as the electron donor. Partial reductive coupling was also observed using reduced Isc ferredoxin, which raises the possibility that Isc ferredoxin is the physiological reductant. The results suggest that reductive coupling of adjacent [2Fe-2S]2+ clusters assembled on IscU provides a general mechanism for the final step in the biosynthesis of [4Fe-4S]2+ clusters. The [4Fe-4S]2+ center on IscU can be reduced to a S = 1/2[4Fe-4S]+ cluster (g∥ = 2.06 and g⊥ = 1.92), but the low midpoint potential (< −570 mV) and instability of the reduced cluster argue against any physiological relevance for the reduced cluster. On exposure to O2, the [4Fe-4S]2+ cluster on IscU degrades via a semistable [2Fe-2S]2+ cluster with properties analogous to those of ...