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, Mössbauer, 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 parallel = 2.06 and g perpendicular = 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 the [2Fe-2S]2+ center in [2Fe-2S]2+ IscU. It is suggested that the ability of IscU to accommodate either [2Fe-2S]2+ or [4Fe-4S]2+ clusters in response to cellular redox status and/or oxygen levels may provide an effective way to populate appropriately cluster-loaded forms of IscU for maturation of different types of [Fe-S] proteins.
Chandramouli
Kala
K
Department of Chemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, Georgia 30602, USA.
Unciuleac
Mihaela-Carmen
MC
Naik
Sunil
S
Dean
Dennis R
DR
Huynh
Boi Hanh
BH
Johnson
Michael K
MK
D005288:Ferredoxins / Q000737:chemistry / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D034521:Nuclear Matrix-Associated Proteins / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D015204:Spectroscopy, Mossbauer
D005288:Ferredoxins; D007506:Iron-Sulfur Proteins; D034521:Nuclear Matrix-Associated Proteins