Multimodal Spectroscopic Analysis of the Fe-S Clusters of the as-Isolated Escherichia coli SufBC₂D Complex

Iron-sulfur (Fe-S) clusters are essential inorganic cofactors dedicated to a wide range of biological functions, including electron transfer and catalysis. Specialized multiprotein machineries present in all types of organisms support their biosynthesis. These machineries encompass a scaffold protein, on which Fe-S clusters are assembled before being transferred to cellular targets. Here, we describe the first characterization of the native Fe-S cluster of the anaerobically purified SufBC₂D scaffold from Escherichia coli by XAS and Mössbauer, UV-visible absorption, and EPR spectroscopies. Interestingly, we propose that SufBC₂D harbors two iron-sulfur-containing species, a [2Fe-2S] cluster and an as-yet unidentified species. Mutagenesis and biochemistry were used to propose amino acid ligands for the [2Fe-2S] cluster, supporting the hypothesis that both SufB and SufD are involved in the Fe-S cluster ligation. The [2Fe-2S] cluster can be transferred to ferredoxin in agreement with the SufBC₂D scaffold function. These results are discussed in the context of Fe-S cluster biogenesis.