Evidence for non-cysteinyl coordination of the [2Fe-2S] cluster in Escherichia coli succinate dehydrogenase

The consequences of replacing Cys65 in the FrdB subunit of Escherichia coli fumarate reductase by Asp or Ala have been investigated in terms of bacterial growth, enzymatic activity, and the EPR/redox properties of the [2Fe-2S] cluster. An aspartic acid residue occupies the equivalent position in E. coli succinate dehydrogenase, and the FrdBCys65Asp mutation has little effect on cell growth, enzyme activity or the physical properties of the Frd [2Fe-2S] cluster. In contrast, the [2Fe-2S] cluster was not observed in the FrdBCys65Ala mutant showing that a coordinating residue is required at this position for assembly of this cluster and significant levels or enzymatic activity. These results support the presence of one non-cysteinyl, oxygenic ligand for the [2Fe-2S] cluster in E. coli succinate dehydrogenase.