Here, crystallography, native ion-mobility mass spectrometry,and chromatography methods reveal the Fe–S assembly subcomplexexists as an equilibrium mixture of these different quaternary structures.Isotope labeling and native mass spectrometry experiments show thatthe NFS1-ISD11-ACP complexes disassemble into protomers, which canthen undergo exchange reactions and dimerize to reform native complexes.Single crystals isolated in distinct architectures have the same activityprofile and activation by the Friedreich’s ataxia (FRDA) proteinfrataxin (FXN) when rinsed and dissolved in assay buffer. This evidence concerns the gene NDUFAB1 and Friedreich ataxia.