Structural and Functional Studies on Human Mitochondrial Iron-Sulfur Cluster Biosynthesis
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Iron-sulfur (Fe-S) clusters are critical protein cofactors found in all life forms. In eukaryotes, a well-conserved biosynthetic pathway located in the mitochondria is used to assemble Fe-S clusters. Although proteins required for Fe-S cluster biosynthesis have been identified, their precise function and mechanism remain elusive. In this study, biochemical and biophysical methods are applied to understand molecular details for the core components of the human Fe-S cluster biosynthesis: Nfs1, Isd11, Isu2, and frataxin (Fxn). Nfs1 is a cysteine desulfurase that converts cysteine into alanine and transfers the sulfur to a scaffold protein Isu2 for Fe-S clusters. Fxn depletion is associated with the neurodegenerative disease Friedreich?s ataxia (FRDA), and results in a complicated phenotype that includes loss of Fe-S clusters. The results presented here provide the first in vitro evidence for a stable protein complex that exists in at least two forms: an inactive complex with Nfs1, Isd11, and Isu2 (SDU) components and an active form that also includes Fxn (SDUF). Fxn binding dramatically changes the catalytic efficiency (kcat/KM) of Nfs1 from 25 to 10,100 M-1s-1 and enhances the rate of Fe-S cluster biosynthesis 25 fold. Oxidizing conditions diminish the levels of both complex formation and Fxn-based activation, whereas Fe2 further stimulates Nfs1 activity. Mutagenesis coupled to enzyme kinetics indicate that one of the three conserved cysteines (C104) on Isu2 accepts the sulfane sulfur from Nfs1 and that this transfer event likely requires prior binding of Fxn. In vitro interrogation of FRDA I154F and W155R and related Fxn variants revealed the binding affinity to SDU followed the trend Fxn ~ I154F > W155F > W155A ~ W155R. The Fxn variants also have diminished ability to facilitate both sulfur transfer and Fe-S cluster assembly. Fxn crystallographic structures reveal specific rearrangements associated with the loss of function. Importantly, the weaker binding and lower activity of the W155R variant compared to I154F explains the earlier onset and more severe disease progression. Finally, these experimental results coupled with computational docking studies suggest a model for how human Fxn functions as an allosteric activator and triggers sulfur transfer and Fe-S cluster assembly.