The complex world of proteins: Structure, function, and oligomerization of frataxin

Abstract: Patients suffering from the deadly progressive neurodegenerative disease Friedreich's ataxia have reduced levels of expression of the protein frataxin. Yeast frataxin has been extensively studied in vitro, and has been shown to deliver iron to heme and iron-sulfur cluster biosynthesis, as well as to oligomerize in the presence of iron. In this dissertation metal-dependent oligomerization of frataxin from human, S. cerevisiae and E. coli was studied using small angle X-ray scattering (SAXS).The structures of yeast frataxin oligomers that formed in the presence of iron were successfully modeled, and the relative distribution of different oligomers in solution at varying iron concentration was determined. The results suggest that trimeric yeast frataxin has a central role in frataxin function, as an oligomer building block as well as an iron delivery unit and a plausible iron sensor. In the light of these results, structural and kinetic characterization of yeast frataxin iron delivery to ferrochelatase was performed. EM reconstruction of the complex between ferrochelatase and frataxin showed that one yeast frataxin trimer binds to each of the subunits within the ferrochelatase dimer. The interface between yeast frataxin and ferrochelatase contained both polar and charged amino acids, and potential hydrogen bond interactions as well as salt-bridge interactions were identified. Furthermore, single-turnover stopped-flow kinetics measurements showed increased rates of heme production when iron was delivered from monomeric or trimeric yeast frataxin, as compared to free iron in solution. It was concluded that, yeast frataxin delivers iron to ferrochelatase, and in addition facilitates ferrochelatase activity, presumably through partial dehydration of the hydrated iron ion.