The neurodegenerative disorder FRDA (Friedreich's ataxia) results from a 
deficiency in frataxin, a putative iron chaperone, and is due to the presence of 
a high number of GAA repeats in the coding regions of both alleles of the 
frataxin gene, which impair protein expression. However, some FRDA patients are 
heterozygous for this triplet expansion and contain a deleterious point mutation 
on the other allele. In the present study, we investigated whether two 
particular FRDA-associated frataxin mutants, I154F and W155R, result in unfolded 
protein as a consequence of a severe structural modification. A detailed 
comparison of the conformational properties of the wild-type and mutant proteins 
combining biophysical and biochemical methodologies was undertaken. We show that 
the FRDA mutants retain the native fold under physiological conditions, but are 
differentially destabilized as reflected both by their reduced thermodynamic 
stability and a higher tendency towards proteolytic digestion. The I154F mutant 
has the strongest effect on fold stability as expected from the fact that the 
mutated residue contributes to the hydrophobic core formation. Functionally, the 
iron-binding properties of the mutant frataxins are found to be partly impaired. 
The apparently paradoxical situation of having clinically aggressive frataxin 
variants which are folded and are only significantly less stable than the 
wild-type form in a given adverse physiological stress condition is discussed 
and contextualized in terms of a mechanism determining the pathology of FRDA 
heterozygous.