Effects of maturation on the conformational free-energy landscape of SOD1

Amyotrophic lateral sclerosis (ALS) is a devastating fatal syndrome characterized by very rapid degeneration of motor neurons. A leading hypothesis is that ALS is caused by toxic protein misfolding and aggregation, as also occurs in many other neurodegenerative disorders, such as prion, Alzheimer's, Parkinson's, and Huntington's diseases. A prominent cause of familial ALS is mutations in the protein superoxide dismutase (SOD1), which promote the formation of misfolded SOD1 conformers that are prone to aberrant interactions both with each other and with other cellular components. We have shown previously that immature SOD1, lacking bound Cu and Zn metal ions and the intrasubunit disulfide bond (apoSOD1^2SH), has a rugged free-energy surface (FES) and exchanges with four other conformations (excited states) that have millisecond lifetimes and sparse populations on the order of a few percent. Here, we examine further states of SOD1 along its maturation pathway, as well as those off-pathway resulting from metal loss that have been observed in proteinaceous inclusions. Metallation and disulfide bond formation lead to structural transformations including local ordering of the electrostatic loop and native dimerization that are observed in rare conformers of apoSOD1^2SH; thus, SOD1 maturation may occur via a population-switch mechanism whereby posttranslational modifications select for preexisting structures on the FES. Metallation and oxidation of SOD1 stabilize the native, mature conformation and decrease the number of detected excited conformational states, suggesting that it is the immature forms of the protein that contribute to misfolded conformations in vivo rather than the highly stable enzymatically active dimer.