Mechanical Probes of SOD1 Predict Systematic Trends in Metal and Dimer Affinity of ALS-Associated Mutants

Mutations and oxidative modification in the protein Cu,Zn superoxide dismutase (SOD1) have been implicated in the death of motor neurons in amyotrophic lateral sclerosis (ALS), a presently incurable, invariably fatal neurodegenerative disease. Here we employ steered, all-atom molecular dynamics simulations in implicit solvent to investigate the significance of either mutations or post-translational modifications (PTMs) to SOD1 on metal affinity, dimer stability, and mechanical malleability. The work required to induce moderate structural deformations as a function of sequence index constitutes a “mechanical fingerprint” measuring structural rigidity in the native basin, from which we are able to unambiguously distinguish wild-type SOD1 from PTM variants and measure the severity of a given PTM on structural integrity. The cumulative distribution of work values provided a way to cleanly discriminate between SOD1 variants. Disulfide reduction destabilizes dimer stability more than the removal of either metal, but not moreso than the removal of both metals. Intriguingly, we found that disulfide reduction mechanically stabilizes apo-SOD1 monomer, underscoring the differences between native basin mechanical properties and equilibrium thermodynamic stabilities and elucidating the presence of internal stress in the apo state. All PTMs and ALS-associated mutants studied showed an increased tendency to lose either Cu or Zn and to monomerize—processes known to be critical in the progression of ALS. The valence of Cu strongly modulates its binding free energy. As well, several mutants were more susceptible to loss of metals and monomerization than the disulfide-reduced or apo forms of SOD1. Distance constraints are required to calculate free energies for metal binding and dimer separation, which are validated using thermodynamic cycles. When distance constraints are removed, the results agree with those obtained from direct application of the Jarzynski equality.