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SOD1 Exhibits Allosteric Frustration to Facilitate Metal Binding Affinity

Superoxide dismutase-1 (SOD1) is a ubiquitous, Cu and Zn binding, free radical defense enzyme whose misfolding and aggregation play a potential key role in amyotrophic lateral sclerosis, an invariably fatal neurodegenerative disease. Over 150 mutations in SOD1 have been identified with a familial form of the disease, but it is presently not clear what unifying features, if any, these mutants share to make them pathogenic. Here, we develop several new computational assays for probing the thermo-mechanical properties of both ALS-associated and rationally-designed SOD1 variants. Allosteric interaction free energies between residues and metals are calculated, and a series of atomic force microscopy experiments are simulated with variable tether positions, to quantify mechanical rigidity "fingerprints" for SOD1 variants. Mechanical fingerprinting studies of a series of C-terminally truncated mutants, along with an analysis of equilibrium dynamic fluctuations while varying native constraints, potential energy change upon mutation, frustratometer analysis, and analysis of the coupling between local frustration and metal binding interactions for a glycine scan of 90 residues together reveal that the apo protein is internally frustrated, that these internal stresses are partially relieved by mutation but at the expense of metal-binding affinity, and that the frustration of a residue is directly related to its role in binding metals. This evidence points to apo SOD1 as a strained intermediate with "self-allostery" for high metal-binding affinity. Thus, the prerequisites for the function of SOD1 as an antioxidant compete with apo state thermo-mechanical stability, increasing the susceptibility of the protein to misfold in the apo state.