SOD1 Exhibits Allosteric Frustration to Facilitate Metal Binding Affinity

TL;DR

This study uncovers how internal frustration in apo SOD1 facilitates metal binding through allosteric mechanisms, revealing a balance between functional activity and stability that influences misfolding in ALS.

Contribution

The paper introduces new computational assays to analyze the thermo-mechanical properties and allosteric frustration in SOD1 variants, linking internal stress to metal binding and misfolding.

Findings

Apo SOD1 is internally frustrated, creating a strained intermediate state.

Mutation partially relieves internal stress but reduces metal-binding affinity.

Residue frustration is directly related to its role in metal binding.

Abstract

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.