The Unfolding Journey of Superoxide Dismutase 1 Barrels under Crowding: Atomistic Simulations Shed Light on Intermediate States and Their Interactions with Crowders

The thermal stability of the superoxide dismutase 1 protein in a crowded solution is investigated by performing enhanced sampling molecular simulations. By complementing thermal unfolding experiments done close to physiological conditions (200 mg/mL), we provide evidence that the presence of the protein crowder bovine serum albumin in different packing states has only a minor, and essentially destabilizing, effect. The finding that quinary interactions counteract the pure stabilization contribution stemming from excluded volume is rationalized here by exploring the SOD1 unfolding mechanism in microscopic detail. In agreement with recent experiments, we unveil the importance of intermediate unfolded states as well as the correlation between protein conformations and local packing with the crowders. This link helps us to elucidate why certain SOD1 mutations involved in the ALS disease reverse the stability effect of the intracellular environment.