Computational Insights into How Structural Changes in the Hydrophobic Region of ALS-Linked Mutant SOD1 (G93A) Activate Fluctuations of the Electrostatic Loop: Implications for Aggregation

Cu/Zn-superoxide dismutase (SOD1) is an essential antioxidant enzyme that converts superoxide radicals into oxygen and hydrogen peroxide. On the other hand, mutations in SOD1 cause familial amyotrophic lateral sclerosis (fALS), and these mutant forms tend to aggregate. Our previous research has shown that residue I35, located in a hydrophobic region, plays a critical role in the intracellular aggregation of SOD1. Furthermore, introducing an additional I35S mutation suppressed aggregate formation and eye degeneration in a Drosophila model caused by ALS-linked mutant SOD1. However, the molecular mechanism linking structural changes induced by ALS mutations to aggregation, and how the compensatory I35S mutation suppresses this aggregation, remain unclear. To elucidate the relationship between structural changes and aggregation, molecular dynamics simulations were performed on three types of SOD1 monomers: wild type (WT), G93A (an fALS mutant), and the G93A/I35S double mutant. In WT, the metal-binding loop (loop IV) exhibited slightly larger fluctuations, whereas in G93A, the electrostatic loop (loop VII) fluctuates significantly more. Detailed analysis revealed that the G93A mutation shifts the hydrogen-bonding partner of the 93rd residue from L38 in WT to G37 in G93A. This change destabilizes the V119-L144 hydrogen bond, thereby enhancing fluctuations in {beta}-strand 8 and the electrostatic loop. Notably, the I35S mutation in the G93A background restores the WT-like packing of the hydrophobic core and reestablishes the V119-L144 hydrogen bond, reducing the abnormal electrostatic loop fluctuations. These findings provide a mechanistic link between a single-point mutation and conformational changes involved in SOD1 aggregation. Our results also suggest that targeting the hydrophobic region may represent a novel therapeutic strategy for SOD1-linked fALS.