Table 1.
SOD1 Serine to BMAA mutation destabilizes SOD1 dimers.
Folding free energy differences between wild type and mutant SOD1, ΔΔG. Values are mean ± standard deviation among 20 independent runs. Calculations are performed using Eris with both fixed and flexible backbone algorithms for serine to BMAA, and to lysine, for comparison.
Fig 1.
BMAA modification induces thermodynamic destabilization and structural changes in SOD1.
(A) Specific heat curves generated from replica exchange DMD simulations of BMAA-modified or wild type SOD1. Peaks in specific heat indicate melting events. Dotted lines indicate major melting event for each species. (B) Histogram of potential energy of BMAA-modified or wild type SOD1 gathered from single(low)-temperature DMD simulations. SOD1-BMAA exists at a mean higher potential energy than wild type SOD1, indicating a less favorable structural conformation. Dotted lines indicate peaks. (C) Structural alignment of BMAA-modified (dark blue) and wild type (light blue) structures. Root mean square distance (RMSD) between structures is 3.24 Å. The β-strands in the dimer interface (right, cutaway of each monomer viewed from the center of the dimer interface) can be seen to be elongated and twisted in BMAA-modified SOD1 (dark blue) as compared to wild type (light blue). BMAA is shown as spheres. (D) Structural alignment of Zn- (left) and Cu- (right) binding sites of BMAA-modified (dark blue) and wild type (light blue) structures. Ions belonging to the BMAA-modified structure are in color (dark grey for Zn, orange for Cu), while ions belonging to the wild type are in light blue. Metal-coordinating residues for both structures are shown as lines.
Fig 2.
BMAA-SOD1 features altered dynamics near disulfide bond and metal ion binding sites.
Heat map of changes in correlated motions between residues (standard sequence numbering provided) upon modification with BMAA. Red indicates increased correlated motions, blue indicates decreased correlated motions, and yellow indicates no change. Dotted line indicates the divide between the two monomer chains. Select significantly affected structural features critical for SOD1 stability are labeled, with location on the SOD1 structure highlighted. A comparison of root mean square fluctuations (RMSF) at each residue for SOD1 with and without BMAA modification is included (top), highlighting regions of increased or decreased flexibility due to misincorporation of BMAA.
Fig 3.
Proposed mechanism of BMAA toxicity in ALS pathology.
(A) Chemical structure of BMAA molecule. (B) Misincorporation of BMAA for serine causes structural rearrangement and strain that propagates to the dimer interface and metal-binding residues. BMAA is show as spheres colored by atom type; copper (orange) and zinc (cerulean) ions are shown as spheres. (C) From left to right: misincorporation of BMAA into SOD1 promotes dimer dissociation and destabilization of the metal-binding sites; metal binding is further destabilized in the monomeric form, leading to metal loss; without metal ions, the SOD1 monomer fold is destabilized misfolds; misfolding promotes oligomerization and the formation of non-native SOD1 trimer, previously shown to be neurotoxic; misfolded SOD1 monomer can also form fibrils observed in ALS patients.