The structure of the native SOD1 dimer (PDB code 1HL5) and the loop regions removed by protein engineering. The structure of the native SOD1 dimer (PDB code 1HL5) and the loop regions removed by protein engineering. (A) In the native SOD1 dimer the long loops IV and VII adapt a compact and highly ordered structure around the active site, where loop IV also forms part of the dimer interface (green). The left-hand monomer is shown as accessible surface (1.4 Å probe radius) whereas the right-hand monomer is represented as a cartoon. Highlighted are the residues coordinating the active-site Cu1+/2+ and Zn2+ ions and the C57–C146 disulfide linkage between loop IV and the central β-barrel. (B) Removal of loops IV and VII from apoSOD1 reduces the dimer interface as well as the metal binding moieties and leads to soluble apoSOD1barrel monomers (PDB code 4BCZ). The truncated loops IV and VII are highlighted in green and blue, respectively. (Adapted from ref. 26.)‏ Jens Danielsson et al. PNAS 2015;112:40:12402-12407 ©2015 by National Academy of Sciences