Figure 3.

(A) Stereo figures showing the superposition of Zn-H46R (blue), wtSOD1 (red), and apo wtSOD1 (green) for the copper site. The N^η1 atom of Arg 46 occupies a position 2.8 Å from the position occupied by copper in the native hSOD structure, while the C^ɛ atom resides some 2.9 Å away. The new conformation of His 120 in Zn-H46R allows the copper-ligating N^ɛ2 atoms of histidines 48 and 120 to form a ~3 Å hydrogen bond. His 48 remains in a very similar conformation in the mutant and wtSOD1 structures. This preserved conformation is likely to be a consequence of the hydrogen bond between its N^δ1 atom and the O atom of Gly 61. The side chain of His 120 is shifted, occupying a position slightly closer to the vacant copper position (N^ɛ2 to copper distance would be ~1.8 Å). (B) The zinc site. In Zn-H46R zinc is ligated by only three protein residues where Asp 83 acts as a bidentate ligand. His 71 is absent in Zn-H46R and is omitted from this figure for clarity, as is the water ligand observed in the Zn-H46R structure. (C) The secondary bridge. In wtSOD1, residue Asp 124 forms a hydrogen-bonded link between the copper ligand His 46 and the zinc ligand His 71. Asp 124 O^δ2 forms a strong (~2.6 Å) hydrogen bond to His46 N^ɛ2, while its O^δ1 atom bonds to His71 N^ɛ2. In addition, the O^δ1 atom is close to the backbone nitrogen atoms of residues 125 (3.1 Å) and 126 (2.8 Å) of the electrostatic loop, providing additional stabilization of the structure. In Zn-H46R, the mutant residue 46 maintains hydrogen bonding to Asp 124, but residue His 71 is disordered and the bridge is broken. In apo-wtSOD1, in the absence of metals His 46 hydrogen bonds to Asp 124, which adopts a different rotamer to that present in wtSOD1.