Crystal structure of the cyanide-inhibited Xenopus laevis Cu,Zn superoxide dismutase at 98 K

The crystal structure of cyanide-inhibited X. laevis Cu,Zn superoxide dismutase has been studied and refined based on diffraction data collected at 98 K. The final R-factor for the 27,299 reflections in the 10.0-1.7 Å resolution range is 0.170. The cyanide anion, which is a competitive inhibitor expected to mimic the superoxide binding mode, binds directly to the active site copper atom, replacing the coordinated water molecule. Moreover, the anion establishes a strong electrostatic interaction with the guanidinium group of the conserved active site residue Arg¹⁴¹. The coordination sphere of Cu²⁺ is partly altered with respect to the uninhibited enzyme: a displacement of 0.41 Å in subunit A, and 0.27 Å in subunit B of the dimeric enzyme is observed for the Cu²⁺ ions. Only two ligands in the Cu²⁺ coordination sphere (His⁴⁶ and His¹¹⁸) are significantly affected by cyanide binding, whereas virtually no rearrangement of the Zn²⁺ ligands is reported.