Figure 2 - full size

 

Figure 2.
Fig. 2. [A]-weighted 2 F[o] - F[c] electron density maps of the Ni ion environment. (A-C) Structures of subunit F captured at successively increasing x-ray doses. (A) The fifth ligand His-1 N (2.5 Å distant to Ni here) is revealed at low x-ray exposure (map resolution 2.2 Å, 1.0 contour level). (B) After longer exposure of the same crystal as in A, the imidazolate ligation is disrupted. (C) Map at 1.6-Å resolution obtained from a different crystal as in A and B, applying a maximum total x-ray dose. The ligands show a distorted cis square-planar geometry, equaling the thiosulfate-reduced NiSOD. The average angle between planesdefined by N(His-1)-Ni-N(Cys-2) and S(Cys-2)-Ni-S(Cys-6) is 7.5°.(D) Superposition of models from A in green, B in magenta, and C in gray illustrates the His-1 imidazole rotation upon which N reaches a distance of 2.9 Å to Val-8 O, thereby maintaining the hydrogen-bond triangle of His-1 N to Glu-17 O and Arg-47 N of the adjacent subunit. (E) Electron density map of thiosulfate-reduced NiSOD contoured at 1.1 showing the square-planar Ni(II) coordination. One thiosulfate-ion (S[2]O[3]^2-) per subunit is found 7-8 Å away from each metal center (subunit A). The precise bonding pattern of these ions varies among the 12 subunits in the asymmetric unit, indicating a high degree of disorder or low-binding specificity.