Crystallization of a nickel-containing superoxide dismutase and preliminary phase determination by MAD at the Ni K edge

Superoxide dismutases are metalloenzymes which catalyse the disproportion of superoxide radicals and thus play an important role in the protection of biomolecules from oxidative damage. Redox-active metal ions known to act as the catalytic centre of these enzymes are Cu, Mn or Fe. Recently, enzymes containing Ni have been found in various Streptomyces species, introducing a fourth type of metal ion to the superoxide dismutase family. NiSOD has been crystallized for the purpose of structure determination by X-ray crystallography using Ni as an anomalous scatterer in multiple-wavelength anomalous dispersion (MAD) experiments. Two crystal forms belonging to space group P212121 and one belonging to space group R3 were obtained using ammonium sulfate as a precipitant. Patterson maps of one of the orthorhombic forms revealed the presence of pseudo-translation, which could be removed for the other orthorhombic form by using 10% glycerol in its crystallization conditions. In addition, this reduced the unit cell by half. Phase information which led to interpretable electron-density maps was derived from MAD data to 2.0 A resolution after density modification applying solvent flattening, histogram matching and NCS averaging. Phases were extended to 1.68 A resolution with a data set collected at a wavelength of 1 A. Model building based on the resulting electron-density maps is in progress.