Overexpression, purification, crystallization and preliminary X-ray diffraction analysis of Cu,Zn superoxide dismutase from Peking duck

The cDNA encoding Peking duck Cu,Zn superoxide dismutase (dSOD) was cloned and sequenced. The recombinant enzyme was overexpressed in Escherichia coli, purified to homogeneity and crystallized using the sitting-drop vapour-diffusion technique. Trigonal crystals of dSOD were obtained at 278 K at low ionic strength and around neutral pH. These crystals belong to space group P3₂21, with unit-cell parameters a = 124.4, c = 163.5 Å, γ = 120°. The asymmetric unit contains four dimers (eight monomers of Cu,Zn dSOD) and has a 56% solvent content, with a V_M of 2.8 ų Da⁻¹. On a Rigaku R-AXIS IIc image-plate area-detector system, the crystal diffracted to 2.9 Å. Unusual supermolecular double-helix packing with 9₂2 non-crystallographic symmetry in crystals has been observed in the initial structural analysis.