Crystallization and preliminary X-ray analysis of AzoR (azoreductase) from Escherichia coli

AzoR (azoreductase), an FMN-dependent NADH-azo compound oxidoreductase from Escherichia coli, has been crystallized in the presence of FMN by the sitting-drop vapour-diffusion method using 2-propanol as a precipitant. AzoR catalyzes the reductive cleavage of azo groups. The crystals were found to diffract X-rays to beyond 1.8 Å resolution using a synchrotron-radiation source. The crystals belonged to the tetragonal space group P4₂2₁2, with unit-cell parameters a = b = 92.2, c = 51.9 Å. The crystals are expected to contain one subunit of the homodimer in the asymmetric unit (V_M = 2.6 Å³ Da^-1) and to have a solvent content of 51.6%. Data sets were also collected from heavy-atom derivatives for use in phasing. As a result, crystals soaked in a solution containing K₂PtCl₄ for 23 d were found to be reasonably isomorphous to the native crystals and the presence of Pt atoms could be confirmed. The data sets from the native crystals and the K₂PtCl₄-derivatized crystals are being evaluated for use in structure determination by single isomorphous replacement with anomalous scattering.