Overexpression, isotopic labeling, and spectral characterization of Enterobacter cloacae nitroreductase

Bacterial nitroreductases have generated much interest recently due to their central roles in both nitroaromatic bioremediation and nitroaromatic toxicity, mutagenicity, and carcinogenicity. Enterobacter cloacae nitroreductase (NR) has been subcloned into the pET overexpression system and purified to homogeneity via a four-step procedure resulting in a final yield of 65.7 mg per liter. Overexpression in minimal media containing ¹⁵NH₄C1 as the sole source of nitrogen yielded 37.6 mg per liter of homogenous NR containing >99 atom % ¹⁵N. A series of melting curves generated under a variety of solvent conditions established the optimal conditions for NR stability as pH 7.5, low ionic strength phosphate buffer. A two-dimensional ¹H-¹⁵N heteronuclear single quantum coherence nuclear magnetic resonance spectrum demonstrates this enzyme to be amenable to study by high-resolution multidimensional NMR in combination with amino-acid-specific isotopic labeling. Optical spectra of the purified enzyme suggest that the noncovalently bound flavin mononucleotide cofactor binds in a hydrophobic environment and is in the neutral and anionic protonation states in the oxidized and two-electron reduced oxidation states, respectively. NR exhibits a novel visible region circular dichroism spectrum which has a small distinct negative band at 366 nm and a large positive ellipticity at 454 nm with a shoulder centered at 480 nm.