Two-electron reduction of nitroaromatic compounds by Enterobacter cloacae NAD(P)H nitroreductase: Description of quantitative structure-activity relationships

Enterobacter cloacae NAD(P)H: nitroreductase catalyzes the reduction of a series of nitroaromatic compounds with steady-state bimolecular rate constants (k_cat/K_m) ranging from 10⁴ M^-1s^-1 to 10⁷ M^-1s^-1, and oxidizing 2 moles NADH per mole mononitrocompound. Oxidation of excess NADH by polynitrobenzenes including explosives 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenyl-N-methylnitramine (tetryl), has been observed as a slower secondary process, accompanied by O₂ consumption. This type of 'redox cycling' was not related to reactions of nitroaromatic anion-radicals, but was caused by the autoxidation of relatively stable reaction products. The logs k_cat/K_m of all the compounds examined exhibited parabolic dependence on their enthalpies of single-electron- or two-electron (hydride) reduction, obtained by quantum mechanical calculations. This type of quantitative structure-activity relationships shows that the reactivity of nitroaromatics towards E. cloacae nitroreductase depends mainly on their hydride accepting properties, but not on their particular structure, and does not exclude the possibility of multistep hydride transfer.