Quantitative structure-activity relationships in two-electron reduction ofnitroaromatic compounds by Enterobacter cloacae NAD(P)H : nitroreductase

Enterobacter cloacae NAD(P)H:nitroreductase (NR; EC 1.6.99.7) catalyzes the reduction of a series of nitroaromatic compounds with steady-state bimolec ular rate constants (k(cat)/K-m) ranging from 10(4) to 10(7) M-1 s(-1). In agreement with a previously proposed scheme of two-step four-electron reduc tion of nitroaromatics by NR (Koder, R. L., and Miller, A.-F, (1998) Biochi m. Biophys, Acta 1387, 395-405), 2 mol NADH per mole mono-nitrocompound wer e oxidized. An oxidation of excess NADH by polinitrobenzenes, including exp losives 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenyl-N- methylnitra mine (tetryl), has been observed as a slower secondary process, accompanied by O-2 consumption. This type of "redox cycling" was not related to reacti ons of nitroaromatic anion-radicals, but was caused by the autoxidation of relatively stable reaction products. The initial reduction of tetryl and ot her polinitrophenyl-N-nitramines by E. cloacae NR was analogous to a two-st ep four-electron reduction mechanism of TNT and other nitroaromatics, The l ogs 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 str ucture-activity relationship shows that the reactivity of nitroaromatics to wards E. cloacae nitroreductase depends mainly on their hydride accepting p roperties, but not on their particular structure, and does not exclude the possibility of multistep hydride transfer. (C) 2001 Academic Press.