Steady-state kinetic mechanism, stereospecificity, substrate and inhibitor specificity of Enterobacter cloacae nitroreductase

Enterobacter cloacae nitroreductase (NR) is a flavoprotein which catalyzes the pyridine nucleotide-dependent reduction of nitroaromatics. Initial velocity and inhibition studies have been performed which establish unambiguously a ping-pong kinetic mechanism, NADH oxidation proceeds stereospecifically with the transfer of the pro-R hydrogen to the enzyme and the amide moiety of the nicotinamide appears to be the principal mediator of the interaction between NR and NADH. 2,4-Dinitrotoluene is the most efficient oxidizing substrate examined, with a k(cat)/K-M an order of magnitude higher than those of p-nitrobenzoate, FMN, FAD or riboflavin. Dicoumarol is a potent inhibitor competitive vs. NADH with a K-i of 62 nM. Several compounds containing a carboxyl group are also competitive inhibitors vs. NADH. Yonetani-Theorell analysis of dicoumarol and acetate inhibition indicates that their binding is mutually exclusive, which suggests that the two inhibitors bind to the same site on the enzyme. NAD(+) does not exhibit product inhibition and in the absence of an electron acceptor, no isotope exchange between NADH and P-32-NAD(+) could be detected. NR catalyzes the 4-electron reduction of nitrobenzene to hydroxylaminobenzene with no optically detectable net formation of the putative two-electron intermediate nitrosobenzene. (C) 1998 Elsevier Science B.V. All rights reserved.