Two-electron reduction of quinones by Enterobacter cloacae NAD(P)H:nitroreductase: Quantitative structure-activity relationships

Enterobacter cloacae NAD(P)H:nitroreductase (NR; EC 1.6.99.7) catalyzes two-electron reduction of a series of quinoidal compounds according to a "ping-pong" scheme, with marked substrate inhibition by quinones. The steady-state catalytic constants (k_cat) range from 0.1 to 1600 s^-1, and bimolecular rate constants (k_cat/K_m) range from 10³ to 10⁸ M^-1 s^-1. Quinones, nitroaromatic compounds and competitive to NADH inhibitor dicumarol, quench the flavin mononucleotide (FMN) fluorescence of nitroreductase. The reactivity of NR with single-electron acceptors is consistent with an "outer-sphere" electron transfer model, taking into account high potential of FMN semiquinone/FMNH^- couple and good solvent accessibility of FMN. However, the single-electron acceptor 1,1′-dibenzyl-4,4′-bipyridinium was far less reactive than quinones possessing similar single-electron reduction potentials (E₇¹). For all quinoidal compounds except 2-hydroxy-1,4-naphthoquinones, there existed parabolic correlations between the log of rate constants of quinone reduction and their E₇¹ or hydride-transfer potential (E⁷(Q/QH^-)). Based on pH dependence of rate constants, a single-step hydride transfer seems to be a more feasible quinone reduction mechanism. The reactivities of 2-hydroxy-1,4-naphthoquinones were much higher than expected from their reduction potential. Most probably, their enhanced reactivity was determined by their binding at or close to the binding site of NADH and dicumarol, whereas other quinones used the alternative, currently unidentified binding site.