Reactions of Plasmodium falciparum Type II NADH: Ubiquinone Oxidoreductase with Nonphysiological Quinoidal and Nitroaromatic Oxidants

In order to detail the antiplasmodial effects of quinones (Q) and nitroaromatic compounds (ArNO2), we investigated their reduction mechanism by Plasmodium falciparum flavoenzyme type II NADH:ubiquinone oxidoreductase (PfNDH2). The reactivity of Q and ArNO2 (n = 29) follows a common trend and exhibits a parabolic dependence on their single-electron reduction potential (E71), albeit with significantly scattered data. The reactivity of quinones with similar E71 values increases with their lipophilicity. Quinones are reduced by PfNDH2 in a two-electron way, but ArNO2 are reduced in a single-electron way. The inhibition studies using NAD+ and ADP-ribose showed that quinones oxidize the complexes of reduced enzyme with NADH and NAD+. This suggests that, as in the case of other NDH2s, quinones and the nicotinamide ring of NAD(H) bind at separate sites. A scheme of PfNDH2 catalysis is proposed, consistent with both the observed ‘ping-pong’ mechanism and the presence of two substrate binding sites. Molecular docking showed that Q and ArNO2 bind in a similar manner and that lipophilic quinones have a higher affinity for the binding site. One may expect that PfNDH2 can be partially responsible for the previously observed enhanced antiplasmodial activity of aziridinylbenzoquinones caused by their two-electron reduction, as well as for the redox cycling and oxidative stress-type action of ArNO2.