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 <i>Plasmodium falciparum</i> flavoenzyme type II NADH:ubiquinone oxidoreductase (<i>Pf</i>NDH2). The reactivity of Q and ArNO₂ (<i>n</i> = 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 <i>Pf</i>NDH2 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 <i>Pf</i>NDH2 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 <i>Pf</i>NDH2 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.