HQNO (2-heptyl-4-hydroxyquinoline-N-oxide) is a natural quinolone secreted by P. aeruginosa. HQNO is a potent inhibitor of the electron transport chain with Kd of 64 nM for the ubiquinol-ferricytochrome c oxidoreductase segment (Complex III) of the respiratory chain. HQNO is also an inhibitor of bacterial and mitochondrialType II NADH : quinone oxidoreductase (NDH-2) with IC50 of 7.3 μM in the presence of 50μM menadione.