Abstract : The phagocytic NADPH oxidase, responsible for superoxide anions production, becomes activated after assembly of cytosolic proteins with cytochrome b558, the transmembrane redox core of the enzyme composed of Nox2 and p22phox. We have previously observed that replacement of the D-loop of Nox2 with D-loop of Nox4 (mutant D-loopNox4-Nox2) induced a NADPH oxidase super-activity. The present work was to elucidate molecular mechanisms responsible for the overproduction of superoxide anions by the D-loopNox4-Nox2 mutant. The super-mutant exhibits an ex vivo oxidase activity which is 2 to 8 times higher than that of the WT-Nox2 PLB-985 cells, after stimulation by soluble and particulate agonists. The highest NADPH oxidase activity is obtained after ionomycin and fMLF chemotactic peptide activation, including signalling pathways with an increase of Ca2+i. This overproduction is also observed in vitro in a cell free system using only purified proteins of the oxidase complex, and activated by phosphatidic acid. Ex vivo oxidase activity of the mutant demonstrated an enhanced sensitivity to calcium influx. The mutated cytochrome b558 is less dependent on phosphorylation events ERK1/2-dependent occurring during activation with fMLF. Then, the super-oxidase activity of the D-loopNox4-Nox2 mutant could become from a conformational change of Nox2 improving the NADPH oxidase activated state. Moreover, superoxide anions overproduction by the mutant improves its killing activity against attenuated strain of P. aeruginosa PAO1. At last, we have found that the bactericidal process of this microorganism by phagocytes consists in two distinct and interdependent phases.