Pseudomonas aeruginosa exopolyphosphatase (paPpx; EC 3.6.1.11) catalyzes the hydrolysis of polyphosphates (polyP), producing polyPn-1 plus inorganic phosphate (Pi). In a recent work we have shown that paPpx is involved in the pathogenesis of P. aeruginosa. The present study was aimed at performing the biochemical characterization of this enzyme. We found some properties that were already described for E. coli Ppx (ecPpx) but we also discovered new and original characteristics of paPpx: (i) the peptide that connects subdomains II and III is essential for enzyme activity; (ii) NH4 (+) is an activator of the enzyme and may function at concentrations lower than those of K(+); (iii) Zn(2+) is also an activator of paPpx and may substitute Mg(2+) in the catalytic site; and (iv) paPpx also has phosphotransferase activity, dependent on Mg(2+) and capable of producing ATP regardless of the presence or absence of K(+) or NH4 (+) ions. In addition, we detected that the active site responsible for the phosphatase activity is also responsible for the phosphotransferase activity. Through the combination of molecular modeling and docking techniques, we propose a model of the paPpx N-terminal domain in complex with a polyP chain of 7 residues long and a molecule of ADP to explain the phosphotransferase activity.