Ping-pong kinetic mechanism of PqsD in HHQ biosynthesis. A) Lineweaver-Burk Plot and B) Hanes-Woolf Plot representing the activity of PqsD as a function of ACoA concentration in the presence of 60 (diamonds), 120 (black circles), 240 (triangles), 480 (open squares) and 1000 μM (open circles) ßK. All data are mean values of four replicates. C) Kinetic parameters of PqsD. D) “Real-time” PqsD kinetics by SPR experiments: 1. ACoA injection; 2. buffer run - response units remain high due to anthranilate-Cys112; 3. βK injection; 4. Buffer run – response line goes to zero due to HHQ formation and restored apo PqsD. E) Schematic view of the putative ping-pong kinetic mechanism of the HHQ biosynthesis (E - PqsD, ECSJ - PqsD with Cys112-ligated anthranilate).