Schematic of the Xanthomonas axonopodis pv. citri NPP active site in a transition state. Red and blue curved arrows represent nucleophilic addition (AN) and elimination (DN) steps, respectively, of the phosphoryl transfer reaction. These two steps could occur in either order, and probably overlap. The active site's threonine nucleophile is regenerated via hydrolysis. This schematic does not contain any bond length or bond order information. Adapted from Zalatan et al. 2006.