Proposed mechanism of binuclear metallohydrolase-catalyzed esterolysis. Following the binding of the substrate in a pre-catalytic complex, structural rearrangements lead to "quasi-monodentate" and bidentate coordination of the μ-hydroxide and phosphate groups, respectively (a-c). Nucleophilic attack by the μ-hydroxide is followed by the release of the leaving group, and the active site is returned to its resting state by the exchange of the bound phosphate group by two water molecules (d-h). A terminal M1-bound hydroxide is observed in the structure of rat PAP (b), which appears to be an artefact of crystallization  and is not supported by solution studies on resting PAP . Where available crystallographic pictures of relevant active site structures are included. (a) rkbPAP-sulfate complex; (b) rat PAP-sulfate complex ; (c) pig PAP-phosphate complex ; (d) sweet potato PAP-phosphate complex  ; (e) rkbPAP-phosphate complex ; (h) rkbPAP  (the bridging and terminal M2-bound water ligands were modelled based on ENDOR studies ).
Schenk et al. BMC Structural Biology 2008 8:6 doi:10.1186/1472-6807-8-6