Given the multiplicity of possible reaction mechanisms, the number of chemical steps and the need to capture active site reorganization along the reaction path, we chose to use a pathbased method-The Adaptive Finite Temperature String Method 57 (ASM) developed by one of us-to dynamically explore the reaction free energy landscape in a multi-dimensional space of collective variables (see Methods). Given the computational cost of such QM/MM-MD The different conformations of the active site previously identified seem prone to favor distinct mechanisms. In particular, the conformation with a single water molecule-in the Mg 2+ hydration shell-bridging the carboxylate group of Glu96 and the reactive phosphate (see starting point "Glu96_1wat" on Figure and Figure ) seems ideally pre-organized for deprotonation of the nucleophilic water molecule directly by Glu96. Structures from the secondary basin ("A118_1wat" on Figure and Figure ) with a single water molecule bridging the Mg 2+