Using the ASM methodology, we characterized the reaction pathway associated to all these possible mechanisms, starting in each case from the a priori most favorable conformation. The computed free-energy barriers are summarized in Table . Note that we could not converge any reaction pathway starting with deprotonation by the Mg 2+ -bound Glu96 oxygen (purple arrow in Fig ) which thus seems highly unfavored. Of all the explored mechanisms, direct deprotonation by the Glu96 oxygen non involved in Mg 2+ coordination appears by far the most favorable, with a free energy barrier ∆G ‡ ≃ 18.2 kcal mol -1 , while all the other mechanisms are associated with barriers over 30 kcal mol -1 (Figures S12, S13, S14).