the main reaction is an sn2 displacement of the halogen for a hydroxyl group derived from water. to begin, aspartate 124 is perfectly aligned with the substrate. it will drive off the halogen and form an ester functionality carbon-oxygen bond. following this displacement is a hydrolysis reaction by utilizing the imidazole ring of histidine 289 as the general base. this will deprotonate water, form a tetrahedral intermediate at the original ester, and create an imidazolium cation at histidine. the final step is beta-elimination. with a newly formed imidazolium cation ready to be an acid, aspartate 124 reverts to its original acidic state and breaks the ester linkage, as well as deprotonating histidine 289. the alcohol is eliminated and the halogen is now a free anion.