The animation shows histidine acting as an acid protonates the hydroxyl on the substrate, allowing it to leave as a water molecule. Serine then acts as a base by extracting a hydrogen atom from the opposite side of the substrate, forming the intermediate cis-aconitate. Cis-aconitate then flips upside down, and the complementary hydration reaction is performed.
Histindine grabs a hydrogen atom from a passing water molecule, placing the resulting hydroxyl group back onto the substrate. Serine then returns its hydrogen atom and the final product isocitrate is released.