Chemical environment of calcium coordinating water molecule HOH 524. The electron density Fobs-Fcalc residual map (light grey) is at 2.2 σ contour level. The two possible protonation states of HOH 524 are modelled (protonated form in light grey (hydrogen) and red (oxygen); deprotonated activated form in grey (hydrogen) and pink (oxygen)). The interactions performed by HOH 524 are shown as black dashed lines. The calcium coordination is highlighted by blue dashed lines. Distances are indicated in Ångstrom.