Superpostion of the monomer structures with the lower radius region highlighted in the red box. The Ca2+/acidic pH state (grey) shows an additional α-helical segment when compared with the water models (green). Comparison of the E95-E97 interaction (top row) and of the Ca2+ binding site in the Ca2+/acidic pH model (bottom row). Close proximity of E95 and E97 in the Ca2+/acidic pH state whereas in water E97 flips towards E106 and is rather flexible. No Ca2+ ion and corresponding coordination is evident in the water model. Residues that change conformation are marked in red. Adjacent subunit models are displayed in lighter shade of the main color. Model 3 is not shown due to high structural similarity with model 2 in the displayed region. 