Mg2+ used in crystallization, Ca2+ was replaced by Mg2+. In the closed state, the Ca2+ bridges three central cleft β-strands through the involvement of residue atoms in its coordination: Asp332 Oδ2 (β10), Asn380 Oδ1 (β14) and Val435 O (β18). Additionally, the loop 316–329 is involved in the binding of the calcium ion via a hydrogen bond between Tyr323 and Asp332. By contrast, the Mg2+ ion found in open state is coordinated only by residues from β18, Asp436 Oδ1 and Val435 O. Therefore, the opening of loop 316–329 and the concomitant disruption of the hydrogen bond between Tyr323 and Asp332 likely allows for a solvent accessibility of the central pore, which resulted in a displacement of the bound Ca2+ by Mg2+ at high concentration in the crystallization liquor.
