Force-probe MD simulations of a wtA2 model with or without a Ca²⁺ bound at the metal-binding site. (A) Force profiles of force-probe MD simulations of the wtA2 model. The α3-β4 region adopts a loop conformation with or without a Ca²⁺ ion bound at the site equivalent to the Ca²⁺-binding site of ssA2. The Ca²⁺ ion is replaced with a water molecule (left panel) or kept at the binding site (right panel). (B) Selected snapshots of the trajectory of force-probe MD simulations of the wtA2 model bound with a Ca²⁺ ion. For the A2 domain, the α-helices are colored in pink, the β strands in yellow, and the α3-β4 loop in cyan. The β4 strand is marked. The residues that participate in the formation of the Ca²⁺-binding site are shown as ball-and-stick models and labeled. The bound Ca²⁺ is shown as a golden sphere.