Molecular dynamics simulation technique is employed to investigate the transition of protein calbindin-AFF in Ca2+-free and Ca2+-bound from N-terminal-folded conformation to C-terminal-folded conformation,respectively.The sequence of protein calbindin-AFF and the corresponding two conformations were analyzed and are shown in Figure 1(a)and(b).Visual inspection of the trajectories by using targeted MD simulation shows that their transition processes in Ca2+-free and Ca2+-bound resemble each other.The restraint energies extracted from the simulation trajectories for both transitions in Ca2+-free are very similar.This energy for the transition from N-terminal-folded conformation to C-terminal-folded conformation in Ca2+-bound is significantly lower than that from C-terminal-folded conformation to N-terminal-folded conformation(see Figure 1(c)).These results predict that the Ca2+ ion as an induced agent can favor the stability of C-terminal-folded conformation,which suggests that the calbindin-AFF is expected to adopt C-terminal-folded conformation in the presence of calcium.It is expected that the AFF-engineered conformational change as optical biosensor is feasible.