Exploring and Engineering the Conformational Landscape of Calmodulin through Specific Interactions.

Conformational fluctuations often play paramount role in the function and activity of proteins. Calmodulin (CaM) is a calcium sensing protein that shows significant conformational flexibility on going from a ligand-free open state to a ligand-bound closed state. By employing large-scale equilibrium molecular dynamics simulations and free energy calculations, we have shown that apo CaM frequently visits a state that is neither fully open nor fully closed and referred to as half-open half-closed (HOHC) state. Such states are functionally relevant as they structurally resemble a ligand-bound closed state. Here, we have envisaged that the inherent conformational dynamics of CaM is primarily triggered by a dual salt bridge interaction between the glutamates and lysine at two different domains (N- and C-terminal domains) of the protein. Upon abolition of the dual salt bridge interaction, conformational dynamics is restricted and centered only near the open state. When a cation-π interaction is introduced while replacing the dual salt bridge interaction, the HOHC state tends to reappear as a stable state. Since salt bridge and cation-π interactions are frequently encountered in the protein dynasty, our work may illuminate an interesting direction toward controlling the conformational landscape of proteins through the modulation of minimal specific interactions.