Functional Calmodulin States are Selected from an Electrostatically Tuned Free Energy Landscape

Abstract

Calmodulin (CaM) is a versatile calcium-binding protein whose structural flexibility enables regulation of diverse cellular processes. Capturing its full conformational landscape remains challenging due to high energy barriers between states. Here we employ well-tempered metadynamics simulations using key collective variables to explore CaM conformations under calcium-bound and calcium-free states at physiological and low salt concentrations. We identify four principal conformations that shift in population depending on calcium binding and ionic strength. Calcium binding favors compact states, while low salt conditions flatten the energy landscape, facilitating transitions, but also causing kinetic trapping due to salt-bridge interactions. Comparison with experimental CaM-protein complexes reveals that target binding stabilizes extended conformations distinct from minima accessible to free CaM. These findings elucidate how calcium and ionic environment orchestrate CaMs conformational dynamics, enhancing understanding of its functional adaptability in cellular calcium signaling.