Ca(2+) dissociation from the C-terminal EF-hand pair in calmodulin: a steered molecular dynamics study.

We used steered molecular dynamics (SMD) to simulate the process of Ca(2+) dissociation from the EF-hand motifs of the C-terminal lobe of calmodulin. Based on an analysis of the pulling forces, the dissociation sequences and the structural changes, we show that the Ca(2+)-coordinating residues lose their binding to Ca(2+) in a stepwise fashion. The two Ca(2+) ions dissociate from the two EF-hands simultaneously, with two distinct groups among the five Ca(2+)-coordinating residues affecting the EF-hand conformational changes differently. These results provide new insights into the effects of Ca(2+) on calmodulin conformation, from which a novel sequential mechanism of Ca(2+)-calmodulin dissociation is proposed.