Effects of Calcium Binding on Structure and Autolysis Regulation in Trypsins. A Molecular Dynamics Investigation.

The calcium ion was proposed to be involved in protein structure stabilization against thermal and proteolytic degradation, such as autolysis phenomena, in trypsin-like serine proteases. However, molecular details related to the role played by the metal ion are still largely unknown. Several molecular dynamics simulations of 6 ns have been used to investigate the dynamic behavior of bovine and salmon trypsins in calcium-bound and calcium-free forms, with the aim of evaluating the role of the calcium ion in trypsin three-dimensional structure and autoproteolysis propensity. It turned out that the calcium-free trypsins are characterized by a more flexible structure, revealing structure-function relationships connecting Ca(2+) binding and autoproteolysis propensity. In particular, the removal of Ca(2+) not only increases the flexibility of regions around its binding site, in the N-terminal domain, but also leads to channeling of the fluctuations to remote sites in the C-terminal domain, possibly involving the interdomain loop. Two primary autolysis sites are strongly influenced by calcium binding (R117 and K188) in bovine trypsin, whereas Ca(2+) plays a less crucial role in salmon trypsin.