Structural dynamic and thermodynamic analysis of calcineurin B subunit induced by calcium/magnesium binding

The structural dynamics and thermodynamics of the interaction of Ca(2+)/Mg(2+) with the calcineurin B subunit (CNB) were monitored by Fourier transform infrared spectroscopy (FT-IR) and isothermal titration calorimetry (ITC). The results suggest that CNB activation by Ca(2+) binding involves significant conformational changes with a marked increase in the α-helix content, whereas Mg(2+) binds to CNB without inducing changes in secondary structure. The results of hydrogendeuterium (HD) exchange and GdnHCl-induced unfolding show that the overall conformation of Ca(2+)-loaded CNB (CNB-Ca(2+)) is more stable and has more hydrophobic areas than that of Ca(2+)-free CNB (apo-CNB) or Mg(2+)-loaded CNB (CNB-Mg(2+)). The thermodynamic characterization suggests that there is no competition between Ca(2+) and Mg(2+) in their binding to the main CNB Ca(2+) binding sites. Mg(2+) is more likely to bind the auxiliary cation-binding sites present on CNB.