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

The structural dynamics and thermodynamics of the interaction of Ca2+/Mg2+ 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 Ca2+ binding involves significant conformational changes with a marked increase in the α-helix content, whereas Mg2+ 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 Ca2+-loaded CNB (CNB-Ca2+) is more stable and has more hydrophobic areas than that of Ca2+-free CNB (apo-CNB) or Mg2+-loaded CNB (CNB-Mg2+). The thermodynamic characterization suggests that there is no competition between Ca2+ and Mg2+ in their binding to the main CNB Ca2+ binding sites. Mg2+ is more likely to bind the auxiliary cation-binding sites present on CNB.