Shaking the EF-Hand. Dynamics and Ion-Binding to Calmodulin and Calbindin D9k

Abstract: The work described in this thesis represents a biophysical approach, mainly using nuclear magnetic resonance (NMR) spectroscopy, to investigate the impact of magnesium ions on the EF-hand calcium-binding proteins calmodulin and calbindin D9k, as well as the dynamics of these proteins at low calcium levels. The x-ray structure of magnesium-saturated calbindin D9k shows a magnesium ion in only one of the two ion binding sites. This site is also significantly populated by magnesium at physiological conditions, in agreement with the different structural and dynamic characteristics of the two sites in this protein. Further, the coordination of magnesium is different from that of calcium, which is also reflected in a different overall conformation and a negative allosteric interaction between the two types of ions. Different picosecond to nanosecond dynamics of the ion binding sites in the absence of ions were shown to depend not only on different sequences in the sites, but also on the different surrounding structures. The two ion-binding sites in the N-terminal domain of calmodulin are significantly populated by magnesium at physiological conditions. The protein does not exhibit the same degree of conformational rearrangements that occur upon calcium binding, because of different ion coordination, probably similar to that in calbindin D9k. The apparent calcium affinity of this domain at physiological magnesium levels is decreased so that the calcium loaded state is only weakly populated even in an activated cell. In contrast to calbindin D9k the interaction between the two ion types is slightly positively allosteric. At low calcium levels, the C-terminal domain of calmodulin exhibits conformational exchange, as well as exchange with a state with one calcium ion bound, with both processes occurring on the microsecond to millisecond time scale. In both cases the exchange appears similar to the change in overall conformation induced by binding of two ions.