15N NMR assignments and chemical shift analysis of uniformly labeled 15N calbindin D9k in the apo, (Cd2+)1 and (Ca2+)2 states.

15N has been uniformly incorporated into the EF-hand Ca(2+)-binding protein calbindin D9k so that heteronuclear experiments can be used to further characterize the structure and dynamics of the apo, (Cd2+)1 and (Ca2+)2 states of the protein. The 15N NMR resonances were assigned by 2D 15N-resolved 1H experiments, which also allowed the identification of a number of sequential and medium-range 1H-1H contacts that are obscured by chemical shift degeneracy in homonuclear experiments. The 15N chemical shifts are analyzed with respect to correlations with protein secondary structure. In addition, the changes in 15N chemical shift found for the apo----(Cd2+)1----(Ca2+)2 binding sequence confirm that the effects on the protein are mainly associated with chelation of the first ion.