NMR Studies of Calcium and Magnesium in Biological Systems

Abstract

The NMR spectroscopic technique has developed to a state where studies of ²⁵Mg²⁺ and ⁴³Ca²⁺ in the millimolar concentration range are feasible. This development has in turn opened new possibilities in the study of calcium and magnesium binding proteins. In the article three proteins are chosen as examples: troponin C, calmodulin and phospholipase A₂. The type of information that can be obtained is: (i) binding constants in the range 1 to 10⁴ M^-1; (ii) the competition of different cations for the calcium or magnesium binding site; (iii) the effects of other ligands (drugs etc.) on the protein; (iv) pK_a values for the groups involved in the Mg²⁺ or Ca²⁺ binding; (v) the dynamics of the binding site (correlation time). ⁴³Ca NMR studies of proteins with calcium binding constants exceeding 10⁵ M^-1 are generally not possible. In such situations the use of ¹¹³Cd or ¹¹¹Cd NMR is often a powerful alternative. The Cd isotopes have both spin I = 1/2 and NMR signals from the individual ions when bound to the protein may be obtained. The chemical shift of cadmium is very sensitive to the nature of the ligands. Additional information may be obtained from competition experiments, chemical modifications, pH variations etc.