Locating the metal ion in calcium-binding proteins by using cerium(III) as a probe.

The detection and assignment of NMR spectroscopic signals of carbon atoms from carbonyl and carboxylate groups in the loop hosting the Ce(III) ion was performed for the cerium-substituted calcium-binding protein calbindin D9k. This provided a tool to characterize in solution the first coordination sphere of the metal ion. Due to the well-documented possibility of replacing calcium with metal ions of the Ln(III) series, this approach turns out to be extremely efficient for characterizing in solution the coordination of calcium ions in proteins, independently of the availability of X-ray crystal structures. The present approach completes the structural characterization of lanthanide-substituted calcium-binding proteins, for which the role of long-range constraints arising from hyperfine interaction and self-orientation has already been assessed.