Assignment Strategy for Fast Relaxing Signals: Complete Aminoacid Identification in Thulium Substituted Calbindin D_9K

Abstract

Paramagnetic proteins generally contain regions with diverse relaxation properties. Nuclei in regions far from the metal center may behave like those in diamagnetic proteins, but those closer to the metal experience rapid relaxation with accompanying line broadening. We have used a set of NMR experiments optimized to capture data from these various concentric regions in assigning the signals from a paramagnetic Calbindin D_9K derivative in which one of the two calcium ions has been replaced by thulium(III). Normal double- and triple-resonance experiments with ¹H detection were used in collecting data from nuclei in the diamagnetic-like region; these approaches identified signals from fewer than 50% of the amino acid residues (those with d>17.5 Å from thulium(III)). Paramagnetism-optimized two-dimensional NMR experiments with ¹H detection were used in collecting data from nuclei in the next nearer region (d>15 Å). Standard (d>14 Å) and optimized (d>9 Å) ¹³C direct-detection experiments were used to capture data from nuclei in the next layer. Finally nuclei closest to the metal were detected by one-dimensional ¹³C (d>5 Å) and one-dimensional ¹⁵N data collection (d>4.2 Å). NMR signals were assigned on the basis of through-bond correlations and, for signals closest to the metal, pseudocontact shifts. The latter were determined from chemical shift differences between assigned signals in thulium(III) and lanthanum(III) derivatives of Calbindin D_9K and they were interpreted on the basis of a structural model for the lanthanide-substituted protein. This approach yielded assignments of at least one resonance per amino acid residue, including those in the thulium(III) coordination sphere.