Assignment of paramagnetic 15N-HSQC spectra by heteronuclear exchange spectroscopy

Paramagnetic metal ions in proteins provide a rich source of structural information, but the resonance assignments required to extract the information can be challenging. Here we demonstrate that paramagnetically shifted 15N-HSQC cross-peaks can be assigned using NZ-exchange spectroscopy under conditions in which the paramagnetic form of the protein is in dynamic equilibrium with its diamagnetic form. Even slow exchange of specifically bound metal ions may be detected within the long lifetime of 15N longitudinal magnetization of large proteins at high magnetic fields. Alternatively, the exchange can be accelerated using an excess of metal ions. In the resulting exchange spectra, paramagnetic 15N resonances become visible for residues that are not directly observed in a conventional 15N-HSQC spectrum due to paramagnetic 1HN broadening. The experiments are illustrated by the 30 kDa lanthanide-binding ɛ186/θ complex of DNA polymerase III in the presence of sub-stoichiometric amounts of Dy3+ or a mixture of Dy3+ and La3+.