Chapter 5: Protein–Protein Interactions

Paramagnetic NMR methods are excellently suited for the study of protein–protein complexes in solution. Intermolecular pseudocontact shifts (PCSs), residual dipolar couplings (RDCs) and paramagnetic relaxations enhancements (PREs) can be used, ideally in combination, for docking proteins and determining their orientation in the complex. PCSs can be used for breaking the structure symmetry in dimer complexes. PCSs also can be applied to detect structural differences in proteins and protein complexes in solution in comparison to crystal structures. RDCs are sensitive to the degree of alignment of both partners in a protein complex and are thus very useful to detect dynamics within complexes. PREs can detect states in which nuclei approach a paramagnetic centre closely, even if it exists only for a small fraction of the time. Thus, PREs are used to detect minor states and characterize ensembles. PRE studies have been the foundation for characterizing encounter states and the process of protein complex formation. In weak complexes, such as are found in electron transfer chains, proteins can be in an encounter state for a large fraction of the complex lifetime. Paramagnetic NMR tools thus have found many applications for studying protein complexes, and more may be on the horizon.