Understanding Protein–Protein Interactions in Biology: Applications of NMR Spectroscopy and Isothermal Titration Calorimetry

Protein–protein interactions play a central role in cellular signaling, immune recognition, and therapeutic development, yet they are often characterized by weak affinities, transient binding, and pronounced conformational flexibility. These features present significant challenges for conventional structural biology techniques. Biophysical approaches, particularly nuclear magnetic resonance (NMR) spectroscopy, and isothermal titration calorimetry (ITC) have emerged as powerful tools for elucidating peptide interactions under near-physiological conditions. NMR offers residue-level information on interaction interfaces, conformational changes, and protein dynamics in solution, making it uniquely suited for the analysis of weak and transient interactions. In contrast, ITC provides a direct and label-free measurement of binding thermodynamics, yielding quantitative parameters such as affinity, stoichiometry, and the enthalpic and entropic contributions to binding. This review highlights the principles, applications, and limitations of NMR and ITC in protein–protein interaction research, emphasizing how their combined use enables an integrated understanding of structure, dynamics, and energetics. Representative examples from the literature are discussed, including viral peptide–host protein interactions such as those involving Epstein–Barr virus latent membrane protein 1 (LMP1). Together, these studies illustrate the unique ability of NMR and ITC to capture structural and dynamic features of peptide recognition that are critical for understanding biological function and guiding peptide-based therapeutic design.