Book excerpt: Mass spectrometry (MS) has emerged as a powerful tool for epitope mapping, protein-ligand interaction, protein-protein interaction, aggregation, and effect of solution environment on protein conformation because they provide high-throughput data with relatively high structural resolution. Two popular MS-based approaches are hydrogen deuterium exchange-mass spectrometry (HDX-MS) and fast photochemical oxidation of proteins (FPOP), which complement classical biophysical and biochemical techniques in achieving higher structural resolution. The research presented in this dissertation is focused on the application of mass spectrometry-based footprinting techniques in characterizing the biophysical properties of Part I: pH-dependent conformation change of diphtheria toxin T domain (Chapters 2-4)); Part II: Ca2+ binding proteins and the role of Ca2+ regulation (Chapters 5-6); and Part III: protein-protein interaction including epitope mapping of IL-23 (Chapter 7) and Marburg virus protein VP24 (Chapter 8). Chapter 1 serves as an introduction to mass spectrometry instrumentation and standard LC-MS workflow. Two mass spectrometry based-footprinting techniques are introduced: (1) hydrogen deuterium exchange (HDX), and (2) fast photochemical oxidation of proteins (FPOP). Part I focuses on the development of pH-dependent HDX-MS for the conformation study of diphtheria toxin T domain. In Chapter 2, we describe the use pH-dependent HDX to study the pH-dependent conformation change of wild-type diphtheria toxin T domain monomer along its translocation pathway. In Chapter 3, we study the pH-dependent dissociation and reformation of T domain dimer. In Chapter 4, we apply the same method to a T domain mutant H223Q to further investigate the role of key histidine residues in triggering the conformation change. Part II focuses on the application of HDX mass spectrometry for the study of calcium binding proteins. Chapter 5 describes the Ca2+-binding property of ACaM and its Ca2+-regulated interaction with myosin VI. In chapter 6, HDX is also applied to an EF-hand Ca2+ binding protein, DREAM, for the study of its Ca2+ binding sites and stoichiometry. Part III of the dissertation focuses on the development and application of MS-based footprinting methods to investigate protein-protein interaction. Chapter 7 describes the methodology of fast photochemical oxidation of proteins (FPOP) for epitope mapping of IL-23 interacting a therapeutic antibody from Bristol-Myers Squibb. Chapter 8 discusses the use of HDX, FPOP, and NEM chemical labeling for the study of Marburg virus protein VP24 and its interaction with the host protein Keap1 Kelch domain. These seven studies on characterization of protein conformation dynamics, Ca2+ binding protein, and protein-protein interaction show the successful application of mass spectrometry in the structural study of large biomolecules.