ABSTRACT

Because structural biology has its roots in studying globular (ordered) proteins, the classical concepts of protein structure are better suited for the description of ordered than disordered proteins. Usually, four hierarchical levels are distinguished, such as primary structure (sequence of amino acids in the polypeptide chain), secondary structure (local, often repetitive structural elements [i.e., α-helix, β-strand, turn and coil]), tertiary structure (the fold in space of the entire polypeptide chain, also meaning the spatial arrangement of its secondary structural elements), and quaternary structure (stoichiometry and spatial arrangement of subunits in a multi-subunit protein). These basic structural principles are covered in many textbooks (Garrett and Grisham 2007; Stryer 1995) and serve as a starting point for the description of the “structure” of IDPs (Chapter 10). Apparently, the physical principles governing the structural organization of the two classes of proteins are the same; only the balance between various components and dynamics of the emerging structure differ.