Structural aspects of conformational changes in ligand binding by antibody fragments

Conformational changes in immunoglobulin fragments on complexation were first noted in trigonal crystals (ammonium sulphate form) of the Meg light chain dimer in 1974 (Edmundson et al., 1974). Additional studies with Meg crystals demonstrated how induced fit mechanisms increase the diversity of ligands which can be bound by a single active site (Ely et al., 1978; Edmundson el al., 1984, 1987, 1989, 1993; Edmundson and Ely, 1985). The ligandinduced conformational changes in the Meg dimer were all studied in crystals of the same space group. While the crystals sometimes cracked and the X-ray reflections broadened in some cases, the crystals were rarely destroyed and the unit cell dimensions were seldom altered by more than 0.5 % as a result of ligand binding. In a limited number of trials, even the complexes prepared in solution produced crystals that were isomorphous with those of the native protein.