Distinguishing induced fit from conformational selection

The interactions between proteins and ligands often involve a conformational change in the protein. This conformational change can occur before (conformational selection) or after (induced fit) the association with ligand. It is often very difficult to distinguish induced fit from conformational selection when hyperbolic binding kinetics are observed. In light of a recent paper in this journal (Vogt et al., Biophys. Chem., 186, 2014, 13-21) and the current interest in binding mechanisms emerging from observed sampling of distinct conformations in protein domains, as well as from the field of intrinsically disordered proteins, we here describe a kinetic method that, at least in some cases, unequivocally distinguishes induced fit from conformational selection. The method relies on measuring the observed rate constant λ for binding and varying both the protein and the ligand in separate experiments. Whereas induced fit always yields a hyperbolic dependence of increasing λ values, the conformational selection mechanism gives rise to distinct kinetics when the ligand and protein (displaying the conformational change) concentration is varied in separate experiments. We provide examples from the literature and discuss the limitations of the approach.