Conformational selection is a dominant mechanism of ligand binding.

Molecular recognition in biological macromolecules is achieved by binding interactions coupled to conformational transitions that precede or follow the binding step, two limiting mechanisms known as conformational selection and induced fit, respectively. Sorting out the contribution of these mechanisms to any binding interaction remains a challenging task of general interest in biochemistry. Here we show that conformational selection is associated with a vast repertoire of kinetic behaviors, can never be disproved a priori as a mechanism of ligand binding, and is sufficient to explain the relaxation kinetics documented experimentally for a large number of systems. On the other hand, induced fit features a narrow spectrum of kinetic behaviors and can be disproved in many cases in which conformational selection offers the only possible explanation. This conclusion offers a paradigm shift in the analysis of relaxation kinetics, with conformational selection acquiring preeminence as a mechanism of ligand binding. The dominant role of conformational selection supports the emerging structural view of the macromolecule as a conformational ensemble from which the ligand selects the initial optimal fit to produce a biological response.