The early elucidation for the ligand-receptor binding mechanism is the lock-and-key theory
proposed by Fischer [27], in which the ligand fits into the receptor like lock and key. The
earliest reported docking methods [10] were based on this theory and both the ligand and
receptor were treated as rigid bodies accordingly. Then the “induced-fit” theory [28, 29]
created by Koshland takes the lock-and-key theory a step further, stating that the active site
of the protein is continually reshaped by interactions with the ligands as the ligands interact
with the protein. This theory suggests that the ligand and receptor should be treated as
flexible during docking. Consequently, it could describe the binding events more accurately
than the rigid treatment.