The inactivation of a number of enzymes during denaturation by physical and chemical factors precedes detectable global conformational changes of the molecules as monitored by conventional methods. It was suggested that the enzyme active site is more flexible and more sensitive to denaturation than the molecule as a whole. The well-known "induced fit" hypothesis by Koshland implies multiconformational states of enzymes in equilibrium with one another easily perturbed by ligands. Each intermediary step during the entire catalytic process may require the molecule to be in a particular conformation state; rapid interconversion between the different conformation states may well be involved in the catalytic process. As a relative fragile and consequently flexible active site has now been envisaged, it appears that a rapid cycling of the different active site conformation states is essential for the full expression of enzyme activity.