The first step is a reversible unfolding of the native catalytically active protein, followed by an irreversible alteration of the unfolded protein to give the final irreversible state. If the rate constant k3 of the second irreversible step U → I is much higher than the rate constant k2 of the reversible step of U → N then most of the U molecules will be converted to I, reducing the Lumry and Eyring model to: