As mentioned earlier, a difference between an enzyme and a chemical catalyst is that an enzyme is flexible. Its slight changes in shape (often arising from the binding of the substrate itself) help to position substrates for reaction after they bind. These changes in shape are explained, in part, by Koshland’s Induced Fit Model of Catalysis, which illustrates that not only do enzymes change substrates, but that substrates also transiently change enzymes. At the end of the catalysis, the enzyme is returned to its original state. Enzyme flexibility also is important for control of enzyme activity. Two distinct structures are typically described– the T (tight) state, which is a lower activity state and the R (relaxed) state, which has greater activity.