The first model of enzyme action was proposed in 1894 by Emil Fischer. In this model, the lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme). Much later, in 1958, the induced fit model was suggested by Daniel Koshland. The induced-fit theory assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible. This explains why certain compounds can bind to the enzyme but do not react because the enzyme has been distorted too much. Which model BEST represents the newer version of the enzyme action model?

The first model that looked to explain the enzyme and substrate interaction was the "lock and key" model. Although this model helped scientists to understand the specificity nature of the interaction enzyme-substrate, it does not reflect reality since enzyme was portrayed as a lock with a structure that doesn't change during the catalysis. Then half century later "The induced-fit theory" was proposed which fits much better the newer version of the enzyme action model. In this theory the enzyme is partially flexible and its structure changes as a result of its interaction with the substrate.