Enzymes are highly specific. They must bind to a specific substrate before they can catalyze a chemical reaction. At present, there are two models, which attempt to explain enzyme specificity: (1) lock-and-key model and (2) induced fit model. In lock-and-key model, the enzyme-substrate interaction suggests that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. Like a key into a lock, only the correct size and shape of the substrate (the key) would fit into the active site (the key hole) of the enzyme (the lock). As for the induced fit model suggested by Daniel Koshland in 1958, it suggests that the active site continues to change until the substrate is completely bound to the active site of the enzyme, at which point the final shape and charge is determined. Unlike the lock-and-key model, the induced fit model shows that enzymes are rather flexible structures.
The lock and key model theory first postulated by Emil Fischer in 1894 shows the high specificity of enzymes. However, it does not explain the stabilization of the transition state that the enzymes achieve.