A model for enzyme-substrate interaction suggesting 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).
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.
Compare: induced fit model.