Which model describes the idea that the enzyme active site and substrate have complementary shapes?

The lock and key model describes the concept that the enzyme's active site is shaped specifically to fit the substrate, much like a key fits into a lock. This model suggests that the enzyme and the substrate possess complementary shapes that allow for a precise binding. In this view, the active site is already in the appropriate form to accommodate the substrate without any change in the structure of the enzyme. This model emphasizes the specificity of enzymes and how they interact only with particular substrates whose shapes match.

In contrast, the induced fit model suggests that the active site of the enzyme undergoes a conformational change upon binding to the substrate, allowing for a tighter fit. The substrate theory and catalytic model do not directly address the specific mechanism of shape complementarity as characterized in the lock and key model. Thus, the correct identification of the lock and key model illustrates the fundamental concept of enzyme specificity based on the structural compatibility between the enzyme's active site and the substrate.