What defines the fit of a substrate in an enzyme according to the lock and key model?

The lock and key model of enzyme activity describes a specific interaction between an enzyme and its substrate, where the enzyme's active site is precisely shaped to fit a particular substrate, much like a lock fits a specific key. In this model, the interaction is characterized as rigid; the substrate fits into the enzyme's active site without necessitating any significant alterations or adjustments in shape from either the substrate or the enzyme. This specificity ensures that enzymes catalyze only particular reactions, highlighting the importance of the precise fit and structural compatibility.

The rigidity of the interaction emphasizes that the enzyme remains unchanged during the binding process, allowing the enzyme to effectively catalyze the reaction with the substrate. This contrasts with other models, such as the induced fit model, where the enzyme may adapt its shape to better accommodate the substrate. In the context of the lock and key model, only substrates that match the specific complementary shape of the active site can bind effectively, reinforcing the model's characterization of a rigid interaction.