There are two main models of enzyme action: Lock and Key model and Induced Fit model.
The Lock and Key Model
According to the lock and key model, the active site of an enzyme bears a specific conformation that complements the substrate precisely so that the substrate fits a particular site in a manner that’s similar to a lock and key.
This model explains how enzymes exhibit specificity for a particular substrate. The drawback of this model is that it fails to explain the stabilization of the transition state that enzymes achieve.
The Induced Fit Model
According to the induced fit model, the active site of an enzyme is not a precise fit for the substrate. Instead, when exposed to a substrate, the active site undergoes a structural change to improve binding.
This model offers a better explanation of how enzymes may exhibit broader specificity. For example, it explains how the enzyme lipase is able to bind to a variety of lipids.
The induced fit model also offers a better explanation of how catalysis may occur. According to this theory, bonds in the substrate are stressed due to conformational changes, which increases reactivity.