How Do Enzymes Work? Lock and Key vs Induced Fit

Enzymes are biological catalysts: they speed up reactions by lowering the activation energy, the minimum energy needed for a reaction to start. A substrate binds to an enzyme’s active site, the reaction takes place, and the products leave. The enzyme is not used up and can catalyse another reaction.

Lock and key vs induced fit

The lock-and-key model describes the active site as a fixed shape that already fits the substrate exactly, like a key fitting a lock. It helps explain why an enzyme is specific: only certain substrates can bind to its active site. However, it does not account for the active site changing shape during binding.

The induced-fit model describes the active site as flexible. When a suitable substrate begins to bind, it causes the active site to change shape so that it fits the substrate more closely. This forms an enzyme–substrate complex. The change can strain bonds in the substrate, making them easier to break and lowering the activation energy. The reaction produces products, which leave the active site.

The active site’s shape depends on the enzyme’s tertiary structure, its overall three-dimensional folding. If that folding changes, for example at an unsuitable pH, the substrate may no longer bind as effectively. Induced fit still requires a suitable substrate: the active site does not mould itself around every molecule.

For example, if an enzyme catalyses the breakdown of a molecule, binding may distort a bond in that molecule. Less energy is then needed to break the bond than would be needed without the enzyme.

Exam mistake to avoid

For AQA A-level Biology, use induced fit when explaining how an enzyme works. Do not describe the active site as permanently rigid, or say that the enzyme is used up. Both models help illustrate specificity, but induced fit explains the change in active-site shape that helps catalyse the reaction.