In the lock-and-key model of enzyme action, the fits into the of the enzyme

In the lock-and-key model of enzyme action, the substrate fits into the active site of the enzyme. Enzyme catalyzes a chemical reaction which starts with the binding of the substrate with the active site (specific region) present in the surface of the enzyme.

Some changes occur in the distribution of electrons in the chemical bonds which are present on the substrate and cause the reaction to leads the formation of products by the binding of the substrate to the enzyme.

The product which is released from the enzyme surface to regenerate the enzyme for next reaction cycle. The active site has unique ability means only one or few enzymes can react with similar compounds.

In the lock and key theory, the enzyme is the lock and key is the substrate. Which means only correct size substrate can fit into the active site of the enzyme.