Answer
The lock and key enzyme model equates the substrate-enzyme binding to how a key binds to its lock.
In the original model, enzymes were considered to be rigid, but it is now accepted that enzymes are flexible and undergo conformational change when they bind to their substrates (the induced fit model).
Work Step by Step
Enzymes catalyze reactions by binding with substrates. Since enzymes are also mostly proteins, they have specific amino acid sequences where the substrate can bind, called the active site. Each enzyme is structured such that the active site is exposed when there is a substrate. The binding of a substrate causes a conformational change in the enzyme such that the substrate can fit more tightly within the enzyme, speeding up the reaction.
