The lock and key analogy is used because each enzyme has specific substrates that fit into it. This is like how locks only work with a specific key. The enzyme is the lock and the substrates are the key.
Work Step by Step
This analogy is not perfect because there are often multiple substrates for each enzyme reaction. It only takes one key to turn the lock. The reaction is also taking place between the substrates themselves and not the enzyme. In the lock and key analogy, the lock is affected by the key, but in an enzyme reaction, the substrates are affected by the enzyme.