The lock-and-key mechanism refers to

The lock and key mechanism refer to the specific enzyme-substrate reaction mechanism.

Explanation:

The lock and key mechanism explain the specific action of the enzyme on the substrate in a specific reaction. The enzyme is the lock and the key is the substrate. The correctly shaped and sized key that is the substrate, fits into the keyhole which is the active site of the lock that is enzyme as per the lock and key mechanism.

As we know the only a specific key can fit in the keyhole and unlock the lock same action goes with enzyme and substrate, only a specific substrate fits in with the active site of the enzyme in particular reaction.

Thus, the lock and key mechanism refer to the specific enzyme-substrate reaction mechanism.