Induced fit is the change in shape of the active site of an enzyme as it embraces its substrate.
The "lock and key" theory has been largely replaced by the "induced fit" model
It suggests that the active site does not at first fit the substrate perfectly.
Rather, as the substrate approaches, the active site changes its shape until the substrate is completely bound to it.
This change in shape of the active site is known as an induced fit.
In its new shape, the active site then catalyzes the reaction
For example, in the phosphorylation of glucose to glucose-6-phosphate, the glucose enters the active site of the enzyme.
The enzyme closes around the glucose, and the reaction takes place
When the products are released, the enzyme reverts to its original form.
Here's an animation of the induced fit process.