induced-fit model
/ in-DOOST fit MOD-el /
An old, simple picture said an enzyme and its substrate fit together like a rigid key in a rigid lock — only the exactly right key turns. That captures specificity, but it is too stiff to be true. A better everyday picture is a glove and a hand: the glove looks loose and shapeless until you put your hand in, and only then does it close snugly around your fingers. The induced-fit model says the enzyme behaves more like the glove than the lock.
In the induced-fit model, when the correct substrate enters the active site, the enzyme changes shape slightly, wrapping more tightly around the substrate. This subtle reshaping does two things: it grips the substrate firmly, and it bends and strains the substrate's bonds toward the transition state — the high-energy in-between form. By actively molding itself around the substrate, the enzyme does more than just hold it; it helps push the reaction over the activation-energy barrier. The fit becomes ideal during the reaction, not before it.
Induced fit explains why enzymes are both selective and powerful catalysts: the shape change can be triggered only by the right substrate, and the change itself contributes to lowering the activation energy. It refines, rather than replaces, the lock-and-key idea — specificity is still real, but the lock gently reshapes around the key. This is also why a wrong-shaped molecule may slip in but fail to trigger the productive fit.
When the enzyme hexokinase binds glucose, its two lobes swing shut around the sugar like a clam closing — squeezing out water and clamping the glucose into exactly the position needed for the next step of breaking down sugar for energy.
The enzyme reshapes around the right substrate, like a glove around a hand.
Induced fit refines the old 'lock and key' idea — it does not throw out specificity; the wrong molecule simply fails to trigger the productive shape change.