How does the induced-fit model modify the classic lock-and-key concept?

• A. It maintains a completely rigid active site that never changes.
• B. It requires the substrate to be a perfect chemical match but allows no movement.
• C. It proposes that the receptor or enzyme changes shape upon binding to achieve a better fit.
• D. It denies any conformational changes during binding.

Answer: (C)

The main idea is that proteins are flexible and can mold themselves to better accommodate a substrate. In the classic lock-and-key view, the active site is already a perfect match and remains rigid. But in the induced-fit model, the initial contact between enzyme and substrate triggers movement in the enzyme, reshaping the active site so the substrate fits more precisely and the catalytic groups are positioned optimally. This induced conformational change helps stabilize the transition state, lowers the activation energy, and can enhance specificity. After the reaction, the enzyme often returns toward its original shape, ready to bind another substrate. So, the induced-fit idea replaces a completely fixed pocket with a dynamic one that adapts as binding occurs.