OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What effect does a mismatch between an enzyme and its substrate have?

1. The reaction is accelerated

2. The activation energy is lowered

3. The reaction is inhibited

4. The reaction is catalyzed

The correct answer is: The reaction is inhibited

When there is a mismatch between an enzyme and its specific substrate, the enzyme is unable to properly bind with the substrate to form the enzyme-substrate complex. This unsuccessful binding means that the enzyme cannot facilitate the chemical reaction that it is designed to catalyze. As a result, the overall reaction is effectively inhibited, meaning that the reaction rate decreases significantly or may halt entirely. Enzymes are highly specific, and this specificity is crucial for their function. If the substrate does not fit into the active site of the enzyme, the enzyme cannot lower the activation energy required for the reaction. Therefore, the expected catalytic action does not occur, which reinforces the concept that a proper match is essential for enzymatic activity.