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

What is the consequence of a substrate not fitting into an enzyme's active site?

1. The enzyme will catalyze the reaction anyway

2. The substrate will be modified

3. The reaction will not occur

4. The enzyme will increase its specificity

The correct answer is: The reaction will not occur

An enzyme's active site has a specific shape that is complementary to the shape of its substrate, much like a key fits into a lock. When a substrate does not fit into the enzyme's active site, it cannot properly bind to the enzyme. This lack of binding means that the enzyme will not be able to facilitate the chemical reaction associated with that substrate. The consequence is that the reaction will not occur because the necessary interaction between the enzyme and substrate necessary for catalysis is absent. The idea behind enzyme specificity is crucial here; enzymes are designed to interact with specific substrates, and if the correct substrate cannot bind, then the entire process of catalysis is prevented. Other choices imply possible scenarios that do not align with the fundamental principles of enzyme function, like the enzyme continuing to work without a substrate, changing a substrate without a proper fit, or altering its specificity, which falls outside the established context of enzyme-substrate interactions.