How It Works: The Lock-and-Key Mechanism
Part of Enzymes in Digestion — GCSE Biology
This how it works covers How It Works: The Lock-and-Key Mechanism within Enzymes in Digestion for GCSE Biology. Enzyme structure and function, digestive enzymes, factors affecting enzyme activity, lock and key model, and practical investigations It is section 13 of 19 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 13 of 19
Practice
20 questions
Recall
25 flashcards
How It Works: The Lock-and-Key Mechanism
Enzyme specificity comes entirely from shape. The active site is a pocket or groove on the enzyme's surface with a precise three-dimensional shape. Only a substrate with a complementary shape can fit into that pocket — in the same way only the correct key will turn a lock.
When the substrate slots into the active site, it forms an enzyme-substrate complex. Being held in this position brings reactive parts of the substrate close together and weakens specific chemical bonds. This lowers the activation energy — the minimum energy needed to start the reaction. Because less energy is needed, the reaction proceeds far more quickly than it would without the enzyme.
Once the reaction is complete, the products no longer fit the active site shape and are released. The enzyme returns to its original state, completely unchanged, and is free to bind another substrate molecule. This is why enzymes are described as biological catalysts — they speed up reactions without being used up.
Denaturation explained: High temperatures and extreme pH values break the bonds that hold the enzyme in its precise three-dimensional shape. The active site distorts permanently. No substrate can bind, so the enzyme is non-functional. This is irreversible — unlike slowing down at low temperature, which is temporary.