Enzymes: Question 6

Worked solution

Step 1: Recall the lock-and-key model

The lock-and-key model proposes that an enzyme’s active site already has a shape that is precisely complementary to its substrate, rather like a specific key fitting a specific lock. On this model, the active site does not need to change shape at all. The substrate simply slots into a pre-formed site, and this rigid complementary shape explains why an enzyme is specific to one substrate (or a small group of very similarly shaped substrates).

Step 2: Recall the induced-fit model

The induced-fit model instead proposes that the active site is not initially a perfect match to the substrate. The active site is flexible, and as the substrate begins to bind, the tertiary structure of the enzyme adjusts slightly, moulding the active site more closely around the substrate. This small conformational change can also strain particular bonds in the substrate, contributing to the lowering of activation energy. Both models still explain specificity, because only a substrate with a broadly compatible shape and chemical groups can trigger this fit in the first place.

Step 3: Evaluate each option

• A: correct, this captures the key difference: a pre-formed, unchanging active site (lock-and-key) versus a flexible active site that moulds around the substrate as it binds (induced fit).
• B: incorrect, this swaps the two models around; it is the induced-fit model, not lock-and-key, in which the active site changes shape.
• C: incorrect. In the induced-fit model it is the enzyme’s active site that changes shape around the substrate, not the substrate changing shape to fit a rigid site.
• D: incorrect, both models describe how a normal substrate binds to the active site; they are not one model for inhibitors and one for substrates.

Final answer

• The induced-fit model differs from the lock-and-key model because its active site is flexible and moulds around the substrate as it binds, rather than already being an exact fit, option A.