By the end of this lesson
- Define an enzyme as a biological catalyst.
- Explain specificity using active sites and enzyme–substrate complexes.
- Use and evaluate models of enzyme action.
BIG QUESTION
How can a tiny amount of protein make a reaction fast enough for life without being consumed?
Catalysis changes the reaction pathway and lowers activation energy; it does not supply energy or alter the final products.
Catalysts make cellular chemistry fast enough
An enzymeA protein that acts as a biological catalyst, speeding up a chemical reaction without being used up. is a protein that acts as a biological catalyst. It increases reaction rate without being used up overall, so one enzyme molecule can catalyse many substrate conversions.
Enzymes lower the activation energyThe minimum energy needed for a chemical reaction to start; enzymes lower the activation energy. by providing an alternative reaction pathway. They do not create energy, make impossible reactions possible or change the equilibrium yield; they help equilibrium be reached faster.
Binding produces an enzyme–substrate complex
A substrateThe molecule an enzyme acts on, which binds to the enzyme's active site. binds at the enzyme’s active siteThe region of an enzyme with a specific shape, complementary to its substrate, where the substrate binds and the reaction is catalysed.. Interactions between them form an enzyme–substrate complex, position reactive groups and stabilise the transition towards products.
Products no longer fit or bind in the same way and are released. The enzyme’s overall structure is restored and it can catalyse another reaction.
Specificity depends on chemistry and shape
An active site is complementary to its substrate in shape and chemical properties such as charge. Other molecules may fail to bind or may bind without undergoing the same reaction.
The lock-and-key model communicates specificity but suggests a rigid active site. The induced-fit modelThe idea that an enzyme's active site changes shape slightly as the substrate binds, so the two fit closely together. is richer: substrate binding can cause a small conformational change that improves catalysis.
Enzymes coordinate metabolism
Cells use different enzymes for different steps in respiration, photosynthesis, synthesis, digestion and many other pathways. Regulating enzyme amount or activity can regulate an entire pathway.
Specificity is high but not always absolute. Some enzymes act on a family of related substrates, so exam answers should avoid the universal claim “one enzyme can act on exactly one molecule.”
Following one catalytic cycle
The enzyme participates in each stage but is not consumed overall.
| Stage | Molecular event | Significance |
|---|---|---|
| 1. Collision | Substrate reaches the active site in a suitable orientation. | A successful collision becomes possible. |
| 2. Binding | An enzyme–substrate complex forms. | Interactions position and strain reacting groups. |
| 3. Catalysis | Bonds break or form through a lower-activation-energy pathway. | The reaction proceeds faster. |
| 4. Release | Products leave the active site. | The unchanged enzyme is available again. |
Language that earns precision
- enzyme
- A protein that acts as a biological catalyst.
- catalyst
- A substance that increases reaction rate without being used up overall.
- substrate
- A reactant molecule upon which an enzyme acts.
- active site
- The region of an enzyme where the substrate binds and catalysis occurs.
- activation energy
- The minimum energy barrier that reacting particles must overcome.
- enzyme–substrate complex
- The temporary association formed when substrate binds to an enzyme.
Common misconceptions
Claim: Enzymes give reactions energy.
Correction: They lower the activation-energy barrier; they do not create or supply energy.
Claim: An enzyme is used up each time it works.
Correction: It is regenerated after product release and can catalyse repeated cycles.
Claim: The lock-and-key model is a literal picture.
Correction: It is a simplified model; real proteins move and active sites can change conformation.
Claim: All enzymes break molecules down.
Correction: Enzymes catalyse both breakdown and synthesis reactions.
Match the answer to the command word
Include protein, biological catalyst, faster rate and not used up overall.
Sequence substrate binding, complex formation, catalysis and product release.
State what a model explains and the molecular behaviour it oversimplifies.
Recall, apply, then justify
Recall, apply, then justify (continued)
Define the term enzyme.
Hint
State what kind of molecule it is and what it does.
Suggested marking points
- A protein or biological catalyst.
- Increases reaction rate without being used up overall.
Explain why an enzyme can catalyse many substrate molecules.
Hint
Follow one cycle to product release.
Suggested marking points
- Substrate binds and is converted to product.
- Products leave the active site.
- The enzyme is not consumed and can bind another substrate.
Explain enzyme specificity.
Hint
Use complementarity and complex formation.
Suggested marking points
- The active site has a particular shape and chemical properties.
- A complementary substrate binds to form an enzyme–substrate complex.
- Non-complementary molecules bind poorly or are not catalysed.
Evaluate one limitation of the lock-and-key model.
Hint
Contrast a rigid lock with a dynamic protein.
Suggested marking points
- It usefully represents complementarity and specificity.
- It fails to show active-site flexibility or induced fit during binding.
Explain how an enzyme can increase reaction rate without changing the equilibrium position of a reversible reaction.
Planning hint
Distinguish pathway speed from relative stability of reactants and products.
What a strong design includes
- The enzyme lowers activation energy by providing an alternative pathway.
- It accelerates both forward and reverse reactions.
- It does not change the free-energy difference between reactants and products.
- Equilibrium is reached sooner but at the same position under unchanged conditions.