How Enzymes Work: Active Site and Induced Fit
How enzymes speed up reactions by lowering activation energy at the active site, why each enzyme is specific, and how the induced fit model replaced lock and key. Includes the 4-mark exam question, a worked model answer and the examiner trap on enzymes providing energy.
Chapters (9)
Exam question [4 marks]
Enzymes are biological catalysts. Explain how an enzyme speeds up a reaction, and why one enzyme usually catalyses only one reaction.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Active site has a specific tertiary structure / shape that is complementary to the substrate, so only that substrate fits (specificity) | "specific shape", "complementary", "other molecules do not fit / cannot bind" | "same shape as the substrate"; "the enzyme is a lock and the substrate a key" alone; "secondary structure of the active site" |
| 2 | Substrate binds to the active site forming an enzyme-substrate complex | "ES complex", "substrate binds / collides with active site" | "enzyme and substrate join permanently"; "enzyme is used up" |
| 3 | Active site changes shape slightly / moulds around the substrate (induced fit), which puts strain on the bonds in the substrate | "induced fit", "active site moulds", "bonds stressed / weakened / distorted" | "enzyme changes shape completely"; "enzyme denatures" |
| 4 | This lowers the activation energy, so more molecules have enough energy to react / reaction is faster | "less energy needed to start the reaction", "more successful collisions" | "enzyme provides / gives / adds energy"; "enzyme speeds up by heating"; "lowers the energy of the products" |
Mark 1 answers the "why only one reaction" half of the question; marks 2 to 4 answer the "speeds up" half. Mark 4 requires the idea of activation energy: the term or the paraphrase in the Accept column both score. Stating that an enzyme "gives energy" never scores, and cancels mark 4 if it appears next to a correct statement. For mark 3, the shape change (induced fit) alone earns the mark; mention of strain on the bonds is an optional addition. For mark 1, "complementary shape" alone earns the mark. Lock and key may be described, but the specification names induced fit.
Model answer (4/4)
The active site has a specific tertiary structure, so its shape is complementary to only one substrate, and other molecules cannot fit. The substrate binds to the active site, forming an enzyme-substrate complex. The active site changes shape slightly and moulds around the substrate, which is induced fit, and this puts strain on the bonds in the substrate. This lowers the activation energy, so more substrate molecules have enough energy to react and the rate of reaction increases.
Examiner trap
- Saying the substrate is 'the same shape' as the active site in the induced-fit model (it is complementary and the active site moulds)
- Saying enzymes 'give energy' to a reaction rather than lowering activation energy
- Saying the active site is a hole in the enzyme's secondary structure rather than tertiary
Key terms
- enzyme
- substrate
- active site
- enzyme-substrate complex
- activation energy
- induced fit
- lock and key
- specificity
- complementary
- product
- intracellular
- extracellular
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:26The enzyme and its active site
An enzyme is a globular protein, folded into a precise tertiary structure. That folding creates a small region on the surface called the active site. Remember, the active site is part of the tertiary structure, the three dimensional folding of the whole chain. It is not a hole in the secondary structure. The molecule the enzyme acts on is the substrate.
The shape of the active site is complementary to the shape of its substrate. Complementary means matching in shape and in charges, not identical.
1:00The enzyme-substrate complex
When the substrate enters the active site, they form an enzyme-substrate complex. The reaction happens, the products leave, and the enzyme is unchanged, ready to be used again. This is why an enzyme can be so specific. Only a substrate with the right shape fits the active site.
A different molecule has a different shape, so it cannot bind, and no complex forms. If the tertiary structure changed, the shape of the active site would change too, and the enzyme would lose its job. We will see that in the next lesson.
1:34Activation energy
Now, why does the reaction speed up? Most reactions need a push before they start. That push is the activation energy, the energy needed to get the reaction going. Think of it as a hill the substrate has to climb before it can become product.
Without a catalyst, a reaction in a cell could be far too slow for life, because the hill is too high for the energy available at body temperature.
1:59How enzymes lower it
An enzyme lowers the activation energy. The hill gets smaller. So at the same temperature, many more substrate molecules have enough energy to react, and the reaction goes faster. Notice what does not change. The reactants and products sit at the same levels.
The enzyme makes the hill smaller. It does not push anything up it.
2:23Lock and key vs induced fit
So how does the enzyme do that? The old idea was the lock and key model. The active site is a rigid shape, and the substrate fits it exactly. The newer, better model is induced fit, and it is the one the specification names. The active site is not quite a perfect fit at first.
When the substrate binds, the active site changes shape slightly and moulds around it. That change puts a strain on the bonds in the substrate, which makes them easier to break, and that lowers the activation energy.
2:57Changing models; intracellular and extracellular enzymes
Notice the models changed, because new evidence showed that proteins are flexible. Science models get replaced when better evidence appears. The spec expects you to appreciate that. And enzymes work everywhere. Intracellular enzymes act inside cells. Extracellular enzymes, such as digestive enzymes, act outside the cell that made them.
4:02Examiner trap and recap
The trap. Never say an enzyme gives energy to a reaction. It lowers the activation energy, and also, do not say the substrate is the same shape as the active site. It is complementary.
Recap. A specific active site gives specificity, the complex forms, induced fit strains the bonds, and activation energy falls.
Next time, we heat an enzyme up and see what goes wrong.
Watch first
This lesson builds on:
- Protein Structure: Primary to Quaternary · AQA 7402 · 3.1.4.1
- Globular vs Fibrous Proteins and Biuret Test · AQA 7402 · 3.1.4.1
Keep going
More from 3.1 Biological molecules
- Amino Acids and Peptide Bonds Explained · AQA 7402 · 3.1.4.1
- Protein Structure: Primary to Quaternary · AQA 7402 · 3.1.4.1
- Globular vs Fibrous Proteins and Biuret Test · AQA 7402 · 3.1.4.1
- Enzymes: Temperature and pH Effects · AQA 7402 · 3.1.4.2
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Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-010.